GASTEROPODA. 



203 



{'pulmonifera), tlie other water {branchifera). 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 j)aLr of ciliated fins springing from the sides of tho 

 ] ead ; and by this means are often more widely 

 dispersed than we should be led to expect from 

 theii" adult habits ; thus some sedentary species 

 of ccdyptroea and chiton haye a greater range 

 than the "paper- sailor," or the eyer-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 Fig. 67.* 



eolis and aplysia, or hucdnum and vermetus. (ISI. Edwards.) 



The development of the branchiferous gasteropods may be 

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

 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. 68. Paludina vivipara.i 



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 their ciliated head-lobes (or veil) are superseded by the creeping 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 Eolts (from Alder and Hancock), o, tlie operculum; the original 

 \3 not larger than the letter o. 



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

 if seen through the shell. The ovary, dist-ended with eggs and embryos, occupies tho 

 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, ISW. 



