SCAPHOPODA.] MOLLUSCA 663 "mesoblasts," which bud off from the invaginated arch- enteron, partly to cells derived from the ectoderm, which at a very early stage is connected by long processes with the invaginated endoderm, as shown in fig. 3, D. The ex ternal form of the embryo goes through the same changes as in other Gastropods, and is not, as was held previously to Lankester s observations, exceptional. When the middle and hinder regions of the blastopore are closing in, an equatorial ridge of ciliated cells is formed, converting the embryo into a typical " Trochosphere " (fig. 4, E, F). The foot now protrudes below the mouth (fig. 4), and the post-oral hemisphere of the Trochosphere grows more rapidly than the anterior or velar area. The young foot shows a bilobed form (fig. 4, D, /). Within the velar area the eyes and the cephalic tentacles commence to rise up (fig. 4, D, t), and on the surface of the post-oral region is formed a cap- like shell and an encircling ridge, which gradually increases in prominence and becomes the freely depending mantle- skirt. The outline of the velar area becomes strongly emarginated and can be traced through the more mature embryos to the cephalic lobes or labial processes of the adult Limnseus (fig. 70). This permanence of the distinction of the part known as the velar area through embryonic life to the adult state is exceptional among Mollusca, and is therefore a point of especial interest in Limnseus. None of the figures of adult Limnoms in recent works on Zoology show properly the form of the head and these velar lobes, and accordingly the figures here given have been specially sketched for the present article. The increase of the visceral dome, its spiral twisting, and the gradual closure of the space over hung by the mantle-skirt so as to convert it into a lung-sac with a small contractile aperture, belong to stages in the development later than any represented in our figures. We may now revert briefly to the internal organization at a period when the Trochosphere is beginning to show a prominent foot growing out from the area where the mid- region of the elongated blastopore was situated, and having therefore at one end of it the mouth and at the other the anus. Fig. 72*** represents such an embryo under slight compression as seen by transmitted light. The ciliated band of the left side of the velar area is indicated by a line extending from v to v ; the foot / is seen between the pharynx ph and the pedicle of invagination pi. The mass of the arch-enteron or invaginated endodermal sac has taken on a bilobed form (compare Pisidium, fig. 151), and its cells are swollen (gs and tge). This bilobed sac becomes entirely the liver in the adult ; the intestine and stomach are formed from the pedicle of invagination, whilst the pharynx, oesophagus, and crop form from the stomodseal invagination ph. To the right (in the figure) of the rectal peduncle is seen the deeply invaginated shell-gland ss, with a secretion sh protruding from it. The shell-gland is destined in Limnseus to become very rapidly stretched out, and to disappear. Farther up, within the velar area, the rudiments of the cerebral nerve-ganglion ng are seen separating from the ectoderm. A remarkable cord of cells having a position just below the integument occurs on each side of the head. In the figure the cord of the left side is seen, marked re. This paired organ consists of a string of cells which are perforated by a duct. The opening of the duct at either end is not known. Such cannulated cells are characteristic of the nephridia of many worms, and it is held that the organs thus formed in the embryo Limnseus are embryonic nephridia. The most important fact about them is that they disappear, and are in no way connected with the typical nephridium of the adult. In reference to their first observer they are conveniently called "Stiebel s canals." Other Fulmonata possess, when embryos, Stiebel s canals in a more fully-developed state, for instance, the common slug Limax (fig. 72**, y>). Here too they disap pear during embryonic life. Further knowledge concern ing them is greatly needed. It is not clear whether there is anything equivalent to them in the embryos of marine Gastropoda or other Mollusca, the ectodermal cells called " embryonic renal organs" in some Gastropod embryos hav ing only a remote resemblance to them. The three pairs of transient embryonic nephridia of the medicinal leech, the ciliated cephalic pits of Nemertines, and the anterior nephridia of Gephyrseans, all suggest themselves for com parison with these enigmatical canals. Marine Pulmonata. Whilst the Pulmonata are essen tially a terrestrial and fresh-water group, there is one genus of slug-like Pulmonates which frequent the sea- coast (Peronia, fig. 72), whilst their immediate congeners (Onchidium) are found in marshes of brackish water. Sem per (33) has shown that these slugs have, in addition to the usual pair of cephalic eyes, a number of eyes developed upon the dorsal integument. These dorsal eyes are very perfect in elaboration, possessing lens, retinal nerve-end cells, retinal pigment, and optic nerve. Curiously enough, however, they differ from the cephalic Molluscan eye (for an account of which see fig. 118) in the fact that, as in the vertebrate eye, the filaments of the optic nerve pene trate the retina, and are connected with the surfaces of the nerve-end cells nearer the lens instead of with the opposite end. The significance of this arrangement is not known, but it is important to note, as shown by Hensen, Hickson, and others, that in the bivalves Pecten and Spondylus, which also have eyes upon the mantle quite distinct from typical cephalic eyes, there is the same relationship as in Onchidiadse of the optic nerve to the retinal cells (fig. 145). In both Onchidiadse and Pecten the pallial eyes have prob ably been developed by the modification of tentacles, such as coexist in an unmodified form with the eyes. The Onchidiadae are, according to Semper, pursued as food by the leaping fish Periophthalmus, and the dorsal eyes are of especial value to them in aiding them to escape from this enemy. Class II. SCAPHOPODA. Characters. Mollusca Glossophorawith the FOOT adapted to a BURROWING life in sand (figs. 73, 74, /). The body, D E Fio. "i3. T>entalium vulgare, Da C. (after Lacaze Duthiers). A. Ventral view of the animal removed from its shell. B. Dorsal view of the same. C. Late ral view of the same. D. The shell in section. E. Surface view of the shell with gill-tentacles exserted as in life, n, mantle ; a , longitudinal muscle ; a", fringe surrounding the anterior opening of the mantle-chamber ; a" , the posterior appendix of the mantle ; b, anterior circular muscle of the mantle ; V, posterior do. ; c, c , longitudinal muscle of mantle ; f, liver ; f, gonad ; I , buccal mass (showing through the mantle) ; t], left nephridium ; s , club-shapeii extremity of the foot ; v, w , longitudinal blood-sinus of the mantle. and to a much greater extent the mantle-skirt and the foot,
are elongated along the primitive antero-posterior (oro-anal)