110 ZOOLOGY. 



These gills are attached along their dorsal margins. 

 Sometimes there is only one gill 011 each side. When 

 viewed by a microscope, these gills are seen to consist 

 of a number of minute "tubular rods supporting a 

 network of blood-vessels. " They are covered with 

 cilia, which, by their perpetual movements, keep up 

 a constant current of water. This current not only 

 supplies fresh water for the purification of the blood, 



FIG. 46. ANATOMY OF THE OYSTER. 



s, mouth ; e, stomach ; i i, intestinal canal ; a, anus ; b g, nervous gang-lia; 

 m t, mantle ; & (figure to the left), branchiae ; m, adductor muscle ; o, ovary. 



but also brings minute particles of food for the sustenance 

 of the animal. 



Those lamellibranchs that live buried in sand or mud 

 are provided with two respiratory siphons or tubes. One 

 of these admits a current of water which passes over the 

 gill plates, supplies the mouth with such food as the 

 water may contain, and is then discharged through the 

 other siphon, carrying with it the excretions from the 

 intestine. These siphons are sometim.es longer than the 

 shell, and in other cases are very short. When the ani- 

 mal buries itself, it always leaves the extremities of the 



