126 



MOLLUSCA 



"mesoblasts," which bud off from the invaginated arch- 

 enteron, partly to colls 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, <), 

 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 Limnasus. None of the figures of 

 adult Limnseus 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 f 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 sk protruding from it. The shell-gland 

 is destined in Linmseus 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 Pulmonata possess, when embryos, Stiebel's 

 canals in a more fully-developed state, for instance, the 



common slug Limax (fig. 72**, pk). 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 

 Onchidiadce 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 



Flo. 73.r>enMium rvlgare, 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, a, 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 ; e, liver ; / gonad ; k, 

 bucca] mass (showing through the mantle) ; q, left nephridium ; s\ club-shaped 

 extremity of the foot ; iv, 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) 



