658 MOLLUSCA [OPISTHOBRANCHIA. With regard to internal organization we may commence with the disposition of the renal organ (nephridium), the external opening of which has already been noted. The position of this opening and other features of the renal organ have been determined recently by Mr. J. T. Cunning ham, Fellow of University College, Oxford, who writes as follows from Naples, February 1883 : "There is considerable uncertainty with respect to the names of the species of Aplysia. There are two forms which are very common in the Gulf of Naples, and which I have used in studying the ana tomy of the renal organ in the genus. One is q"ite black in colour, and measures when outstretched eight or nine inches in length. The other is light brown and somewhat smaller, its length usually not exceeding seven inches. The first is flaccid and sluggish in its movements, and has not much power of contraction; its epipodial lobes are enormously developed and extend far forward along the body; it gives out when handled an abundance of purple liquid, which is derived from cutaneous glands situated on the under side of the free edge of the mantle. In the Zoological Station this form is known as Ap. leporina; but according to Blochmann it is iden tical with A. Camelus of Cuvier. The other species is A. dcpilans; it is firm to the touch, and contracts forcibly when irritated; the secretion of the mantle-glands is not abundant, and is milky white in appearance. The kidney has similar relations in both genera, and is identical with the organ spoken of by many authors as the triangular gland. Its superficial extent is seen when the folds covering the shell are cut away and the shell removed; the external surface forms a triangle with its base bordering the pericardium and its apex directed posteriorly and reaching to the left-hand posterior corner of the shell-chamber. The dorsal surface of the kidney extends to the left beyond the shell-chamber beneath the skin in the space between the shell-chamber and the left epipodium. " When the animal is turned on its left-hand side and the mantle- chamber widely opened, the gill being turned over to the left, a part of the kidney is seen beneath the skin between the attachment of the gill and the right epipodium (fig. 63). On examination this is found to be the under surface of the posterior limb of the gland, the upper surface of which has just been described as lying beneath the shell. In the posterior third of this portion, close to that edge which is adjacent to the base of the gill, is the external opening (fig. 63, o). "When the pericardium is cut open from above in an animal otherwise entire, the anterior face of the kidney is seen forming the posterior wall of the pericardial chamber; on the deep edge of this face, a little to the left of the attachment of the auricle to the floor of the pericardium, is seen a depression; this depression con tains the opening from the pericardium into the kidney. "To complete the account of the relations of the organ : the right anterior corner can be seen superficially in the wall of the mantle- chamber above the gill. Thus the base of the gill passes in a slant ing direction across the right-hand side of the kidney, the posterior end being dorsal to the apex of the gland, and the anterior end ventral to the right-hand corner. " As so great a part of the whole surface of the kidney lies adjacent to external surfaces of the body, the remaining part which faces the internal organs is small; it consists of the left part of the under surface; it is level with the floor of the pericardium, and lies over the globular mass formed by the liver and convoluted intestine. "Mere dissection does not give sufficient evidence concerning such communications as these of the kidney in Aplysia. I studied the external opening by taking a series of sections through the sur rounding region of the gland; to demonstrate the internal aperture injected a solution of Berlin blue into the pericardium; it did not fill the whole kidney easily, but ran down into the part adjacent to the base of the gill." Thus the renal organ of Aplysia is shown to conform to the Molluscan type. The heart lying within the adjacent pericardium has the usual form, a single auricle and ven tricle. The vascular system is not extensive, the arteries soon ending in the well-marked spongy tissue which builds up the muscular foot, epipodia, and dorsal body-wall. The alimentary canal commences with the usual buccal mass; the lips are cartilaginous, but not armed with horny jaws, though these are common in other Opisthobranchs; the lingual ribbon is multidenticulate, and a pair of salivary glands pour in their secretion. The oesophagus expands into a curious gizzard, which is armed internally with large horny processes, some broad and thick, others spinous, fitted to act as crushing instruments. From this we pass to a stomach and a coil of intestine embedded in the lobes of a voluminous liver; a caecum of large size is given off near the commencement of the intestine. The liver opens by two ducts into the digestive tract. The generative organs lie close to the coil of intestine and liver, a little to the left side. When dissected out they appear as represented in fig. 64. The essential reproductive FIG. 65. Follicles of the hermaphrodite gonads of Euthyneurous Anisopleura. A, of Helix; B, of Eolidia. a, ova; b, developing spermatozoids; c, com mon efferent duct. organ or gonad consists of both ovarian and testicular cells (see fig. 65). It is an ovo-testis. From it passes a common or hermaphrodite duct, which very soon becomes entwined in the spire of a gland the albuminiparous gland. The latter opens into the common duct at the point x, and here also is a small diverticulum of the duct y. Passing on, we find not far from the genital pore a glandular spherical body (the spermatheca a) opening by means of a longish duct into the common duct, and then we reach the pore (fig. 63, &). Here the female apparatus terminates. But when the male secretion of the ovo-testis is active, the seminal fluid passes from the genital pore along the spermatic groove (fig. 63,) to the penis, and is by the aid of that eversible muscular organ introduced into the genital pore of a second Aplysia, whence it passes into the spermatheca, there to await the activity of the fe male element of the ovo-testis of this second Aplysia. After an interval of some days possibly weeks the ova of the second Aplysia commence to descend the hermaphrodite duct; they become enclosed in a viscid secre- FIG. 66. Enteric canal of firm at tlip nnint wliprp fhp nl papillosa. pl, pharynx; m, mid- .e point Wiiert the al- gutj with its hepatic appendages bliminiparOUS gland Opens into h , a11 of which are not figured; ,1 i , . , i -ii -. e , hind gut; an, anus. (From the duct intertwined With it; Gegenbaur, after Alder and Han- and on reaching the point where cock -) the spermathecal duct debouches they are impregnated by the spermatozoa which escape now from the spermatheca and meet the ova. The development of Aplysia from the egg presents many points of interest from the point of view of comparative embryology, but in relation to the morphology of the Opisthobranchia it is sufficient to point to the occurrence of a trochosphere and a veliger stage (fig. 60), and of a shell-gland or primitive shell-sac (fig. 68, *As), Avhich is suc ceeded by a nautiloid shell. The nervous system of Aplysia will be found on com parison of fig. 20, which represents it, with our schematic Mollusc (fig. 1, D) to present but little modification. It is in fact a nervous system in which the great ganglion-pairs are well developed and distinct. The Euthyneurous visceral loop is long, and presents only one ganglion (in Aplysia
cainelus, but two distinct ganglia joined to one another in