II. ACEPHALA 



317 



Class II. Acephala (Lamellibranchiata, Pelecypoda). 



These have, among the molluscs, the least powers of locomotion. 

 Some are fixed, the majority burrow slowly through sand or mud; only 

 a few spring by means of the foot or swim by strokes of the valves. 

 Hence they need more protection than other species, and this is afforded 

 by the strong shells in which the body can usually be completely enclosed. 

 This shell recalls that of the brachiopod in that it consists of two halves or 

 valves, but these valves are right and left and hence are usually similar 

 in shape. Only when the animal rests permanently on one side is this 

 symmetry lost (the extreme is reached in the fossil Ritdistes), and then 

 the symmetry of the soft parts is affected. 



FIG. 318. 



a 



FIG. 319. 



FIG. 318. Left valve of Crassatella plumbea, inner and outer surfaces (from Zittel). 

 The outer surface showing lines of growth; no pallial sinus. 



FIG. 319. Right valve of Mactra still tonim, with pallial sinus (from Ludwig- 

 Leunis). Letters for both figures: a', anterior, a", posterior adductor scar; e, hinge; 

 /, internal ligamental groove; in, pallial line; s, pallial sinus. 



The two lobes of the mantle which secrete the shell on their outer 

 surface arise from the back of the animal and grow downwards, forwards, 

 and backwards, so that they envelop the whole (fig. 322). Hence the 

 oldest and thickest part of the shell, the wnbo, occurs near the back 

 (fig. 318). Around this the lines of growth are arranged concentrically, 

 lines which show how, by gradual growth of the mantle, the shell has in- 

 creased in size. On the back the valves approach each other, and in the 

 majority are movably connected by a hinge, which consists of projections 

 or teeth on one valve fitting into depressions in the other. The valves 



