LAMELLIBRANCHIATA. 6S5 



sedentary Bivalves it is more or less completely aborted. In many 

 cases, especially in the young, there is developed, on the under 

 surface of the foot, in the middle line, a peculiar " byssal gland." 

 This gland secretes a viscid material which is moulded into threads 

 by grooves on the external surface of the foot, and which gives rise 

 to a tuft of silky fibres (the " byssus "), serving to moor the animal 

 to foreign objects, and often having a special notch or aperture in 

 the shell for its emission. 



The shell of the Bivalves is the result of the deposition of lime- 

 salts in the outer layer of the mantle, chiefly along its ventral mar- 

 gins ; and as the mantle is divided into a right and left " lobe," so 

 the shell also is divided into a right and left " valve." The outer 

 surface of the shell is usually covered by thinner or thicker horny 

 " epidermis " or cuticle, but this may become obsolete in the adult. 

 The calcareous tissue of the shell itself is very generally disposed in 

 two distinct strata, of which the outer is prismatic, and the inner is 







B 



Fig. 553. — a, External surface of Pinna, showing the ends of the prisms of the outer shell- 

 la ver, greatly enlarged ; b, Polished surface of mother-of-pearl, enlarged eighty-five times. 

 (After W\ B. Carpenter.) 



laminated. The outer prismatic layer is secreted by the free edges 

 of the mantle-lobes, and grows, therefore, at the circumference of 

 the shell only. It consists, in the majority of cases, of polygonal 

 calcareous prisms, placed side by side in close contact, and directed 

 perpendicularly to the surface of the shell. Hence sections of this 

 layer taken parallel with the surface exhibit the transversely-divided 

 prisms as a pavement of hexagonal or polygonal cells ; and the same 

 appearance is seen on examining the surface of this layer under a 

 sufficient magnifying power (fig. 553, a). The prisms of the outer 

 layer of the shell vary considerably in diameter, length, regularity, 

 and precise mode of arrangement in different Bivalves. The inner 



