202 MANUAL OF THE MOLLUSCA. 
CHAPTER II. 
CLASS II.—GASTEROPODA. 
The gasteropods, including land-snails, sea-snails, whelks, 
limpets, and the like, are the types of the mollusca ; that 1s to 
say, they present all the leading features of molluscous organisa- 
tion in the most prominent degree, and make less approach to 
the appearance and condition of fishes than the cephalopods, and 
less to the crustaceans and zoophytes than the bivalves. 
Their ordinary and characteristic mode of locomotion 1s 
exemplified by the common garden-snail, which creeps by the . 
successive expansion and contraction of its broad muscular foot. 
These muscular movements may be seen followimg each other in 
rapid waves when a snail is climbing a pane of glass. 
The nucleobranchs are “aberrant” gasteropods, haying the 
foot thin and vertical; they swim near the surface of the sea im 
a reversed position, or adhere to floating sea-weed. 
Fig. 66. A nucleobranch.* 
The gasteropods are nearly all unsymmetrical, the body being 
coiled up spirally, and the respiratery organs of the left side 
being usually atrophied. In chifon and dentalium the branchice 
and reproductive organs are repeated on each side. 
A few species of cymba, littorina, paludina, and helix, are yivi- 
parous; 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.t 
The gasteropods form two natural groups; one breathing air 
* Fig. 66. Carinarva cymhium, Desh. = C. cristata, L. sp. (after Blainville), Medi- 
terranean. , proboscis; t, tentacles; 0, branchie; s, shell; 7, foot; d, disk. 
+ M. Lovén believes that the embryo shell of the nudibranchs falls off at the time 
they acquire a locomotive foot. 
