420 



MOLLUSC A : ACEPHALA. 



tightly shut by one or two strong muscles called adductors 

 (Fig. 620, , a'). When the animal relaxes these muscles 



FIG. 619. 



f S 



An Acephal Clam. Mya truncata, L. Reduced one-halt'. 



rs, respiratory tube or siphon ; the water flows into the branchial chamber, 



through this tube, aud out through the excurreat siphon s. 



FIG. 620. 

 I u Dorsal margin. 



Anterior aide. 



Posterior side. 



Ventral margin or base. 



Names of the parts of a Bivalve Shell. 



a, place of anterior adductor muscle ; a', place of posterior adductor muscle ; t, lat- 

 eral teeth ; c, cardinal tooth ; /, lunale ; u, unibo ; /i, hinge ligament; s, sinus occu- 

 pied by the retractor muscles of the siphons; p, pallial impression or place of the 

 attachment of the mantle. 



the shell is opened by an elastic pad or ligament situated 

 at the hinge (Fig. 620, 7<). 



The Acephals abound in both salt and fresh waters; and 

 living as well as dead specimens are always easily obtained 

 for study. Every student can obtain either an Oyster 



