DEVELOPMENT OF THE EXTERNAL FORM OPISTHOBRANCHIA. 161 



significance is ascribed to them by TRIXCHESE in the Aeolidae and 

 by FOL 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 foot appears as a swelling (/") ; behind it can 

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

 developed further. The Trockophore 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 LANKESTER and 

 TRINCHESE and other observers in Opisthobranchs which have been 



a. 



mr. 



v 



FIG. 71. Two stages in the development of Aplysia Umaciiia (after BLOCHMANN). 

 anal cells ; ect, ectoderm ; ent, entoderm ; /, foot ; M, mouth ; mes, mesoderm ; 

 margin of the mantle ; s, shell ; sd, shell-gland ; sh, shell-integument ; v, velum. 



investigated by them. An embryo of Aplysia figured by RAY 

 KESTER * shows the greatest resemblance to the embryos of flra* 

 loi<l<i depicted in Fig. 65 A. 



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

 passes 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 Lamellibranchia, No. 29, PK. 8,. Fig.. 17.) 

 M 



