PTEROPODA AND GASTROPODA. 567 



tion alternately approximating and stretching outwards. On the 

 hinder part of the foot is an operculum, which closes the mouth of 

 the shell when the embryo retracts itself. Among the internal 

 organs the acoustic capsules appear first, then the eyes. The ten- 

 tacles next protrude, and the border of the mantle appears. The 

 mouth is established between the vela. On the 36th day the stomach 

 and a looped portion of intestine come into view out of the germ- 

 mass, the remnant of which is chiefly changed in the hepatic and 

 genital glands. A longitudinal muscle for retracting the body into 

 the shell also now appears. 



During this course of development the nidamental band has become, 

 by endosmose of sea-water, three times as thick as before. The al- 

 buminous substance is absorbed by the embryos : they respire by the 

 reaction of their ciliated surface on the imbibed water ; as they grow 

 they, with difficulty, find room for their evolutions, and, between the 

 32nd and 38th ^days, rupture the delicate membrane of their nest, 

 and struggle out. They are now about one-eighth of a line in 

 length, and swim by ciliary action, the vela being kept stiffly out- 

 stretched. They survived in the vessels of daily renewed sea-water 

 two weeks, then died, their embryo shells floating on the surface. 



The ova of the Aplysia are excluded in a long string, enveloped 

 by a transparent flexible mucus, in the centre of which they are 

 aggregated in several irregular series. When examined at this 

 period, the yolk has apparently divided itself into six, seven, or more 

 numerous globules ; or, in other words, as many germinal vesicles 

 included in the same mass of albumen and in a common chorionic 

 coat, have given origin to as many aggregations of vitelline cells ; 

 these, therefore, may be regarded as so many independent yollvs, in 

 each of which the same progressive fissiparous multiplications have 

 been observed, as in the single vitellus of the ovum of the Planorbis, 

 and of animals in general. Fig, 209. exhibits one 

 of these yolkleis prior to the commencement of the 

 fissiparous action, by which subdivision of the 

 mass is produced. Fig. 210. shows the quadrifid 



Aplysia. 



product of that action and of the assimilative 



powers of the resulting divisions. The directive cell is indicated by 



Van Beneden at a* 



In Jig. 211. the multiplication of the globules has increased, and 

 two of them, of larger size than the rest, indicate, one, the seat of the 

 future branchial organs, the other that of the muscular mass. 



The ciliated epithelium, with which the vitellus is now almost 



* CCCLXVIII. pi. 123. p. I. 

 o o 4 



