126 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



Lamellibranchs breathe by four hollow, platelike gills 

 (whence the name), two on each side underneath the 

 mantle (Fig. 275), the water being drawn into the cavi- 

 ties in the gills by the action of ciliated cells. In the 

 higher forms, the margin of the mantle is rolled up into 

 two tubes, or siphons, for the inhalation and exhalation 

 of water. They feed on microscopic organisms gathered 

 from the water by the ciliated inner surface of the mantle, 

 the cilia producing a flow of particles toward the mouth. 



CL 



FIG. 86. Lamellibranch (Mactra): a, foot; b, c, siphons. 



A few are fixed ; the oyster, e.g. habitually lying on its 

 left valve, and the salt-water mussel hanging to the rocks 

 by a cord' of threads called " byssus " ; but most have a 

 " foot," by which they creep about. Unlike the oyster, 

 also, the majority live in an erect position, resting on the 

 edges of their shells. About five thousand living species 

 are known. These are fresh-water and marine, and range 

 from the shore to a depth of a thousand feet. 



The chief characters for distinguishing lamellibranchs 

 are the muscular impressions, 26 whether one or two ; the 

 presence of a pallial sinus, which indicates the possession 

 of siphons ; the structure of the gills, and the symmetry 

 of the valves (Fig. 296). 



The following are the more important orders, classi- 

 fied according to gill structure : - 



