DEVELOPMENT OF THE EXTEBNAL FOBM — OPISTHOBBANCHIA. 161 



significance is ascribed to' them by Tbinchese in the Aeolidcu and 

 by For, in the Heteropoda and Pteropoda. 



The ciliated cells of the pre-oral ring have already become differ- 

 entiated, the velar area being thus marked off (Fig. 71, v). Ventrally 

 behind the mouth, the font appears as a swelling (/) ; behind it ran 

 be recognised the anal cells ((c). The shell-integument has already 

 developed further. The Trochophort stage is here less marked than 

 in many other Gastropods, as the embryo undergoes certain modifica- 

 tions in consequence of the richer supply of yolk. Such a stage, 

 however, has been distinctly recognised by Ray. Ijankester and 

 Tbinchese and other observers in Opisthobranchs which have been 



tt. 



J5 



jn«>. 



Fig. 71.— Two stages in tin- development of Aplysia limacina (after Blochmann), «'., 

 anal cells ; ect, ectoderm ; ent, entoderm ; /. foot : »<. mouth : mes, mesoderm ; mr, 

 margin of the mantle ; ••■'. shell; sd, shell-gland; sh, shell-integument; <•. velum. 



investigated by them. An embryo of Aplysia figured by Bay Lan- 

 kesteb* shows the greatest resemblance to the embryos of Firo- 

 /<■;,/,! depicted in Fig. ii") .1. 



The Trochnphore stage, by the transverse extension of the velum, 

 pa>srs into the Veliger stage, in which, owing to processes of growth 

 similar to those already described, the symmetrical shape undergoes 

 certain modifications. In most respects, indeed, the ontogenetic 

 processes which now follow closely resemble those described for the 

 Prosobranchia, so that we need here only touch upon the principal 

 features. 



• (See Lit. to Lamellibramhia. No. 29, PI. 8, Pig. 17. | 



M 



