CLASS OF THE ACEPHALA. 



521 



the body is generally prolonged so as to form a fleshy foot, 

 having some analogy with the gasteropodes, but not so well 

 formed for locomotion. Sometimes it is the inner surface of 

 the mantle, as in the terebratulse, which takes the place of 

 the respiratory organ, and for this purpose shows a highly 

 developed vascular network; but in general there exists a 

 very well developed branchial apparatus, composed of two 

 pairs of large membranous plates, finely striated, and floating 



Fig. 546. Pearl oyster. 



Fig. 547. Buccardium. 



Fig. 548 Shell of the 

 Terebratula. 



Fig. 549. The Animal of the 

 Terebratula. 



between the foot and the mantle (Fig. 544). The mouth is 

 also concealed between the folds of the mantle, and is found at 

 one of the extremities at the base of the abdomen ; it has 

 never any teeth, but is furnished laterally with two pairs of 

 labial prolongations, constituting laminated tentacles. The 

 stomach is sufficiently developed, and the intestine forms 

 around the liver circumvolutions before reaching the posterior 



