MOLLUSKS 



241 



foot mouth. 



A gastropod (snail). Why is this a mollusk? 



maintained through the mantle cavity, bearing oxygen to the gills 

 and carbon dioxide away from them. In most mollusks, this 

 current of water passes into 

 and out from the mantle 

 cavity through muscular 

 tubes called siphons. 



The food of clams or 

 oysters consists of tiny 

 organisms which are carried 

 in the current of water to 

 the mouth of the animal, this water current being maintained in 

 part by the action of cilia on the palps or liplike flaps surround- 

 ing the mouth. A single muscular foot enables the clam to move 

 about slowly. 



Gastropods. Snails, whelks, slugs, and the like are called 

 gastropods (stomach-footed) because the foot occupies so much 

 space that most of the organs of the body, including the stomach, 

 are covered by it. 



Cephalopods. Another class of mollusks are those known as 

 cephalopods (sefa-16-podz). The name means head-footed. As 



__Ffelecxpoc£ct 



Ampkineura 



dhitcm, 



ScaphocCcc 



tu&ukxr^ shelled: 



ctccm: 



Cephalop^Gcr 



-s^uict 



Class i. Pelecyp'oda (hatchet-footed). Shells of two valves or parts. Clams, oysters, scallops, 



mussels, etc. 

 Class II. Cephalop'oda (head-footed). Foot partly surrounds head and bears tentacles or 



grasping organs. Squid, octopus, cuttlefish, etc. 

 Class III. Gastrop'oda (belly-footed). With or without shells, which are usually of one piece 



and coiled. Snail, whelk, slug. 

 Class IV. Scaphoda. With a tapering tubular shell, with a spadelike foot for burrowing. 



Tooth shells. 

 Class V. Amphineura. Simple marine mollusks. Protected by a shell of eight arched 



segments. Chiton. 



