Encyclopædia Britannica, Ninth Edition/Tunicata

by William Abbott Herdman

TUNICATA

THIS group of animals was formerly regarded as constituting along with the Polyzoa and the Brachiopoda the invertebrate class Molluscoidea. It is now known to be a degenerate branch of the Chordata, and to be more nearly related to the Vertebrata than to any group of the Invertebrata.

History.^[1]

Simple Ascidians.More than two thousand years ago Aristotle gave a short account of a Simple Ascidian under the name of Tethyum. He described the appearance and some of the more important points in the anatomy of the animal. From that time onwards to little more than a century ago, although various forms of Ascidians had been briefly described by writers on marine zoology, comparatively little advance was made upon the knowledge of Aristotle. Compound Ascidians.Schlosser and Ellis, in a paper containing a description of Botryllus, published in the Philosophical Transactions of the Royal Society for 1756, first brought the Compound Ascidians into notice; but it was not until the commencement of the 19th century, as a result of the careful anatomical investigations of Cuvier and Savigny.Cuvier (1) upon the Simple Ascidians and of Savigny (2} upon the Compound, that the close relationship between these two groups of the Tunicata was conclusively demonstrated. Up to 1816, the date of publication of Savigny's great work (2), the few Compound Ascidians then known had been generally regarded as Alcyonaria or as Sponges; and, although many new Simple Ascidians had been described by O. F. Müller (4) and others, their internal structure had not been investigated. LamarckLamarck (3) in 1816, chiefly as the result of the anatomical discoveries of Savigny and Cuvier, instituted the class Tunicata, which he placed between the Radiata and the Vermes in his system of classification. The Tunicata included at that time, besides the Simple and the Compound Ascidians, the pelagic forms Pyrosoma, which had been first made known by Péron in 1804, and Salpa, described by Forskal in 1775.

Chamisso and alternation of generations.Chamisso in 1820 made the important discovery that Salpa in its life-history passes through the series of changes which were afterwards more fully described by Steenstrup in 1842 as "alternation of generations"; and a few years later Kuhl and Van Hasselt's investigations upon the same animal resulted in the discovery of the alternation in the directions in which the wave of contraction passes along the heart and in which the blood Circulation.circulates through the body. It has since been found that this observation holds good for all groups of the Tunicata. In 1826 H. Milne-Edwards and Audouin made a series of observations on living Compound Ascidians, and amongst other discoveries they found the free-swimming tailed larva, and traced its development into the young Ascidian. Milne-Edwards.Milne-Edwards (5) also founded the group of "Social" Ascidians, now known as the Clavelinidæ, and gave a classification of the Compound Ascidians which was universally accepted for many years. From the year 1826 onwards a number of new and remarkable forms were discovered, as, for instance, some of the Bolteninæ (Macleay), Chelyosoma (Broderip and Sowerby, and afterwards Eschricht), Oikopleura (Mertens), Perophora (Lister), Pelonaia (Forbes and Goodsir), Chondrostachys and Diplosoma (Denis Macdonald), Diazona (Forbes and Goodsir), and Rhodosoma (Ehrenberg, and afterwards Lacaze-Duthiers).

Carl Schmidt.In 1845 Carl Schmidt (6) first announced the presence in the test of some Ascidians of "tunicine," a substance very similar to cellulose, and in the following year Löwig and Kölliker (7) confirmed the discovery and made some additional observations upon this substance and upon the structure of the test in general. Huxley.Huxley (8), in an important series of papers published in the Transactions of the Royal and Linnean Societies of London from 1851 onwards, discussed the structure, embryology, and affinities of the pelagic Tunicates Pyrosoma, Salpa, Doliolum, and Appendicularia. These important forms were also investigated about the same time by Gegenbaur, Vogt, H. Müller, Krohn, and Leuckart. The most important epoch in the history of the Tunicata is the date of the publication of KowalevskyKowalevsky's celebrated memoir upon the development of a Simple Ascidian (9). Tailed larva.The tailed larva had been previously discovered and investigated by several naturalists notably H. Milne-Edwards (5), J. P. van Beneden (10), and Krohn (11); but its minute structure had not been sufficiently examined, and the meaning of what was known of it had not been understood. Relationship to Vertebrates.It was reserved for Kowalevsky in 1866 to demonstrate the striking similarity in structure and in development between the larval Ascidian and the vertebrate embryo. He showed that the relations between the nervous system, the notochord, and the alimentary canal are much the same in the two forms, and have been brought about by a very similar course of embryonic development. This discovery clearly indicated that the Tunicata are closely allied to Amphioxus and the Vertebrata, and that the tailed larva represents the primitive or ancestral form from which the adult Ascidian has been evolved by degeneration, and this led naturally to the view usually accepted at the present day, that the group is a degenerate side-branch from the lower end of the phylum Chordata, which includes the Tunicata (Urochorda), Amphioxus (Cephalochorda), and the Vertebrata. Kowalevsky's great discovery has since been confirmed and extended to all other groups of the Tunicata by Kupffer, Giard, &c.Kupffer (12), Giard (13 and 15), and others. Important observations upon the process of Gemmation.gemmation and the formation of colonies in various forms of Compound Ascidians have been made by Krohn, Metschnikoff, Kowalevsky, Ganin, Giard, Della Valle, and others, and have gradually led to the establishment of the general principle, that all the more important layers of the bud are derived more or less directly from the corresponding regions in the body of the parent.

Fol, &c.In 1872 Fol (14) added largely to the knowledge of the Appendiculariidæ, and Giard (15) to that of the Compound Ascidians. The latter author described a number of new forms and remodelled the classification of the group. The most important additions which have been made to the Compound Ascidians since Giard's work have been those described by Von Drasche (16} from the Adriatic and those discovered by the "Challenger" expedition (17). The structure and the systematic arrangement of the Simple Ascidians have been mainly discussed of recent years by Alder and Hancock (18], Heller (19), Lacaze-Duthiers (20), Traustedt (21), and Herdman (17, 22}. Sub-neural gland and dorsal tubercle.In 1874 Ussoff (23) investigated the minute structure of the nervous system and of the underlying gland, which was first discovered by Hancock, and showed that the gland has a duct which communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or "olfactory") tubercle. In an important paper published in 1880 Julin (24) drew attention to the similarity in structure and relations between this gland and the hypophysis cerebri of the vertebrate brain, and insisted upon their homology. He suggests that they perform a renal function. The Thaliacea Thaliacea.have of late years been the subject of several very important memoirs. The researches of Todaro, Brooks (25), Salensky (26) and others have elucidated the embryology, the gemmation, and the life-history of the Salpidæ; and Grobben, Barrois (27), and more especially Uljanin (28) have elaborately worked out the structure and the details of the complicated life-history of the Doliolidæ. Finally, in an important work published in 1886 on the morphology of the Tunicata, Van Beueden and Julin.E. van Beneden and Julin (30) have, mainly as the result of a close comparison of the embryology of Ascidians with that of Amphioxus and other Chordata, added considerably to our knowledge of the position and affinities of the Tunicata, and of the exact relations of their organs to the corresponding parts of the body in the Vertebrata.

Anatomy.

Ascidia mentula.As a type of the Tunicata, Ascidia mentula, one of the larger species of the Simple Ascidians, may be taken. This species is found in most of the European seas, generally in shallow water on a muddy bottom. External characters.It has an irregularly ovate form, and is of a dull grey colour. It is attached to some foreign object by one end (fig. 1 ).

Fig. 1.—Ascidia mentula from the right side. at, atrial aperture; br, branchial aperture; t, test. (Original.)

The opposite end of the body is usually narrow, and it has a terminal opening surrounded by eight rounded lobes. This is the mouth or branchial aperture, and it always indicates the anterior end^[2] of the animal. About half-way back from the anterior end, and on a rounded projection, is the atrial or cloacal aperture—an opening surrounded by six lobes—which is always placed upon the dorsal region. When the Ascidian is living and undisturbed, water is being constantly drawn in through the branchial aperture and passed out through the atrial. If coloured particles be placed in the water near the apertures, they are seen to be sucked into the body through the branchial aperture, and after a short time some of them are ejected with considerable force through the atrial aperture. The current of water passing in is for respiratory purposes, and it also conveys food into the animal. The atrial current is mainly the water which has been used in respiration, but it also contains all excretions from the body, and at times the ova and spermatozoa or the embryos.

The test.The outer grey part of the body, which is attached at or near its posterior end and penetrated by the two apertures, is the "test." This is a firm gelatinous cuticular secretion from the outer surface of the ectoderm, which is a layer of flat cells lining its inner surface. Although at first produced as a cuticle, the test soon becomes organized by the migration into it of cells derived from the ectoderm (see fig. 2). These test cells may remain as rounded or fusiform or stellate cells imbedded in the gelatinous matrix, to which they are constantly adding by secretions on their surfaces; or they may develop vacuoles in their protoplasm, which become larger and fuse to form a huge ovate clear cavity (a "bladder cell"), surrounded by a delicate film of protoplasm and having the nucleus still visible at one point; or they may form pigment granules in the protoplasm; or, lastly, they may deposit carbonate of lime, so that one or several of them together produce a calcareous spicule in the test.

Fig. 2. Diagrammatic section of part of mantle and test of an Ascidian, showing the formation of a vessel and the the structure of the test, m, mantle; e, ectoderm; tc test cell; tm, matrix; blc, bladder cell; s, s', blood sinus in mantle being drawn out into test; mc, mantle cells; y, septum of vessel. (From Herdman, Challenger Report.

Only the unmodified test cells and the bladder cells are found in Ascidia mentula. Calcareous spicules are found chiefly in the Didemnidæ, amongst Compound Ascidians; but pigmented cells may occur in the test of almost all groups of Tunicata. The matrix in which these structures are imbedded is usually clear and apparently homogeneous; but in some cases it becomes finely fibrillated, especially in the family Cynthiidæ. It is this matrix which contains tunicine. At one point on the left side near the posterior end a tube enters the test, and then splits up into a number of branches, which extend in all directions and finally terminate in rounded enlargements or bulbs, situated chiefly in the outer layer of the test. These tubes are known as the "vessels" of the test, and they contain blood. Each vessel is bounded by a layer of ectoderm cells lined by connective tissue (fig. 3, B), and is divided into two tubes by a septum of connective tissue.

Fig. 3.—A a vessel from the test. B, diagrammatic transverse section of a vessel. ec, ectoderm; ct, connective tissue; s, s' the two tubes; y, septum; tk, terminal bulb (Original.)

The septum does not extend into the terminal bulb, and consequently the two tubes communicate at their ends (fig. 3, A). The vessels are formed by an outgrowth of a blood sinus (derived originally from the blastocœle of the embryo) from the body wall (mantle) into the test, the wall of the sinus being formed by connective tissue and pushing out a covering of ectoderm in front of it (fig. 2, s' ). The test is turned inwards at the branchial and atrial apertures to line two funnel-like tubes,—the branchial siphon leading to the branchial sac and the atrial siphon leading to the atrial or peribranchial cavity.

Mantle.The body wall, inside the test and the ectoderm, is formed of a layer (the somatic layer of mesoderm) of connective tissue, inclosing muscle fibres, blood sinuses, and nerves. This layer (the mantle) has very much the shape of the test outside it, but at the two apertures it is drawn out to form the branchial and atrial siphons (fig. 4). In the walls of these siphons the muscle fibres form powerful circular bands, the sphincter muscles. Throughout the rest of the mantle the bands of muscle fibres form a rude irregular network. They are numerous on the right side of the body, and almost totally absent on the left. The muscles are all formed of very long fusiform non-striped fibres. The connective tissue of the mantle is chiefly a clear gelatinous matrix, containing cells of various shapes; it is frequently pigmented and is penetrated by numerous lacunæ, in which the blood flows. Inside the mantle, in all parts of the body, except along the ventral edge, there is a cavity,—the atrial or peribranchial cavity,—which opens to the exterior by the atrial aperture. This cavity is lined by a layer of cells derived originally from the ectoderm ^[3] and directly continuous with that layer through the atrial aperture (fig. 5); consequently the mantle is covered both externally and internally by ectodermal cells.

Branchial sac and neighbouring organs.The branchial aperture (mouth) leads in to the branchial siphon (buccal cavity or stomodæum), and this opens into the anterior end of a very large cavity (the branchial sac) which extends nearly to the posterior end of the body (see figs. 4 and 5).

Fig. 4. Diagrammatic dissection of A. mentula to show the anatomy.at, atrial aperture; br, branchial aperture; a, anus; brs, branchial sac; dl, dorsal lamina; dt, dorsal tubercle end, endostyle; h, heart; I, intestine; m mantle; ng, nerve ganglion; œ, oesophagus; œa, œsophageal aperture; ov, ovary; pbr peribranchial cavity; r, rectum; st, stomach; t, test; tn, tentacles; vd, vas deferens; ngl, subneural gland.(Original.)

This branchial sac is an enlarged and modified pharynx, and is therefore properly a part of the alimentary canal. The œsophagus opens from it far back on the dorsal edge (see below, p. 612). The wall of the branchial sac is pierced by a large number of vertical slits,—the stigmata,—placed in numerous transverse rows. These slits place the branchial sac in communication with the peribranchial or atrial cavity, which lies outside it (fig. 5, B).

Fig. 5. Diagrammatic longitudinal (A) and transverse (B) sections through Ascidia to show the position of the ectoderm and the relations of the branchial and peribranchial cavities. The lettering is the same as for fig. 4. B represents a section taken along the dotted line A-B in A.(Original.)

Between the stigmata the wall of the branchial sac is traversed by blood-vessels, which are arranged in three regular series (fig. 6), (1) the transverse vessels, which run horizontally round the wall and open at their dorsal and ventral ends into large longitudinal vessels, the dorsal and ventral sinuses; (2) the fine longitudinal vessels, which run vertically between adjacent transverse vessels and open into them, and which bound the stigmata; and (3) the internal longitudinal bars, which run vertically in a plane internal to that of the transverse and fine longitudinal vessels. These bars communicate with the transverse vessels by short side branches where they cross, and at these points are prolonged into the lumen of the sac in the form of hollow papillæ. The edges of the stigmata are richly set with cilia, which drive the water from the branchial sac into the peribranchial cavity, and so cause the currents that flow in through the branchial aperture and out through the atrial.

Fig. 6.—A. Part of branchial sac of Ascidia from inside. B. Transverse section of same, tr, transverse vessel; cd, connecting duct; hm, horizontal membrane; 'il, internal longitudinal bar; Iv, fine longitudinal vessels; p, p′, papillæ; sg, stigmata. A and B are drawn to different scales. (From Herdman, Challenger Report.)

Along its ventral edge the wall of the branchial sac is continuous externally with the mantle (fig. 5, B), while internally it is thickened to form two parallel longitudinal folds bounding a groove, the "endostyle," hypobranchial groove, or ventral furrow (figs. 4, 5, end}.Endostyle. The endoderm cells which line the endostyle are greatly enlarged at the bottom and on parts of the sides of the furrow so as to form projecting pads, which bear very long cilia. It is generally supposed that this organ is a gland for the pro duction of the mucous secretion which is spread round the edges of the branchial sac and catches the food particles in the passing current of water; but it has recently been pointed out that there are comparatively few gland cells in the epithelium of the endostyle, and that it is more probable that this furrow is merely a ciliated path along which the mucous secretion (produced possibly by the subneural gland) is conveyed posteriorly along the ventral edge of the branchial sac. Peripharyngeal bands.At its anterior end the edges of the endostyle become continuous with the right and left halves of the posterior of two circular ciliated ridges,—the peripharyngeal bands, which run parallel to one another round the front of the branchial sac. Dorsal lamina.The dorsal ends of the posterior peripharyngeal band bend posteriorly (enclosing the epibranchial groove), and then join to form the anterior end of a fold which runs along the dorsal edge of the branchial sac as far as the œsophageal aperture. This fold is the dorsal lamina (figs. 4, 5, dl). It probably serves to direct the stream of food particles entangled in a string of mucus from the anterior part of the dorsal lamina to the œsophagus. Dorsal languets.In many Ascidians this organ, instead of being a continuous membranous fold as in A. mentula, is represented by a series of elongated triangular processes—the dorsal languets,—one attached in the dorsal median line opposite to each transverse vessel of the branchial sac. The anterior peripharyngeal band is a complete circular ridge, having no connexion with either the endostyle or the dorsal lamina. In front of it lies the prebranchial zone, which separates the branchial sac behind from the branchial siphon in front. The prebranchial zone is bounded anteriorly by a muscular band—the posterior edge of the sphincter muscle,—which bears a circle of long delicate processes, the tentacles (figs. 4, 7, 8, tn). Tentacles.These project inwards at right angles so as to form a network across the entrance to the branchial sac. Each tentacle consists of connective tissue covered with epithelium (endoderm), and contains two or more cavities which are continuous with blood sinuses in the mantle. Subneural gland.In the dorsal median line near the anterior end of the body, and imbedded in the mantle on the ventral surface of the nerve ganglion, there lies a small glandular mass—the subneural gland which, as Julin has shown (24), there is reason to regard as the homologue of the hypophysis cerebri of the vertebrate brain. Julin and E. van Beneden have suggested that the function of this organ may possibly be renal.^[4] The subneural gland, which was first noticed by Hancock, communicates anteriorly, as Ussoff (23) pointed out, by means of a narrow duct with the front of the branchial sac (pharynx). The opening of the duct is enlarged to form a funnel-shaped cavity, which may be folded upon itself, convoluted, or even broken up into a number of smaller openings, so as to form a complicated projection, called the dorsal tubercle, situated in the dorsal part of the prebranchial zone (fig. 7).

Fig. 7—Diagrammatic section through anterior dorsal part of A. mentula, showing the relations of the nerve ganglion, subneural gland, &c. Lettering as for fig. 4; n, nerve; n′, myelon; pp, peripharyngeal band; sgl, subneural gland; sgd, its duct; t′, test lining branchial siphon.(Original.)

Dorsal tubercle.The dorsal tubercle in A. mentula is somewhat horse-shoe-shaped (fig. 8); it varies in form in most Ascidians according to the genus and species, and in some cases in the individual also.

Fig. 8.—Dorsal tubercle and neighbouring organs of A. mentula. Lettering as before; egr, epibranchial groove; z, prebranchial zone.(Original.)

Possibly, besides being the opening of the duct from the subneural gland, it may be a sense-organ for testing the quality of the water entering the branchial sac.

Nervous system.The single elongated ganglion in the median dorsal line of the mantle between the branchial and atrial siphons is the only nerve-centre in A. mentula and most other Tunicata. It is the degenerate remains of the anterior part of the cerebro-spinal nervous system of the tailed larval Ascidian (see below, p. 614). The posterior or spinal part has entirely disappeared in most Tunicata. It persists, however, in the Appendiculariidæ, and traces of it are found in some Ascidians (e.g., Clavelina see Julin). The ganglion gives off distributory nerves at both ends, which run through the mantle to the neighbourhood of the apertures, where they divide and subdivide. Sense organs. The only sense-organs are the pigment spots between the branchial and atrial lobes, the tentacles at the base of the branchial siphon, and possibly the dorsal tubercle and the languets or dorsal lamina. These are all in a lowly developed condition. The larval Ascidians on the other hand have well-developed intra-cerebral optic and auditory sense-organs; and in some of the pelagic Tunicata otocysts and pigment spots are found in connexion with the ganglion.

Alimentary canal.The mouth and the pharynx (branchial sac) have already been described. The remainder of the alimentary canal is a bent tube which in A. mentula and most other Ascidians lies imbedded in the mantle on the left side of the body, and projects into the peribranchial cavity. The oesophagus leaves the branchial sac in the dorsal middle line near the posterior end of the dorsal lamina (see fig. 4, œa). It is a short curved tube which leads ventrally to the large fusiform thick-walled stomach. The intestine emerges from the ventral end of the stomach, and soon turns anteriorly, then dorsally, and then posteriorly so as to form a curve—the intestinal loop—open posteriorly. The intestine now curves anteriorly again, and from this point runs nearly straight forward as the rectum, thus completing a second curve—the rectal loop—open anteriorly (see fig. 4). The wall of the intestine is thickened internally, to form the typhlosole, a pad which runs along its entire length. The anus opens into the dorsal part of the peribranchial cavity near to the atrial aperture. The walls of the stomach are glandular; and a system of delicate tubules with dilated ends, which ramifies over the outer wall of the intestine and communicates with the cavity of the stomach by means of a duct, is probably a digestive gland.

Excretory organs.A mass of large clear vesicles which occupies the rectal loop, and may extend over the adjacent walls of the intestine, is a renal organ without a duct. Each vesicle is the modified remains of a part of the primitive cœlom or body-cavity, and is formed of cells which eliminate nitrogenous waste matters from the blood circulating in the neighbouring blood-lacunæ and deposit them in the cavity of the vesicle, where they form a concentrically laminated concretion of a yellowish or brown colour. These concretions contain uric acid, and in a large Ascidian are very numerous. The nitrogenous waste products are thus deposited and stored up in the renal vesicles in place of being excreted from the body. In other Ascidians the renal organ may differ from the above in its position and structure; but in no case has it an excretory duct, unless the subneural gland is to be regarded as a renal organ.

Blood-vascular system and cœlom.The heart is an elongated fusiform tube placed on the ventral and posterior edge of the stomach, in a space (the pericardium) which is part of the original cœlom or body-cavity, the rest of which exists merely in the form of lacunæ and of the cavities of the reproductive organs and renal vesicles in the adult Ascidian. The wall of the heart is formed of a layer of epithelio-muscular cells, the inner ends of which are cross-striated; and waves of contraction pass along it from end to end, first for a certain number of beats in one direction and then in the other, so as to reverse the course of circulation periodically. At each end the heart is continued into a vessel (see fig. 9), a large sinus or lacuna lined with a delicate endothelial layer. The sinus leaving the ventral end of the heart is called the branchio-cardiac vessel,^[5] and the heart itself is merely the differentiated posterior part of this sinus and is therefore a ventral vessel. The branchio-cardiac vessel, after giving off a branch which, along with a corresponding branch from the cardio- visceral vessel, goes to the test, runs along the ventral edge of the branchial sac externally to the endostyle, and communicates laterally with the ventral ends of all the transverse vessels of the branchial sac. The sinus leaving the dorsal end of the heart is called the cardio-visceral vessel, and this, after giving off to the test the branch above mentioned, breaks up into a number of sinuses, which ramify over the alimentary canal and the other viscera. These visceral lacunae finally communicate with a third great sinus, the viscero-branchial vessel, which runs forward along the dorsal edge of the branchial sac externally to the dorsal lamina and joins the dorsal ends of all the transverse vessels of the branchial sac. Besides these three chief systems there are numerous lacunae in all parts of the body, -by means of which anastomoses are established between the different currents of blood. All these blood spaces and lacunae are to be regarded as derived from the blastoccele of the embryo, and not, as has been usually supposed, from the cœlom (30). Course of circulation.When the heart contracts ventro-dorsally, the course of the circulation is as follows: the blood which is flowing through the vessels of the branchial sac is collected in an oxygenated condition in the branchio-cardiac vessel, and, after receiving a stream of blood from the test, enters the heart.

Fig. 9—Diagram of circulation in Ascidia. Lettering as before; bc, branchio-cardiac or ventral vessel; cv, cardio-visceral vessels; vb, viscero-branchial or dorsal vessel; vt, vessels to test. (Original.)

It is then propelled from the dorsal / end of the visceral vessels, and so reaches the test and digestive and other organs; then, after circulating in the visceral lacunae, it passes into the viscero-branchial vessel in an impure condition, and is distributed to the branchial vessels to be purified again. When the heart on the other hand contracts dorso-ventrally, this course of circulation is reversed. As the test receives a branch from each end of the heart, it follows that it has afferent and efferent vessels which ever way the blood is flowing. In some Ascidians the vessels in the test become very numerous and their end branches terminate in swollen bulbs close under the outer surface of the test. In this way an accessory respiratory organ^[6] is probably formed in the superficial layer of the test. The blood corpuscles are chiefly colourless and amoeboid; but in most if not all Ascidians there are also some pigmented corpuscles in the blood. These are generally of an orange or reddish brown tint, but may be opaque white, dark indigo-blue, or of intermediate colours. Precisely similarly pigmented cells are found throughout the connective tissue of the mantle and other parts of the body.

Reproductive organs.A. mentula is hermaphrodite, and the reproductive organs lie, with the alimentary canal, on the left side of the body. The ovary is a ramified gland which occupies the greater part of the intestinal loop (see fig. 4). It contains a cavity which, along with the cavities of the testis, is derived from a part of the original cœlom, and the ova are formed from its walls and fall when mature into the cavity. The oviduct is continuous with the cavity of the ovary and leads forwards alongside the rectum, finally opening near the anus into the peribranchial cavity. The testis is com posed of a great number of delicate branched tubules, which ramify over the ovary and the adjacent parts of the intestinal wall. Those tubules terminate in ovate swellings. Near the commencement of the rectum the larger tubules unite to form the vas deferens, a tube of consider able size, which runs forwards alongside the rectum, and, like the oviduct, terminates by opening into the peribranchial cavity close to the anus. The lumen of the tubules of the testis, like the cavity of the ovary, is a part of the original coelom, and the spermatozoa are formed from the cells lining the wall. In some Ascidians reproductive organs are present on both sides of the body, and in others (Polycarpa) there are many complete sets of both male and female systems, attached to the inner surface of the mantle on both sides of the body and projecting into the peribranchial cavity.^[7]

Embryology^[8] and Life History.

Embryology.In most Ascidians the eggs are fertilized in the peribranchial cavity, and undergo most of their development before leaving the parent; in some cases, however, the eggs are laid, and fertilization takes place in the surrounding water.

Fig. 10.—Stages in the embryology of a Simple Ascidian (after Kowalevsky).A to F. Longitudinal vertical sections of embryos, all placed with the dorsal surface uppermost and the anterior end at the right. A. Early blastula stage, during segmentation. B. Early gastrula stage. C. Stage after gastrula, showing commencement of notochord. D. Later stage, showing formation of notochord and of neural canal. E. Embryo showing body and tail and completely formed neural canal. F. Larva just hatched; end of tail cut off. G. Transverse section of tail of larva.
adp, adhering papillæ of larva; at, epiblastic (atrial) involution; au, auditory organ of larva; ar, archenteron; bc, blastocœle; bp, blastopore; ch, notochord; ep, epiblast; hy, hypoblast; nc, neural canal; nec, neurenteric canal; oc, ocular organ of larva; g, gelatinous investment of embryo; m, muscle cells of tail; mes, mesenteron; mc, mesoderm cells; nv, cerebral vesicle at anterior end of neural canal.

The segmentation is complete and regular (fig. 10, A) and results in the formation of a spherical blastula, which then undergoes invagination (fig. 10, B). The embryo elongates, and the blastopore or iuvagination opening comes to be placed on the dorsal edge near the posterior end (fig. 10, C). The hypoblast cells lining the archenteron are columnar in form, while the epiblast cells are more cubical (fig. 10, B, C, D). The dorsal surface of the embryo now becomes flattened and then depressed to form a longitudinal groove, extending forwards from the blastopore to near the front of the body. This "medullary groove" now becomes converted into a closed canal by its side walls growing up, arching over, and coalescing in the median dorsal line (fig. 10, D). This union of the laminae dorsales to form the neural canal commences at the posterior end behind the blastopore and gradually extends forwards. Consequently the blastopore comes to open into the posterior end of the neural canal (fig. 10, D), while the anterior end of that cavity remains open to the exterior. In this way the archenteron communicates indirectly with the exterior. The short canal leading from the neural canal to the archenteron is known as the neurenteric canal (fig. 10, D, nee). Previous to this stage some of the hypoblast cells at the front edge of the blastopore and forming part of the dorsal wall of the archenteron (tig. 10, C, ch) have become separated off, and then arranged to form an elongated band, two cells wide, underlying the posterior half of the neural canal (fig. 10, D, E, ch.). This is the origin of the notochord. Outgrowths from the sides of the archenteron give rise to laterally placed masses of cells, which are the origin of the mesoblast. These masses show no trace of metameric segmentation. The cavities (reproductive and renal vesicles) which are formed later in the mesoblast represent the ccelom. Consequently the body-cavity of the Tunicata is a modified form of enterocoele. The anterior part of the embryo, in front of the notochord, now becomes enlarged to form the trunk, while the posterior part elongates to form the tail (fig. 10, E). In the trunk the anterior part of the archenteron dilates to form the mesenteron, the greater part of which becomes the branchial sac; at the same time the anterior part of the neural canal enlarges to form the cerebral vesicle, and the opening to the exterior at the front end of the canal now closes. In the tail part of the embryo the neural canal remains as a narrow tube, while the remains of the wall of the archenteron the dorsal part of which becomes the notochord are converted into lateral muscle bands (fig. 10, G) and a ventral cord of cells, which eventually breaks up to form blood corpuscles. As the tail grows longer, it becomes bent round the trunk of the embryo inside the egg-membrane. About this period the epiblast cells begin to form the test as a cuticular deposit upon their outer surface. The test is at first devoid of cells and forms a delicate gelatinous investment, but it shortly afterwards becomes cellular by the migration into it of test cells formed by proliferation from the epi blast. 1

Larval The embryo is hatched about two or three days after fertilization, stage. in the form of a tadpole-like larva, which swims actively through the sea by vibrating its long tail. The anterior end of the body is provided with three adhering papillae (fig. 10, F, adp) in the form of epiblastic thickenings. In the free-swimming tailed larva the nervous system, formed from the walls of the neural canal, becomes considerably differentiated. The anterior part of the cerebral vesicle remains thin-walled (fig. 10, F), and two unpaired sense organs develop from its wall and project into the cavity. These are a dorsally and posteriorly placed optic organ, provided with retina, pigment layer, lens, and cornea, and a ventrally placed auditory organ, consisting of a large spherical partially pigmented otolith, attached by delicate hair-like processes to the summit of a hollow crista acoustica (fig. 10, F, au). The posterior part of the cerebral vesicle thickens to form a solid ganglionic mass traversed by a narrow central canal. The wall of the neural canal behind the cerebral vesicle becomes differentiated into an anterior thicker region, placed in the posterior part of the trunk and having a superficial layer of nerve fibres, and a posterior narrower part which traverses the" tail, lying on the dorsal surface of the notochord, and gives off several pairs of nerves to the muscles of the tail. Just in front of the anterior end of the nervous system a dorsal involution of the epiblast breaks through into the upturned anterior end of the mesenteron and thus forms the mouth opening. Along the ventral edge of the mesenteron, which becomes the branchial sac, the endostyle is formed as a narrow groove with thickened side walls. It probably corresponds to the median portion of the thyroid body of Vertebrata. A curved outgrowth from the posterior end of the mesenteron forms the alimentary canal (oesophagus, stomach, and intestine), which at first ends blindly. An anus is formed later by the intestine opening into the left of two lateral epiblastic involutions (the atria), which rapidly become larger and fuse dorsally to form the peribranchial cavity. Outgrowths from the wall of the branchial sac meet these epiblastic involutions and fuse with them to give rise to the first formed pair of stigmata, which thus come to open into the peribranchial cavity; and these alone correspond to the gill clefts of Amphioxiis and the Vertcbrata.

Metamor- After a short free-swimming existence the fully developed tailed phosis larva fixes itself by its anterior adhering papillre to some foreign to adult object, and then undergoes a remarkable series of retrogressive form. changes, which convert it into the adult Ascidian. The tail atro phies, until nothing is left but some fatty cells in the posterior part of the trunk. The adhering papillse disappear and are replaced functionally by a growth of the test over neighbouring objects. The nervous system with its sense organs atrophies until it is re duced to the single small ganglion, placed on the dorsal edge of the pharynx, and a slight nerve cord running for some distance pos teriorly (Van Beneden and Julin). Slight changes in the shape of the body and a further growth and differentiation of the branchial sac, peribranchial cavity, and other organs now produce gradually the structure found in the adult Ascidian.

The most important points in connexion with this process of development and metamorphosis are the following. (1) In the Ascidian embryo all the more important organs (c.y., notochord, neural canal, archenteron) are formed in essentially the same i Some of the first test cells are also probably derived from the epithelium of the egg follicle. manner as they are in Ampli!o;nis and other dim-data. (2) The free-swimming tailed larva possesses the essential characters of the Chordata, inasmuch as it has a longitudinal skeletal axis (the noto chord) separating a dorsally placed nervous system (the neural canal) from a ventral alimentary canal (the archenteron); and therefore during this period of its life-history the animal belongs to the Chordata,. (3) The Chordate larva is more highly organized than the adult Ascidian, and therefore the changes by which the latter is produced from the former may be regarded as a process of degeneration (ji). The important conclusion drawn from all this is that the Tunicata are the degenerate descendants of a group of the primitive Chordata (see below p. 618).

CLASSIFICATION AND CHARACTEKS OF GHOUPS.

Order I. LARVACEA.

Free-swimming pelagic forms provided with a large locomotory Charappendage (the tail), in which there is a skeletal axis (the urochord). acters of A relatively large test (the "Haus") is formed with Larvaceagreat rapidity as a secretion from the ectoderm; it is merely a temporary structure, which is cast off and replaced by another. The branchial sac is simply an enlarged pharnyx with two ventral ciliated openings (stigmata) leading to the exterior. There is no sej. parate peribranchial cavity. The nervous system consists of a large dorsally placed / ganglion and a long nerve cord, which y stretches backwards over the alimentary canal to reach the tail, along which it runs on the left side of the urochord. The anus opens ventrally on the surface of the body in front of the stig mata. No reproduction by gemmation or metamorphosis is known in the life-history. This is one of the most in teresting groups of the Tuni cata, as it shows more com pletely than any of the rest the characters of the original ancestral forms. It has un dergone little or no degen eration, and conseqiiently corresponds more nearly to the tailed. larval condition than to the adult forms of the other groups. The order Fio. IL-Oikopleura coplwcerca in "Ham" includes a single family the (after Fol), seen from right side, magnified APPENDICULARIIOE, all the six times. The arrows indicate the course members of which are minute 1 reticulated parts of &nd free . swimming . T h ey occur on the surface of the sea in most parts of the world. They possess the power to form Strucwith great rapidity an enormously large investing gelatinous layer ttire of (fig. 11), which corresponds to the test of other groups. This was Appenp,p n.y, dicularia. Fio. 12. Semi-diagrammatic view of Appendicularia from the right, a, anus; at, one of the atrial apertures; app, tail; br, branchial aperture; brs, branchial sac; <it, dorsal tubercle; end, endostyle; h, heart; i, intestine; m, muscle band of tail; n, nerve cord in body; n, nerve cord in the tail; ce, oesophagus; ot, otocyst; ov, ovary; pp, peripharyngeal band; ng, cerebral ganglion; ny, caudal ganglion; tig", enlargement of nerve cord in tail; so, sense organ (tactile) on lower lip; sg, ciliated aperture in pharynx; st, stomach; tes, testis; 11, urochord; it, its cut end. (Original.) first described by Von Mertens and by him named Haus. " It is only loosely attached to the body and is frequently thrown olf soon after its formation. The tail in the AppeiuHculariidze, is at tached to the ventral surface of the body (fig. 12), and usually points more or less anteriorly. It shows distinct traces of metameric segmentation, having its muscle bands broken up into myotomes, while the nerve cord presents a series of enlargements from which distributary nerves are given off (fig. 12, ng"). Near the base of the tail there is a distinct elongated ganglion (fig. 12, ncf). The anterior (cerebral) ganglion has connected with it an otocyst, a pigment spot, and a tubular process opening into the branchial sac and representing the dorsal tubercle and associated parts of an ordinary Ascidian. The branchial aperture or mouth leads into the branchial sac or pharynx. There are no tentacles. The endostyle is short. There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly. The wall of the branchial sac has " only two ciliated apertures. They are homologous with the primary stigmata of the typical Ascidians and the gill clefts of Vertebrates. They are placed far back on the ven tral surface, one on each side of the middle line, and lead into short funnel-shaped tubes which open on the surface of the body behind the anus (fig. 12, at}. These tubes corre spond to the right and. left atrial involutions which, in an ordinary Ascidian, fuse to form the peribranchial cavity. The heart, according to Lankester, is formed of two cells, which are placed at the opposite ends and connected by delicate con tractile protoplasmic fibrils. The large ovary and testis are placed at the posterior end of the body. The remainder of the structural details can be made out from fig. 12.

The family Appendiculariidte comprises the genera, Oikopleura (Mertens), and Appendicularia (Cham.), in both which the body is short and compact and the tail relatively long, while the endostyle is straight; Fritillaria (Q. and G.), in which the body is long and composed of anterior and posterior regions, the tail relatively short, the endostyle recurved, and an ectodermal hood is formed over the front of the body; and Kowalevskia (Fol), a remarkable form de scribed by Fol (14), in which the heart, endostyle, and intestine are said to be absent, while the branchial sac is provided with four rows of ciliated tooth-like processes.

Order II. THALIACEA.

Free-swimming pelagic forms which may be either simple or compound, and the adult of which is never provided with a tail or a notochord. The test is permanent and may be either well developed or very slight. The musculature of the mantle is in the form of more or less complete circular bands, by the contraction of which locomotion is effected. The branchial sac has either two large or many small apertures, leading to a single peribranchial cavity, into which the amis opens. Alternation of generations occurs in the lifehistory, and may be complicated by polymorphism. The Thaliacea comprises two groups, Cyclomyaria and Hemimyaria.

Sub-order 1. Cyclomyaria.

Free-swimming pelagic forms which exhibit alternation of genera tions in their life-history but never form permanent colonies. The body is cask-shaped, with the branchial and atrial apertures at the opposite ends. The test is more or less well developed. The mantle has its musculature in the form of circular bands siirrounding the body. The branchial sac is fairly large, occupying the anterior half or more of the body. Stigmata are xisually present in its posterior part only. The peribranchial cavity is mainly posterior to the branchial sac. The alimentary canal is placed ventrally close to the posterior end of the branchial sac. Hermaphrodite reproductive organs are placed ventrally near the intestine.

This group forms one family, the DOLIOLID^E, including two genera, Doliolum (Quoy and Gaimard) and Anchinia (C. Vogt). Doliolum, of which several species are known from various seas, has a cask-shaped body, usually from 1 to 2 cm. in length. The terminal branchial and atrial apertures (fig. 13) are lobed, and the lobes are provided with sense organs. The test is very slightly developed and contains no cells. The mantle has eight or nine circular muscle bands surrounding the body. The most anterior and posterior of these form the branchial and atrial sphincters. The wide branchial and atrial apertures lead into large branchial and peribranchial cavities, separated by the pos terior wall of the branchial sac, which is pierced by stigmata; con sequently there is a free passage for the water through the body along its long axis, and the animal swims by contracting its ringlike muscle-bands, so as to force out the contained water posteriorly. Stigmata may also be found on the lateral walls of the branchial sac, and in that case there are corresponding anteriorly directed diverticula of the peribranchial cavity. There is a distinct endo style on the ventral edge of the branchial sac and a peripharyngeal hand surrounding its anterior end, but there is no representative of the dorsal lamina on its dorsal edge. The oesophagus com mences rather on the ventral edge of the posterior end of the branchial sac, and runs backwards to open into the stomach, which is followed by a curved intestine opening into the peribranchial cavity. The alimentary canal as a whole is to the right of the middle line. The hermaphrodite reproductive organs are to the left of the middle line alongside the alimentary canal. They open into the peribranchial cavity. The ovary is nearly spherical, while the testis is elongated, and may be continued anteriorly for a long distance. The heart is placed in the middle line ventrally, beat I as for fig. 12; m Fio. 13. Doliolum den iculatum, sexual generation, from the left side. Letter &, muscle bands; ng, nerve ganglion; sg, stigmata; pbr, peribranchial cavity; all, atrial lobes; so, sense ing sgl, subneural gland organs; brl, branchial lobes. (Original.) tween the posterior end of the endostyle and the cesophageal aperture. The nerve ganglion lies about the middle of the dorsal edge of the body, and gives off many nerves. Under it is placed the subneural gland, the duct of which runs forward and opens into the anterior end of the branchial sac by a simple aperture, surrounded by the spirally twisted dorsal end of the peripharyngeal band (fig. 13, dt).

The ova of the sexual generation produce tailed larvae.; these Developdevelop into forms known as "nurses" (blastozooids), which are ment of asexual, and are characterized by the possession of nine muscle Doliobands, an auditory sac on the left side of the body, a ventrally- lum. placed stolon near the heart, upon which buds are produced, and a dorsal outgrowth near the posterior end of the body. The buds give rise eventually to the sexual generation, which is polymor phous, having three distinct forms, in two of which the reproduc tive organs remain undeveloped. The buds while still very young migrate from their place of origin on the stolon, divide by fission, and become attached to the dorsal outgrowth of the body of the nurse, where they develop. The three forms produced are as follows. (1) Nutritive forms (trophozooids), which remain permanently at tached to the nurse and serve to provide it with food; they have the body elongated dorso - ventrally, and the musculature is very slightly developed. (2) Foster forms (phorozooids), which, like the preceding, do not become sexually mature, but, unlike them, are set free as cask-shaped bodies with eight muscle bands and a ventral outgrowth, which is formed of the stalk by which the body was formerly united to the nurse. On this outgrowth the (3) forms (gonozooids) which become sexually mature are attached while still young buds, and after the foster forms are set free these reproductive forms gradually attain their complete development, and are event ually set free and lose all trace of their connexion with the foster forms. They resemble the foster forms in having a cask-shaped body with eight muscle bands, but differ in having no outgrowth or process, and in having the reproductive organs fully developed. 1 Anchinia, of which only one species is known, A. rubra, from Anchinia, the Mediterranean, has the sexual forms permanently attached to portions of the dorsal outgrowth from the body of the unknown nurse. The body is elongated dorso-ventrally. The test is well developed and contains branched cells. The musculature is not so well developed as in Doliolum. There are two circular bands at the anterior end and two at the posterior, and two on the middle of the body. The stigmata are confined to the obliquely placed posterior end of the branchial sac. The alimentary canal forms a U-shaped curve. The reproductive organs are placed on the right side of the body. The life-history is still imperfectly known. As in the case of Doliolum the sexual generation is polymorphous, and has three forms, two of which remain in a rudimentary condition so far as the reproductive organs are con cerned. In Anchinia, however, the three forms do not occur to gether on one stolon or outgrowth, but are produced successively, the reproductive forms of the sexual generation being independent of the foster forms " (see Barrois, 27).

Sub-order 2. Hemimyaria.

Free-swimming pelagic forms which exhibit alternation of genera- Charactions in their life-history and in the sexual condition form colonies, ters of The body is more or less fusiform, with the long axis antero-posterior, //em/ and the branchial and atrial apertures nearly terminal. The test myaria. is well developed. The musculature of the mantle is in the form of a series of transversely -running bands, which do not form com plete independent rings as in the Cyclomyaria. The branchial and 1 For further details see Uljanin (zS). peribranchial cavities form a continuous space in the interior of the body, opening externally by the branchial and atrial apertures, and traversed obliquely from the dorsal and anterior end to the ventral and posterior by a long narrow vascular band, which represents the dorsal lamina, the dorsal blood-vessel, and the neighbouring part of the dorsal edge of the branchial sac of an ordinary Ascidian. The alimentary canal is placed ventrally. It may either be stretched out so as to extend for some distance anteriorly, or as is more usual be concentrated to form along with the reproductive organs a rounded opaque mass near the posterior end of the body, known as the visceral mass or "nucleus." The embryonic development is direct, no tailed larva being formed.

This sub-order contains two very distinct families, the SALPID.S:, which are the typical members, and the OCTACNEMID.E, including a single very remarkable form (Octacnemus bythius), which in some respects does not conform with the characters given above. The Salpidae includes the single genus Salpa (Forskal), which, however, may be divided into two well-marked groups of species, (1) those, such as S. pinnata, in which the alimentary canal is stretched out along the ventral surface of the body, and (2) those, such as S. fusiformis (fig. 14, A), in which the aliment ary canal forms a compact globular mass, the " nucleus," near the posterior end of the body. About fifteen species altogether are known; they are all pelagic forms and are found in nearly all seas. Each species occurs in two forms the solitary asex ual (proles solitaria) and the aggregated sexual proles gregaria) which are usually quite unlike one another. The soli tary form (fig. 14, B) gives rise by internal gemmation to a complex tubular stolon, which contains processes from all the more important organs of the parent body and which becomes seg mented into a series of or embryos. As,dt gem at A B Fio. 14. Salpa rundnata-fusiformte. A. Aggi-e- buds gated form. B. Solitary form. Lettering as the stolon elongates, the before; 1-9, muscle bands; em, embryo; gem, em v, rvO q npar fi, p f rpp geramiparous stolon; m, mantle; vise, visceral * n ir y? s . ne mass (nucleus). (Original.) end which have become advanced in their deve lopment are set free in groups, which remain attached together by processes of the test, each enclosing a diverticulum from the mantle so as to form "chains" (fig. 15). k Each member of the chain is a Salpa I of the sexual or aggregated form, j and when mature may either still / attached to its neighbours or se- h^ parated from them (fig. 14, A) | produce one or several embryos, which develop into the solitary Salpa. Thus the two forms alterStruc- nate regularly. The more importture of ant points in the structure of a Salpa. typical Salpa are shown in fig. 16. The branchial and atrial apertures are at opposite ends of the body, and each leads into a large cavity, the branchial and peribranchial Fi<>. 15. Posterior part of solitary sacs wriirh arp in frpp rommunira- format Salpa denuxratica-mveron<Ua, ics, wmcn aie in n ica- showing a chain of embryos nearly tion at the sides of the obliquely- ready to be set free, gem, young running dorsal lamina or "gill." aggregated Salpse forming the chain; The test is well developed and ^ stolon; m, muscle band of the i, i, .1 L f /. mantle. (Uncmal.) adheres closely to the surface 01 the mantle. The muscle bands of the mantle do not completely encircle the . em j, body. Theyare 5 present dorsally and laterally, but the major ity do not reach the ventral sur face. In many cases neigh bouring bands join in the med ian dorsal line, Pro 16 _ genii-diagrammatic representation of Salpa from (ng. 14). Ihe leftside. Lettering as before; emb, embryo; m, mantle; anterior end of I, languet; sgd, duct of subneural gland; l-ll, muscle tVio rtnroal la bands of mantle; f, thickening of test over nucleus; dl, the dorsal m or branchia< ( (5 ri gj nal .) nuna is prolonged to form a prominent tentacular organ, the languet, proJ 9,o n end t tn jecting into the branchial sac. The nerve ganglion, subneural gland, dorsal lamina, peripharyngeal bands, and endostyle are placed in the usual positions. A pigment spot and an otocyst are found in connection with the ganglion. The large spaces at the sides of the dorsal lamina (often called the gill or branchia of Salpa), by means of which the cavity of the branchial sac i.s placed in free communication with the peribranchial cavity, are to be regarded as gigantic stigmata formed by the suppression of tho lateral walls of the branchial sac. Fig. 16 represents an aggre gated or sexual Salpa which was once a member of a chain, since it shows a testis and a developing embryo. The ova (always few in number, usually only one) appear at a very early period in the developing chain Salpa, while it is still a part of the gemmiparous stolon in the body of the solitary Salpa. This gave rise to the view put forward by Brooks (23), that the ovary really belongs to the solitary Salpa, which is therefore a female producing a series of males by asexual gemmation, and depositing in each of these an ovum, which^will afterwards, when fertilized, develop in the body of the male into a solitary or female Satya. This idea would of course entirely destroy tho view that Salpa is an example of alterna tion of generations. The sexual or chain Salpa, although really hermaphrodite, is always protogynous: i.e., the female elements or ova are produced at an earlier period than the male organ or testis. This prevents self-fertilization. The ovum is fertilized by the Developspermatozoa of an older Salpa belonging to another chain, and ment of the embryo is far advanced in its development before the testis is Salpa. formed. At an early period in its development a part of the embryo becomes separated off, along with a part of the wall of the cavity in which it lies, to form the " placenta," in which the embryonic and the maternal blood streams circulate in close proximity (or actually coalesce during one period) and so allow of the passage of nutriment to the developing embryo. At a somewhat later stage a number of cells placed at the posterior end of the body alongside the future nucleus become filled up with oil-globules to form a mass of nutrient material the elseoblast which is used up later on in the develop ment. Many suggestions have been made as to the homology of the elseoblast. The most probable is that it Js the disappearing rudiment of the tail found in the larval condition of most Ascidians. The family Octacnemidse includes the single remarkable form OctaOctacnemus bythius, found during the " Challenger " expedition, and cnemidee. first described by Moseley (29). It is apparently a deepsea representative of the pelagic Salpidse, and may pos sibly be fixed. The body is somewhat discoid, with its margin prolonged tr> fm-m PitrVit taripr Fl - 17. Diagrammatic vertical longitudinal section l i jt? " of Octacnemus bythiits (after Moseley). br, branchial ing^ processes, on to aperture; TO, opening of oesophagus; r, rectum; at, which the muscle atrial aperture; rm, rm, radiating muscles; n, nucleus; bands of the "** musc l es f nucleus; g, respiratory membrane; 6, thickened margin of base of test; pa, pedicle of mantle are con- attachment, tinned. The ali mentary canal forms a compact nucleus (fig. 17); the endostyle is very short; and the dorsal lamina is apparently absent. The re production and life-history are entirely unknown.

Order III. ASCIDIACEA.

Fixed or free-swimming Simple or Compound Ascidians which in Ascidthe adult are never provided with a tail and have no trace of a iacea. notochord. The free-swimming forms are colonies, the Simple Ascidians being always fixed. The test is permanent and well developed; as a rule it increases with the age of the individual. The branchial sac is large and well developed. Its walls are per forated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the atrial aperture. Many of the forms reproduce by gemmation, and in most of them the sexually-produced embryo develops into a tailed larva. The Ascidiacea includes three groups, the Simple Ascidians, the Compound Ascidians, and the free-swimming colonial Pyrosoma.

Sub-order 1. Ascidise Simplices.

Fixed Ascidians which are solitary and very rarely reproduce by Simple gemmation; if colonies are formed, the members are not buried in Ascida common investing mass, but each has a distinct test of its own. ians. No strict line of demarcation can be drawn between the Simple and the Compound Ascidians, and one of the families of the former group, the Clavelinidss (the Social Ascidians), forms a transition from the typical Simple forms, which never reproduce by gemmation, to the Compound forms, which always do (see p. 618 below). The Ascidise Simplices may be divided into the following families:

Family I. CLAVELINID.E. Simple Ascidians which reproduce by gemmation to form small colonies in which each ascidiozooid has a distinct test, but all are connected by a common blood-system. Buds formed on stolons which are vascular outgrowths from the pos terior end of the body, containing prolongations from the ectoderm, mesoderm, and endoderm of the ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. This family contains three genera: Ectcinascidia ( Herdman ), with internal longitudinal bars in branchial sac; Clavelina (Savigny), with intestine extending behind branchial sac; and Perophora (Wiegmann), with intestine alongside branchial sac. Family II. ASCIDIID.E. Solitary fixed Ascidians with gelatinous test; branchial aperture usually eight-lobed, atrial aperture usually six - lobed. Branchial sac not folded; internal longitudinal bars usually present; stigmata straight or curved; tentacles simple. This family is divided into three sections: Sub-family 1. HYPOBYTHIX.E. Branchial sac with no internal longitudinal bars. One genus, Hypobythius (Moseley). Sub-family 2. ASCIDIX^;. Stigmata straight. Many genera, of which the following are the more important: Ciona (Fleming), dorsal languets present; Astidia (Linnaeus, =Phallusia, Savigny), dorsal lamina present (see figs. 1 to 10); Rhodosoma (Ehrenberg), anterior part of test modified to form operculum; Abyssasddia (Herdman), intestine on right side of branchial sac. Sub-family 3. CORELLIXJE. Stigmata curved. Three genera: Corella (Alder and Hancock), test gelatinous, body sessile; Corynascidia (Herdman), test gelatinous, body pedunculated; Chelyosoma (Brod. and Sow.), test modified into horny plates. Family III. CYXTHIID.S. Solitary fixed Ascidiaus, usually with leathery test; branchial and atrial apertures usually both four-lobed. Branchial sac longitudinally folded; stigmata straight; tentacles simple or compound. This family is divided into three sections: Sub-family 1. STYELIXJE, not more than four folds on each side of branchial sac; tentacles simple. The more important genera are Styela (Macleay), stigmata normal, and Bathyoncus (Herdman), stigmata absent or modified. Sub-family 2. CYXTHIX.E, more than eight folds in branchial sac; tentacles compound; body sessile. The chief n genus is Cynthia (Sa- * vigny), with a large number of species. Sub -family 3. BOLTEXIX.E, more than eight folds in branchial sac; tentacles compound; body pedunculated (fig. 18, A). The chief genera are Boltenia (Savigny), branchial aperture fourlobed, stigmata normal; and Culeolus (Herd- Fi& man), branchial aper ture with less than four lobes, stigmata absent or modified (fig. 18, B). This last is a deep-sea genus discovered by the "Challenger" expedition (see 77). Family IV. MOLGULIDJE. Solitary Ascidians, sometimes not fixed; branchial aperture six-lobed, atrial four-lobed. Test usually incrusted with sand. Branchial sac longitudinally folded; stigmata more or less curved, usually arranged in spirals; tentacles compound. The chief genera are Molgula (Forbes), with distinct folds in the branchial sac, and Eugyra (Aid. and Hanc.), with no distinct folds, but merely broad internal longitudinal bars in the branchial sac. In some of the Molgididas (genus Anurella, Lacaze-Duthiers, 20) the embryo does not become converted into a tailed larva, the development being direct, without metamorphosis. The embryo when hatched assumes gradually the adult structure, and never shows the features characteristic of larval Ascidians, such as the urochord and the median sense-organs. Sub-order 2. Ascidise Composite. Fixed Ascidians which reproduce by gemmation, so as to form colonies in which the ascidiozooids are buried in a common invest ing mass and have no separate tests. This is probably a somewhat artificial assemblage formed of two or three groups of Ascidians which produce colonies in which the ascidiozooids are so intimately united that they possess a common test or investing mass. This is the only character which distinguishes them from the Clavelinidse, but the property of reproducing by gemmation separates them from the rest of the Ascidix Simplices. The Ascidiaz Composite may be divided into the following families: Family I. DISTOMID.B. Ascidiozooids divided into two regions, thorax and abdomen; testes numerous; vas deferens not spirally coiled. The chief genera are Distoma (Gaertner); Distaplia (Delia Valle); Colella (Herdman), forming a pedunculated colony (see fig. 19, A) in which the ascidiozooids develop incubatory pouches, connected with the peribranchial cavity, in which the embryos undergo their development (17}; and Chondrostachys (Macdonald). brf?. Culeolus willemcesi. A. Entire body, natural size. B. Part of branchial sac mag nified, at, atrial aperture; br, branchial aper ture; fed, peduncle; brf, slight fold of branch ial sac; i I, internal longitudinal bar; mh, mesh; sp, calcareous spicules in vessels; tr, transverse vessels. (After Herdman, Challenger Report.) Family II. CO3LOCORMID.B. Colony not fixed, having a large axial cavity with a terminal aperture. Branchial apertures five-lobed. This includes one species, Coslocormus huxleyi (Herdman), which is a transition form between the ordinary Compound Ascidians (e.g., Distomidse) and the Ascidiss Saljnform.es (Pyrosoma). Family III. DIDEMNID.SL. Colony usually thin and incrusting Test containing stel late calcareous spi cules. Testis single, large; vas deferens spirally coiled. The chief genera are Didemnum (Savigny), in which the colony is thick and fleshy and there are only FlG . i 3 ._Colonies of Ascidue Composite (natural size). three TOWS of Stlg- A. Colella quoyi. B. Leptodinum neglect-urn. C. Pharyngodictyon mirabile. D. Botryllus, showing arran g ement of ascidiozooids in circular systems each of which has a central common cloaca. (After Herdman, Challenger Report.) JJ mata on each side of tVio V>ranr>Viial earac and Leptodinum (Milne-Edwards), in which the colony is thin and incrusting (fig. 19, B) and there are four rows of stigmata on each side of the branchial sac. Family IV. DIPLOSOMID.E. Test reduced in amount, rarely con taining spicules. Vas deferens not spirally coiled. In Diplosoma (Macdonald), the most important genus, the larva is gemmiparous. Family V. POLYCLIXID.E. Ascidiozooids divided into three regions, thorax, abdomen, and post-abdomen. Testes numerous; vas deferens not spirally coiled. The chief genera are Pharyngodictyon (Herdman), with stigmata absent or modified, containing one species, Ph. mirabile (fig. 19, C), the only Compound Ascidian known from a depth of 1000 fathoms; Polyclinum (Savigny), with a smooth-walled stomach; Aplidium (Savigny), with the stomach wall longitudinally folded; and Amaroucium (Milne-Edwards), in which the ascidiozooid has a long post-abdomen and a large atrial languet. Family VI. BOTRYLLID.E. Ascidiozooids having the intestine and reproductive organs alongside the branchial sac. Dorsal lamina present; internal longitudinal bars present in branchial sac. The chief genera are Botryllus (Gaertn. and Pall. ), with simple stellate systems (fig. 19, D), and Botrylloides (Milne - Edwards), with elongated or ramified systems. Family VII. POLYSTYELID^;. Ascidiozooids not grouped in systems. Branchial and atrial apertures four-lobed. Branchial sac may be folded; internal longitudinal bars present. The chief genera are Thylacium (Cams), with ascidiozooids projecting above general surface of colony; Goodsiria (Cun ningham), with ascidiozooids completely imbedded in investing mass; and Chorizocormus (Herdmau), with ascidiozooids united in little groups which are connected by stolons. The last genus contains one species, Ch. reticulatus, a transition form between the other Polystyelidas and the Styelinaz amongst Simple Ascidians. The methods of reproduction by gemma tion differ in their details in the various groups of Compound Ascidians; but in all cases the process is essentially a giving off from the parent body of groups of cells re presenting the ectoderm, the mesoderm, and the endoderm, which develop into the corresponding layers of the bud. The first ascidiozooid of the colony produced by the tailed larva does not form sexual repro ductive organs, but reproduces by gemma tion so as to make a colony. Thus there is alternation of generations in the lifehistory. In the most completely formed colonies (e.g., Botryllus) the ascidiozooids are arranged in groups (systems or ccenobii), and in each system are placed with their atrial apertures towards one another, and all communicating with a common cloacal cavity which opens to the exterior in the centre of the system (fig. 19 D). Sub-order 3. Aecidiae Salpiformes. Free-swimming pelagic colonies having ^^^HSlRCi^ Ascidise the form of a hollow cylinder closed at one ^ j " Solpiend. The ascidiozooids forming the colony FlG 20. Pyrosoma ellganf,f ornles are imbedded in the common test in such a natural size. A. Side view manner that the branchial apertures open of entire colony. B. End on the outer surface and theatrial apertures gLU < ** extrenut yon the inner surface next to the central cavity of the colony. The ascidiozooids are produced by gemmation from a rudimentary larva (the cyathozooid) developed sexually. This sub-order includes a single family, the PYROSOMIDJS, con taining one well-marked genus, Pyrosoma (Peron), with several species. They are found swimming near the surface of the sea, chiefly in tropical latitudes, and are brilliantly phosphorescent. A fully developed Pyrosoma colony may be from an inch or two to upwards of four feet in length. The shape of the colony is seen in fig. 20. It tapers slightly towards the closed end, which is rounded. The opening at the opposite end is reduced in size by the presence of a membranous prolongation of the common test (fig. 20, B). The branchial apertures of the ascidiozooids are placed upon short papillae projecting from the general surface, and most of the ascidio zooids have long conical processes of the test projecting outwards beyond their branchial apertures (figs. 20, 21, and 22). There is only a single layer of ascidiozooids in the Pyrosoma colony, as all the fully developed ascidiozooids are placed with their anteroposterior axes at right angles to the surface and communicate by their atrial apertures with the central cavity of the colony (fig. 21). FIG. 21. Part of a longitudinal section through wall of Pyrosoma, showing arrangement of ascidiozooids, magnified (partly after Savigny). at, atrial apertures; br, branchial apertures; osc, young ascidiozooid of a future colony produced by budding from cy, cyathozooid; em, embryos in various stages; t, test; tp, processes of test; brs, branchial sac; yas, young ascidiozooid. Their dorsal surfaces are turned towards the open end of the colony. The more important points in the structure of the ascidiozooid of Pyrosoma are shown in fig. 22. A circle of tentacles, of which one, placed ventrally (fig. 22, in], is larger than the rest, is found just inside the branchial aperture. From this point a wide cavity, with a few circularly-placed muscle bands run ning round its walls, leads back to the large branchial sac, which occupies the greater part of the body. The stigmata are elongated trans versely and crossed by internal longitu dinal bars. The dor sal lamina is repre sented by a series of eight languets (I). The nerve ganglion (on which is placed a small pigmented f sense organ), the subneural gland, the dor sal tubercle, the peripharyngeal bands, and the endostyle are placed in the usual _, iiouHoTi< On panh Fla 22. Mature ascidiozooid of Pyrosoma, from left .ions. VB wen side (partly after Keferstein). Lettering as before; cm, cellular mass, the seat of phosphorescence; cm, posterior cellular mass; gs, gemmiparous stolon; TO&, muscle band; ngl, subneural gland; pig, pigment spot on ganglion; tp, process of test. side of the anterior end of the branchial sac, close to the peripharyngeal bands, is a mass of rounded gland cells which are the source of the phosphores cence. The alimentary canal is placed posteriorly to the branchial sac, and the anus opens into a large peribranchial (or atrial) cavity, of which only the median posterior part is shown (pbr) in fig. 22. The reproductive organs are developed in a diverticulum of the peribranchial cavity, and consist of a lobed testis and a single ovum at a time. The development takes place in a part of the peribranchial Developcavity (fig. 21,?). The segmentation is meroblastic, and an ment of elongated embryo is formed on the surface of a mass of yolk. The Pyroembryo, after the formation of an alimentary cavity, a tubular soma. nervous system, and a pair of laterally placed atrial tubes, divides into an anterior and a posterior part. The anterior part then segments into four pieces, which afterwards develop into the first ascidiozooids of the colony, while the posterior part remains in a rudimentary condition, and was called by Huxley the "cyatho zooid "; it eventually atrophies. As the four ascidiozooids increase in size, they grow round the cyathozooid and soon encircle it (fig. 21, asc and cy). The cyathozooid absorbs the nourishing yolk upon which it lies, and distributes it to the ascidiozooids by means of a heart and system of vessels which have been meanwhile formed. When the cyathozooid atrophies and is absorbed, its original atrial aperture remains and deepens to become the central cavity of the young colony, which now consists of four ascidiozooids placed in a ring, around where the cyathozooid was, and enveloped in a common test. The colony gradually increases by the formation of buds from these four original ascidiozooids. PHYLOGENY. The accompanying diagram shows graphically the probable Phyloorigin and course of evolution of the various groups of Tunicata, geny. and therefore exhibits their relations to one another much more correctly than any system of linear classification can do. The ancestral Proto- Tunicata are here regarded 1 as an offshoot from the Proto-Chordata the common ancestors of the Tunicata (Urochorda), Amphioxus (Cephalochorda), and the Vertebrata. The ancestral Tunicata were probably freeswimming forms, not very unlike, the existing Appcndiculariidse, and are represented in the life -history of nearly all sections of the Tunicata by the tailed lar val stage. The Larvacea are 1 the first offshoot from the ancestral forms which gave rise to the two lines of descendants, the Proto- Thaliacea and the ProtoAscidiacea. The Proto- Thaliacea then split into the ancestors of the existing Cyclomyaria and Hemimyaria. The Proto- Ascidiacea gave up their pelagic mode of life and became fixed. This ancestral process is repeated at the present day when the free-swimming larva of the Simple and Compound Ascidians becomes attached. The Proto- Ascidiacea, after the change, are probably most nearly repre sented by the existing genus Clavelina. They have given rise directly or indirectly to the various groups of Simple and Com pound Ascidians and the Pyrosomidaz. These groups form two lines, which appear to have diverged close to the position of the family Clavelinidas. The one line leads to the more typical Compound Ascidians, and includes the Polyclinidw, Distomida?,, Didemnidse, Diplosomidse, Ccelocormidae, and finally the Ascidite Salpiformes. The second line gave rise to the Simple Ascidians, and to the Sotryllidse and Polystyelidse, which are, therefore, not closely allied to the other Compound Ascidians. The later ProtoAscidiacea were probably colonial forms, and gemmation was re tained by the Clavelinidae and by the typical Compound Ascidians (Distomldse, &c. ) derived from them. The power of forming colonies by budding was lost, however, by the primitive Simple Ascidians, and must, therefore, have been regained independently by the ancestral forms of the BotryllidsR and the Polystyelidit . If this is a correct interpretation of the course of evolution of the Tunicata, we arrive at the following important conclusions. (1) The Tunicata, as a whole, form a degenerate branch of the ProtoChordata; 2) the Ascidise Salpiformes (Pyrosoma) are much more closely related to the typical Compound Ascidians than to the other pelagic Tunicata, viz., the Larvacea and the Thaliacea; and (3) the Ascidise Composites form a polyphyletic group, the sections of which have arisen at several distinct points from the ancestral Simple Ascidians. Bibliography. (/) Cuvier, "Mem. s. les Ascidies," &c., in Mem. d. Mus., vol. ii. p. 10, Paris, 1815; (2) Savigny, Memoires sur les Animaux sans VertSbres, pt. ii. fasc. i., Paris, 1816; (3) Lamarck, Hist. Nat. d. Anim. sans Vertkbres, 1st ed., Paris, 1815-23; (4) O. F. Muller, Zool. Danica, vol. iv., 1806; (f) Milne-Ed wards, "Observ. s. les Ascidies Composees," &c., in Mem. Acad. Sci., Paris, vol. xviii., 1842; (6) Schmidt, Zur vergl. Physiol. d. wirbellos. Tkiere, Bruns1 By Dohrn and others their point of origin is placed considerably further up on the stem of the Chordata, thus causing the Tunicata to be regarded as very degenerate Vertebrata (see 32). wick, 1845; (7) Lo wig and Kolliker, "De la Compos., &c., d. Envel. d. Tun.," in Ann. Sc. Nat., ser. iii. (Zool.), vol. v., 1846; (S) Huxley, Phil. Trans., 1851; (g) Kowalevsky, "Entwickel. d. einf. Ascid.," in Mem. St Petersb. Acad. Sc., ser. vii., vol. x., 1866; (/o) J. P. van Beneden, "Rech. s. 1 Embryolog., &c., I. Asc. Simp.," in Mem. Acad. May. Belg., vol. xx., 1847; (//) Krohn, in Wieginann and Mullet's Archie, 1852; (12) Kupffer, Arch. f. mikr. Anat., 1869, 1872; (13) Gianl, "Etude d. trav. Embryolog. d. Tun., &c.," in Arch. Zool. Kxper., vol. i., 1872; (14) Fol, "Etudes sur les Appendiculaires du Detroit de Messine," in Mem. Soc. Phys. Hist. Nat. Geneve, vol. xxi.; (if) Giard, " Rerherches's. 1. Asc. Comp.," in Arch. Zool. Exper., vol. i., 1872; (i(>) Von Drasche, I lie Synascidien der Bucht von Rovigno, Vienna, 1883; (//) Herdman, "Report upon the Tunicata of the Challenger Expedition," pt. i. in Zool. Chall. Exp., vl. vi., 1SS2; pt. ii. in Zool. Chall. Exp., vol. xiv., 1886; pt. iii., not yet pub lished; (/<$ Alderand Hancock, in Ann. Mag. Nat. Hist., 1863, 1870; (ig) Heller, "Untersuch. u. d. Tunic, d. Adriat. Meeres," in DenkscJtr. d. J:. Akad. Wisx., 1875-77; (20) Lacaze-Duthiers, "Asc. Simp. d. C6tes d. 1. Manche," in Arch. Zool. Exper., 1874, 1877; (21) Traustedt, in Vidensk. Mead. Naturh. For., Copen hagen, 1881-84; (22) Herdman, "Notes on British Tunicata, &c.," in Journ. Linn. Soc., Zool., vol. xv., 1880; (zj) Ussoff, in Proc. Imp. Soc. Nat. Hist., Moscow, vol. xviii., 1876; (24) Julin, "Rech. s. 1 Org. d. Asc. Simp.," in Arch. d. Biol., vol. ii., 1881; (25) Brooks, "Development of Salpa," in Bull. Mils. Comp. Zool., Harvard, vol. iii. p. 291; (*6)Salensky, Ztschr. f. wiss. Zool., 1877; (27) Barrels, Journ. d. VAnat. et Phys., vol. xxi., 1885; (28) Uljanin, Fauna, &c., d. Golfes von Neapel, vol. x., 1884; (29) Moseley, "On Deep-Sea Ascid.," in Trans. Linn. Soc., ser. ii., vol. i., 1876; (j?o) E. van Beneden and Julin, "Morph. d. Tuniciers," in Arch. d. Biol., vol. vi., 1886; (j/) Lankester, Degeneration (Nature series), London, 1880; (32) Dohrn, "Stud. z. Urgesch. d. Wirbclth.," in MUth. Zool. Stat. Neapel. (W. A. HE.)

---

---

1. Only the more important works can be mentioned here. For a more detailed account of the history of the group and a full bibliography, see (17) in the list of works at the end of this article.
2. 1 Some writers use a different nomenclature of regions; see (17).
3. According to E. van Beneden and Julin's recent investigations (30) only the outer wall of the atrium is lined with epiblast, the inner wall being derived from the hypoblast of the primitive branchial sac.
4. See also Herdman, Nature, vol. xxviii. p. 284.
5. On account of the periodic reversal of the circulation none of the vessels can be called arteries or veins.
6. See Herdman, Nature, vol. xxxi. p. 247.
7. For structure of other forms, see p. 614 sq. below.
8. For reproduction by gemmation, see under "Classification," p. 614 sq. below.