166 GASTROPODA. 



nearly to the adult form, but has first to pass through a moult 

 (NORDMANN), during which it remains entirely quiescent, surrounded 

 by the cast skin as by a transparent sheath. This membrane is no 

 doubt to be regarded as the cast off cuticle. 



D. Pteropoda. 



The early development of the Pteropoda closely resembles that of 

 other Gastropods. We have already seen that the embryo at first 

 has the form of an epibolic gastrula and passes from this to an 

 iny agination- gas trula (Fig. 45 A and B, p. 115). The entoderm, at a 

 later stage, by the great increase in number of its cells, is transformed 

 direct into the epithelium of the archenteron ; but, in some forms, 

 the macromeres seem to be retained for a long time, the transition to 

 the definitive entoderm being then less simple. The blastopore is 

 slit-like and situated at the vegetative pole. After its closure, the 

 mouth arises at the same spot through an ectodermal depression. 

 From the published accounts, we may assume that the mouth then 

 shifts its position or, in consequence of the further growth of the 

 embryo, changes its shape. At one end of the embryo a circle of 

 strongly ciliated cells marks off the velar area, immediately behind 

 which the mouth now lies. At a point almost opposite the cephalic 

 area, on the dorsal surface of the embryo, an ectodermal depression 

 appears which varies in size in the different genera ; this is the 

 shell-gland. The whole of the interior of the embryo is filled with 

 yolk-laden macromeres. The velum becomes more distinct, and, 

 behind the mouth, the foot appears as a large outgrowth. When 

 the otocysts arise near the foot and the two anal cells (which also 

 occur in the Pteropoda) behind it, the embryo passes into the 

 Trochophore stage which greatly resembles that met with in the 

 Opisthobranchs, or the corresponding stage in Firoloida (Fig. 65). 



At the stage just described, or even earlier, the embryo may 

 become free and may swim about actively, since it is already provided 

 with a velum. Up to this point the different Pteropods develop in 

 much the same way, but differentiations soon appear in the develop- 

 ment of the larval form, especially with regard to the shape of the 

 velum and the shell. The Gymnosomata also diverge from the other 

 forms in so far as the Veliger stage gives rise to a peculiar larval 

 form encircled with several ciliated rings. 



A certain differentiation in the development of the early larval stages is 

 also caused by the fact (stated by FOL) that the order in which the organs 



