93 



MANUAL OF THE MOLLUSCA. 



A few species of cymba, litorina, paludina, and helix, are viviparous ; the 

 rest are oviparous. 



"When first 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 (pulmonifera) t 

 the other water (brancMfera). 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 ciliated fins springing from the sides of the 

 head ; and by this means are often more widely dispersed 

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

 thus some sedentary species of calyptraa 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 

 vermetus. (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 oviducts in early summer contain young in all stages of growth 

 some being a quarter of an inch in diameter. 



Fig. GO.t 



Fig. 61. Paludina vivipara.% 



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 

 they acquire a locomotive foot. 



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

 s not larger than the letter o. 



I Fig. 61. Paludina vivipara L. (original); the internal organs are represented at 

 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 alimentary canal. Surrey Docks, June, 1850. 



