150 MOLLUSKS. 



The Life Process. Since, in connection with each of 

 the organs studied, the work it has to do has been already 

 mentioned, it only remains to summarize these various 

 operations as parts of the process by which the life of the 

 animal is maintained. 



I. Nutrition. This part of the process is dependent 

 to a marked extent upon the aquiferous system, and the 

 water currents it keeps up. The siphons are the most 

 conspicuous part of this system ; but the cilia lining the 

 openings into the gills, although the least conspicuous, 

 are the most important part of it, for it is by their lash- 

 ing action that the water is drawn into one siphon and 

 driven into and through the gills and out through the 

 other siphon. In the gills the usual exchange of car- 

 bonic-acid gas for oxygen takes place between the 

 water flowing freely over the thin-walled blood vessels 

 distributed throughout the gills and the blood flowing 

 inside those vessels. In other animals that breathe by 

 gills this exchange seems to be the sole purpose of the 

 water currents, but in the mussel these currents also assist 

 in the procuring of food and in excretion. The food con- 

 sists chiefly of low plant organisms found free in the water. 

 These are carried into the branchial chamber by the enter- 

 ing current. The current does not all enter the gills, but 

 a branch of it is driven forward toward the anterior end 

 of the body, and the bits of food carried along by it are 

 directed by the labial palps into the mouth. Excreta are 

 expelled with the current passing through the cloacal 

 siphon. Thus all the operations connected with nutrition 

 are more or less directly dependent on the water currents. 



For convenience in summarizing, we may say that ru- 

 trition is effected in the river mussel through the agency 

 of the following interdependent systems of organs : 



1. A digestive system, comprising the alimentary canal 

 and the large accessory gland called liver. 



