MOLLUSCA 



259 



rated. The heart consists 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 l. Isomya: Two adductor muscles which are essentially equal. 



(a) Siphon well developed, retractile; pallial line (Fig. 109) 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 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. 118. 



Fig. 118. Afytilus edulis, a Mussel. From Binney's Gould. 



Questions on the figure. — Identify the umbo. What are your evidences that 

 it is the umbo? Compare the lines on the shell with those in Pig. 117. What 

 is the significance of the specific name [edulis) ? What are the habits of the species ? 



(Fig. 117). 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 (Unto, Anodonta, 

 Cyclas), and the cockles {Cardium) of the ocean. The Unionidae 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, and pearls of value are not of infrequent occurrence. These 

 are merely the mother-of-pearl, which ordinarily lines the shell, secreted about 

 some infected or irritated point on the epidermis of the mantle. Considerable 



