ACTION OF THE GILLS. 160 



the blood circulates, and in which it is exposed to the 

 dissolved oxygen which is contained in the water. 



The gills have another function besides purifying 

 the blood; they are the food gatherers. Since the 

 animal has no head and no teeth it must depend for 

 its food upon the supply which is brought to it by the 

 surrounding water. Upon the surface of the gills 

 are innumerable cilia, or microscopic hairs. These 

 keep up a continual lashing and thus create a cur- 

 rent. The fresh water flows into the space within the 

 mantle, around the gills, and the old supply is con- 

 stantly being driven out. This circulation accom- 

 plishes two important results; first, fresh oxygen is 

 brought in for the blood, and carbonic acid is 

 expelled; and secondly, hundreds of minute organ- 

 isms which float in the water are caught upon the 

 surfaces of the gills, where they are rolled into a kind 

 of mucilaginous thread and passed on into the mouth. 

 The food of these mollusks consists, therefore, of 

 microscopic animals and vegetables, and is brought to 

 them by the united action of all the little whips 

 which cover the surface of the gills. 



To give a proper direction to the currents of water, 

 different means are employed. In those mollusks 

 which live above the surface of the mud, as the oys- 

 ter, the mantle lobes are divided, and the water cur- 

 rents pass freely through the open edges of the shells. 

 But in the clams which burrow in the mud, the case 

 is different. In these the mantle lobes are united, 

 inclosing the animal in a bag. At the rear end of the 

 shell the folds of the mantle are prolonged into a doub- 

 le tube, or two single ones, which the animal has 

 power to protrude to a considerable length and then 

 retract again into the space indicated by the pallial 

 sinus. 



