128 WILLIAM RAY ALLEN. 



The mussel, regardless of its considerable size, depends entirely 

 for food on the microscopic organisms floating in the water, and 

 offering little resistance to capture. That this diet is sufficient, 

 is probably due to the comparatively inactive life of the animal. 

 Energy is further economized by a partial combination of the 

 functions of respiration and food-getting in the same organ the 



gill. 



THE CILIA AND THEIR ACTION. 



The filaments of the gills are covered with cilia which intercept 

 the particles contained in the water and prevent their passing 

 through the gills with the water. They become entangled in 

 mucus and through the action of these cilia such particles are 

 wafted toward the mouth in streams. If they are of a harmless 

 nature or of food value, they are permitted to enter the alimentary 

 tract. During the incubation of the glochidia, the female gives 

 up a greater or less part of one or both of the gills for marsupial 

 purposes. At this period these parts are of little use for res- 

 piration or for the collection of food. 



Cilia similar to those of the gills line the entire branchial 

 chamber, cover all organs which come into contact with the 

 water, and also line the alimentary tract. They are, as is 

 always true of cilia, in constant motion during life; they act 

 independently of nervous control and in a single plane. Their 

 concerted action is in the form of waves resembling in appear- 

 ance the passing of a breeze over a field of grain, or, as Field ('09) 

 has said, the movement of a bank of oars. The direction which 

 these waves or streams take varies in the several organs. But all 

 of the streams taken together are coordinated to accomplish a 

 certain common end. So vigorous and powerful are the cilia 

 that, when an organ or portion of one is removed and placed 

 upon a smooth surface for study, the whole mass is moved by 

 their action slowly and steadily in a direction opposite to that 

 in which they are directed. Small parts will even climb the 

 side of a watch glass. McAlpine ('88) found that seven twelfths 

 by weight of the soft part of the animal may thus be moved by 

 ciliary action. As long as the part so removed is kept in water 

 their activity continues, even for several days. In one instance 

 McAlpine kept the cilia of marine clams living for eight days after 



