GASTEROPODA. 2038 
(pulmonifera), the 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 pair of ciliated fins springing from the sides of the 
lead; 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 calyptrea 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 Fig. 67.* 
eolis and aplysia, or buccinwm and vermetus. (M. Edwards.) 
The development of the branchiferous gasteropods may be 
observed with much facility in the common rivyer-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. 68. Paludina vivipara.t 
Hnrbryos 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 deyelopment of the pulmoniferous embryo is best seen in 
the transparent eges 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their ciliated head-lobes (or veil) are superseded by the creeping 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 Hols (from Alder and Hancock). o, the operculum; the original 
is not larger than the letter o. 
+ Fig. 68. 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 whorl; the gill is seen on the left; and between them the 
termination of the alimentary canal. Surrey Docks, June, 18), 
