gasteropoda: 



203 



(pulmonifera), the other water {hranchifera). 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 

 ) ead ; and by this means are often more widely 

 disper ed than we should be led to expect from 

 theii' adult habits ; thus some sedentary species 

 of co.Jyptrcea 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. Edwards.) 



The development of the branchiferous gastcropods may be 

 observed with much facility in the common river-snails {palu- 

 dina) ; 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. 67.* 



Fig. 68. Paludina viviparaA 



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

 shell, ornamented with epidermal fringes ; a foot and operculum ; 

 and the head has long delicate tentacula, and very distinct black 

 eyes. 



The development of the pulmoniferous embryo is best seen in 

 the transparent eggs of the fresh-water limneids ; these are not 

 hatched until the young have passed the larval condition, and 

 their ciliated head-lobes (or veil) are superseded by the creeping 

 disk, or foot. 



* Fig. 67. Fry of Eolis (from Alder and Hancock), o, the operculum; the original 

 is not larger than the letter o. 



t Fig. 68. Paludina vivipara, L. (original) ; the internal organs are represented as 

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

 right side of the body whorl ; the gill is seen on the left ; and between them the 

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



