MOLLUSKS 115 



many species lay eggs. Many snails ave lierbiverons, while a few 

 prey upon barnacles or of-lier niollusks. Sense organs are poorly 

 developed in snails, but in land snails and sings wo often lind eyes 

 mounted upon the tip ends of long tentacles which arise from the 

 head. In sea snails, however, the eyes are smaller, and are found 

 at the bases of the tentacles or only half way up. 



The clams, oysters and mussels form a great group called the 

 " La)iiellibivnchiala,'' for their gills project in ciirtaln-lilce sheets 

 from the sides of the body, within the shell. In these niollusks we 

 find two shells or more properly "valves," one on the right and the 

 other on the left side of the body, while the "hinge" between the 

 valves extends along the back of the animal. This hijige is tough 

 and muscular, and tends constantly to open the valves, but this is 

 prevented by the contraction of t^vo powerful sets of muscles which 

 run across from one valve to the other. 



The foot of the animal is often well developed and capable of 

 pushing the creature rapidly tlirough the sand, as in the razor- 

 clam, while in other cases, as in the oyster, it is small and degenerate. 

 There is no distinct head, but the mouth has two leaf-like lips. It 

 should be said that the long, muscular " neck " of the soft-shell clam 

 is not the neck of the animal, but is a tubular outgrowth of the 

 nrantle, which opens b}' two apertures. Tlie one on the lower side 

 is for the admission of water to the gill chambers, and the other is 

 the anal aperture. A continuous current of water flows in at the for- 

 mer and out from the latter aperture. In scientific language the 

 "neck" of the clanr is called the siplion, and it is well to remember 

 that it extends outward from the posterior end of the bodj^; the 

 mouth being at the opposite side of the shell. 



Cllams, oysters and mussels feed upon minute plant and ani- 

 mal organisms, which are drawn in between the edges of the mantle, 

 or through the siphon by means of the constant beating of vast 

 numbers of little hair-like cilia tliat cover the gills. 



The most highly developed moUusks are the Cephalopoda, 

 represented by the squid, octopus, nautilus, and argonaut. In 

 these we find two large e\'es and eight or ten long, muscular, 

 sucker-bearing "arms" which surround the mouth. These arms 

 constitute a portion of what was once the foot of the ancestral forms 

 from which the CcpJialopoda are descended. The remaining part 



