

318 COXCHIFERA. 



orifice of the digestive canal, and may be considered the 

 seat of taste, although these animals appear to be by no 

 means discriminating in the choice of their food, living 

 principally on Infusoria, microscopic vegetables, or par- 

 ticles of decomposed animal matter, which are brought to 

 them by the branchial currents produced by the cilia which 

 clothe the gills. The mouth leads by a short oesophagus to 

 the stomach, which, in some tribes, contains an instrument 

 called the crystalline stylet which appears to serve the 

 same purpose as the calcareous plates in the gizzard of the 

 Bitllidce among the Gasteropods. The intestine, after a few 

 turns, passes through the ventricle of the heart, and ter- 

 minates near the hind part of the body either in an anal 

 tube, as in the Pholadidce, or in a free vent, as in the 

 Pectin idee. 



The organ of hearing is composed of two simple cap- 

 sules, filled with a clear fluid, and each containing a round, 

 polished otolith. The sense of touch appears chiefly to 

 reside in the tentacular filaments which spring from the 

 margin of the mantle. 



The body of the Bivalve Mollusks is enveloped in a mus- 

 cular mantle, which is usually more or less united at the 

 margins, forming a branchial cavity with three openings, a 

 pedal, a branchial or inhalent, and an excretory or anal; 

 the pedal orifice being situated anteriorly, and the others 

 towards the hind part. The mantle secretes the shell, the 

 interior of which it lines, and to which it is fixed by the 

 adductor muscles, which pass through it to be attached to 

 the body of the animal. In some families the pallia! 

 orifices of the branchial chamber are prolonged into tubes, 

 as in the Pholadidte, in which case the ventral margins of 

 the mantle are usually united, leaving, however, an aperture 



