^02 



MAirCTAL OF THE MOLLTJSCA. 



Chaptes II. 

 CLASS II.— GASTEEOPODA. 



The gasteropoda, including land-snails, sea-snails, whelks, 

 limpets, and the like, are the types of the moUusca ; that is 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 is 

 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 following each other in 

 rapid waves when a snail is climbing a pane of glass. 



The nuchohranchs are "aberrant" gasteropods, having the 

 foot thin and vertical ; they swim near the surface of the sea in 

 a reversed position, or adhere to floating sea-weed. 



Fig. G6. A nucleobranch.* 



The gasteropods are nearly aU ur symmetrical, the body being 

 coiled up spirally, and the respiratory organs of the left side 

 being usuaUy atrophied. In chiton and dentalium the hranchioe 

 and reproductive organs are repeated on each side. 



A few species of cymba, Uttorina, jpaludina, and helix, are vivi- 

 '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.f 



The gasteropods form two natural groups ; one breathing air 



* Fig. 66. Carinaria cymbium, Desh. = C. cristata, L. sp; (after Blainville), Medi- 

 terranean, p, proboscis ; t, tentacles ; b, branchiae ; s, shell ; /, foot ; d, disk. 



t M. Loven believes tliat the embryo shell of the nudibranchs falls off at the time 

 they acquire a locomotive foot. 



