MOLLUSCA. 201 



Mussels, (Mytilidcs^) Wing-shells, or Pearl Oysters, (Av- 

 iculid&l} and River Mussels, (Unionidcs^} 



Section B. Siphonida. Having siphons. 



(i.) Integro-pallialia. Siphon short, pallial line sim- 

 ple, as in the families Tridacnidce, Cardiadce, (Cockles,) 

 and Cyprinidce, (Heart-cockles.) 



(2.) Sinu-pallialia. Long siphons, pallial line sinu- 

 ated, as in Venerida, (Clams,) Mactridcz, Solenidce, (Ra- 

 zor-shells,) and Pholadidce, (Boring-shells.) 



6. GASTEROPODA, (Gr., gaster, stomach ; pous, foot.) 

 This class derives its name from the fact that loco- 

 motion is usually effected by a muscular expansion 

 of the under surface of the body, termed the " foot." 

 It includes all the univalve shells, the naked slugs, the 

 Dorsibranchs, the Pteropods, and the Multivalvular 

 Chiton. 



The body of most Gasteropods is unsymmetrical, the 

 organs not being in pairs, but single, and on one side, 

 instead of central. The mantle is continuous round the 

 body, not bilobed, as in Lamellibranchs. A few, as the 

 Garden-snail, have a lung, but the majority breathe by 

 gills. The head is more or less distinct, and is provided 

 with two tentacles, with auditory sacs, or rudimentary 

 organs of hearing at their bases. The eyes are some- 

 times quite conspicuous. The Snail, for example, carries 

 two ocelli, or simple eyes, on the tip of its long tentacles. 

 Each consists of a globular lens, of short focus, which is 

 a part of the transparent cornea, with a colored mem- 

 brane (choroid) and a nervous net-work (retina) behind. 

 The arrangement for retracting the eye and tentacle is 

 seen in Fig. 100. 



