128 



GASTROPODA. 



Lamellibranchs (cf. Figs. 15 and 18, pp. 31 and 36). This re- 

 semblance is not only an external one, but extends to the inner- 

 structure also. There is thus a Trochophore stage in the Gastropods 

 also (BAY LANKESTER, No. 63) ; it is not, indeed, usually developed 

 in so typical a way as in Patella, but shows certain modifications. 

 These modifications are either definite characteristics of the Gastropod 

 larva or are transformations undergone by the primitive larval form 

 as a consequence of altered conditions of life, especially by the 

 presence of a greater abundance of yolk causing an abbreviation or 

 the suppression of the larval stage and many modifications of the 

 processes of development. 



Besides the outward resemblance to other Molluscan larvae 

 (Lamellibranchia, Scaphopoda, Amphineura) which is at once evident, 

 we have the inner organisation correspondingly developed. We have 

 already mentioned the apical plate and the pre-oral ciliated ring (Figs. 

 52 and 53), but we have to add to these the post-oral ciliated ring 

 which has been demonstrated in the Gastropod larvae, e.g., in Crepidnla, 

 Fulgur, Fasciolaria and other Prosobranchia, as well as in Heteropoda, 

 Opisthobranchia and Pteropoda (GEGENBAUR, KROHN, FOL, BROOKS, 

 McMuRRiCH, etc.). It consists of a row of cilia which lie immedi- 

 ately behind the mouth and run parallel with the pre-oral ciliated 

 ring (Fig. 54, p n , p. 130). Between this and the pre-oral ring there 

 are also delicate cilia which correspond to the so-called ad-oral ciliated 

 zone of the Lamellibranch larva. The whole apparatus, in any case, 

 serves, as in the Lamellibranchs, for forwarding particles of food to 

 the mouth, while the pre-oral ring, as the velum proper, is chiefly 

 of locomotory significance. The ciliated tuft at the cephalic pole 

 completes the resemblance to the Trochophore of other Molluscs (Fig. 

 3, p. 6, and Fig. 36, p. 91) and the Annelida (Vol. i., Fig. 118, p. 

 265). In the pre-oral section, in the region of the apical plate, eye- 

 spots may occur. The post-oral otocysts lying at the sides of the 

 mouth have already been mentioned. 



The alimentary canal, like the other organs, shows the same 

 structure as in other Trochophore larvae. It is composed of the 

 entodermal mid-gut, the enteron, and of an ectodermal fore-gut, the 

 stomodaeum, and perhaps also of an ectodermal hind-gut, the procto- 

 daeum (?) ; at a later stage, the radular sac, that special character of 

 the Gastropods which distinguishes them from the Lamellibranchs, 

 appears in the stomodaeum (Fig. 53, r). 



Among the organs found in the Gastropod larva, one is of special 

 significance when comparison is made with the Annelidan Trocho- 



