MOLLUSCA. 253 



metry is shown in the right and left valves of the shell secreted by a 

 bilobed mantle, and in some of the internal organs. There may be one 

 or two adductor muscles. The head is undeveloped. The ventral body 

 region is differentiated into a muscular foot, shaped like a plow-share. 

 The gills are in sheets (see 285) usually two on either side, and are sus- 

 pended in the mantle cavity. Paired labial palps occur about the otherwise 

 unspecialized mouth. The three pairs of ganglia, the cerebro-pleural, 

 the pedal, and the visceral, are usually well separated. The heart con- 

 sists of two auricles and one ventricle surrounded by a pericardial space, 

 which is a portion of the body cavity and communicates with the exterior 

 by a pair of nephridial tubes. The reproductive organs are simple; the 

 sexes are ordinarily separate. Development by a metamorphosis. 



[The primary subdivisions of the group may be made on the basis 

 either of the gill structure, the adductor muscles, or the presence or 

 absence of the siphon.] 



Order I. Isomya : Two adductor muscles which are essentially equal. 



(o) Siphon well developed, retractile; pallial line (Fig. 107) with a 

 sinus. Here occurs Mya arenaria, the common clam of the Atlantic coast. 

 Great heaps of shells of this clam show that it was much used by the 



FIG. 115. 



FIG. 115. Ensis americanus, the razor clam. From Verrill, after Gould. 







Questions on the figure. Where is the hinge, the umbo, etc. ? Trace 

 the lines of growth and compare with other figures of bivalves. 



Indian tribes as food. In New England the clam fisheries are of very 

 considerable importance. Mya burrows in the mud, using its long siphon 

 to keep it in connection with the water from which it gets its food. Of 

 somewhat similar habits is the razor-shell clam (Fig. 115). The "borer" 

 (Pholas} and the "ship-worm" (Teredo} belong to this group and 

 possess the power of boring into wood or stone and are thus of much 

 damage to submerged structures in waters where they abound. 



(6) Siphon usually present but not highly developed; no pallial sinus. 

 In this group are embraced the more abundant fresh water mussels (Unio, 

 Anodonta, Cyclas), and the cockles (Cardium} of the ocean. The 

 Unionidse are very widely distributed and very common in our own fresh 

 waters. They are not much used for food at present, though the Indians 

 used them, probably in times of scarcity of other food. Their shells are 

 widely employed in the making of buttons, knife handles and the like, 



