98 MANUAL OF THE MOLLUSCA. 



A few species of cymha, litorina, paludina, and helit, are viviparous ; the 

 rest are oviparous. 



When fii-st hatched the young are always provided -nath 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 {puhnonifera) , 

 the other water {brancfiifera). The air-breathers undergo no apparent meta- 

 morphosis ; when born, they differ from their parents in size only. The 

 water-breathers have at first a small nautiloid shell, capable of concealing 

 them entirely, and closed by an operculum. Instead of creeping, they swim 

 with a pair of cihated fins springing from the sides of the 

 head ; and by this means are often more mdely dispersed 

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

 thus some sedentaiy species of cahjptrcea and chiton have 

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

 oceanic-snail. 



At this stage, w^hich may faii'ly be compared with the 

 lai-val condition of insects, there is scarcely any difference 

 between the young of eolis and aplysia, or buccinum and 

 vermetm. (M. Edw.) '^'^- ^^'^ 



The development of the branchiferous gasteropods may be observed with 

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

 and whose o^d ducts in early summer contain young in all stages of growth ; 

 some being a quarter of an inch in diameter. 



Fig. 61. Paludina vivipara.X 

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

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

 delicate tentacula, and very distinct black eyes. 



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

 t\\Qy acquire a locomotive foot. 



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

 js not larger than the letter o. 



i 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 ; tlie gill is seen on the left : and between them the termi- 

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



