98 



MANUAL or THE MOLLrSCA. 



A few species of cymha, litor'ma, paludina, and helix, are viviparous ; the | 

 rest are oviparous. \ 



TMieu lii'st hatched the young are always provided with a shell, though in 

 many families it becomes concealed by a fold of the mantle, or it is speedily 

 and wholly lost.* 



The gasteropods form two natural groups; one breathing air {pulmoniferd), •'; 

 the other water {braac/ufem). The air-breathers undergo no apparent meta- | 

 morphosis ; when born, they differ ft'om theii- parents in size only. The « 

 water-breathers have at fii'st a small nautiloid shell, capable of concealing 'i 

 them entii'ely, and closed by an operculum. Instead of creeping, they swim ■j 

 Auth a pair of ciliated fins springing fi'om the sides of the . y 



head ; and by this meaus ai'e often more widely dispersed 

 than we should be led to expect from their adult habits ; 

 thus some sedentary species of calyptrcea and chiton have 

 a greater range than the " paper-sailor," or the ever-drifting 

 oceanic-snail. 



At this stage, which may fairly be compared with the 

 larval condition of insects, there is scarcely any difference 

 between the young of eolis and aplysia, or buccinum and 

 termetus. (M. Edw.) 



The development of the branchiferous gasteropods may be observed with 

 much facility in the common river-snails {j)alndina) ; which are viviparous, 

 and Avhose ovaducts in early simimer contain young in all stages of growth 

 some being a quarter of an inch in diameter. 



GO.t 



Fig. 61. Paludinavivipara.X 

 Embryos scarcely visible to the naked eye have a weU-formed shell, orna- 

 mented with epidermal fringes ; a foot and operculum ; and the head has long 

 deheate tentacula, and very distinct black eyes. 



* M. Loven believes that the embryo shell of the nudibranches falls ofiFat the time 

 they acquire a locomotive foot. 



+ Fig. 60. Fry of Eolis (from Alder and Hancock) ; o, the opeiculum ; the original 

 s not larger than the letter o. 



X Fig. 61. Paludina vivipara L. (original) : the internal organs are represented as 

 if seen through the shell. The ovary, distended with eggs and embryos, occupies the 

 right side of the body whirl ; the gill is seen on the left : and between them the termi- 

 nation of the aUmentary canal. Surrey Docks, June, 1850. 



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