352 



VIVIPAROUS GASTEROPODS. 



or at most three or four ova, has its own proper globule 

 of jelly, contained within a skin or pellicle of the greatest 

 tenuity, and which isolates it from the rest.* The annexed 

 figure (Fig. 70) exhibits this disposition as we see it in 



the spawn of the Periwinkle (Littorina littorea) ; and you 

 perceive also that the minute embryos are already covered 

 each with its shell, so that any metamorphosis they undergo 

 is early and speedily completed. 



A few species of these phytivorous Gasteropods are vivi- 

 parous, the spawn being lodged in the branchial cavity until 

 the young are fitted for a separate existence; and we find 

 some oviparous and some viviparous species in the same 

 genus. Thus the common Periwinkle (Littorina littorea) 

 has the economy of the first, and the Littorina rudis that 

 of the second. The Paludina vivipara affords another illus- 

 tration of the same fact ; and the foetal shell of this, as 

 Swammerdam ascertained, is armed with crystalline spines 

 arranged in regular rows on the whorl, while the mature 

 shell is even and destitute of armature. It is almost the 

 only instance known amongst Mollusks of the embryo shell 

 exhibiting an exterior structure different from the adult. 

 Spallanzani affirms that young individuals of Paludina vivi- 

 para taken from the mother, and kept always in a state of 

 isolation, yet produced in due time young, a fact from 

 which you are not to infer, with him, their monoecious 

 nature, which anatomy disproves, but that a single impreg- 



* The embryo has the same rotatory motion in the ovum of these and 

 other Mollusca as that described in the spawn of the bivalves. See Cams 

 as translated in Zool. Journ. iv. 257. 



