MOLLUSCA 



249 



is a curious wormlike group without any shell, which 

 may be considered the slugs of this series. Chitons are 

 common on rocks between tide marks on our Atlantic 

 and Pacific coasts ; some of the Californian species are 

 quite large, one being about 9 inches in length. An- 

 other bilaterally symmetrical group of mollusks is the 

 Scaphopoda, including the Dentalium or tooth shell. TheScapho- 

 The shell is a long, cylindrical tube, tapering apically. ^thsheiis 

 These are all marine. 



5. The Lamellibranchiata or leaf-gilled mollusks are Bivalve 

 also called Pelecypoda (hatchet-footed). They are the Mollusca 

 bivalves, with two similar parts to the shell ; familiar 

 examples are the oyster, clam, mussel, and cockle. 

 They differ from the Gastropoda in many very impor- structure of 

 tant characters ; there is no well-defined head, and the ablvalve 



u 



m 



Drawing byW.P. Hay (after model in Am. Mus. Natural History) 



FIG. 66. A lamellibranchiate mollusk, the common clam or quahog (Venus mer- 

 cenaria), partly dissected: g, gills, mainly cut away; m, mantle; s l and s 2 , upper 

 and lower siphons ; aa and pa, anterior and posterior adductor muscles ; ht, heart ; 

 h, hinge ; u, umbo. 



