42 CONCHIFEKA. 



are open in front, and united at the back. Each 

 leaf consists of a series of close-set transverse 

 tubes, which open into channels leading into a 

 cavity behind the united leaves. These tubes are 

 clothed with cilia, which maintain a constant cur- 

 rent in the water that bathes them. 



In most of the members of this Class there are 

 two openings, exactly as in the TUNICATA: the 

 one for the admission of water, the other for its 

 rejection. These often take the form of tubes 

 (siphons], more or less lengthened and projected at 

 will from the shell. The receiving siphon com- 

 municates with the cavity included between the 

 gill-leaves, the discharging one with another cavity 

 behind the leaves. In some species, however, the 

 discharging siphon alone is present, the receiving 

 one merging into the wider opening of the mantle- 

 leaves ; while in the Oyster family there are no 

 siphons, the mantle being entirely open. 



Within the innermost pair of gill-leaves are 

 placed the various viscera, and in particular those 

 of the digestive system. As in the TUNICATA, the 

 entrance to the stomach is placed at the bottom of 

 the breathing organ, receiving the minute atoms 

 of food that has been collected from the water- 

 currents. The mouth, thus placed, is guarded by 

 tumid lips, and by one or two pairs of broad mem- 

 branous palps, or organs of a delicate sense. The 

 food of all bivalves is microscopically minute ; it 

 consists very largely of those dubious objects, 

 abounding in both fresh and sea water, which 

 occupy a debatable position between the animal 

 and vegetable kingdoms, the Diatomacece, &c.* 



* The membranous palps, in many species, form so many 

 grooves leading to the mouth, and the particles of food travel 

 along them by means of the cilia with which they are bordered. 



