Anatomy of the Food Mollusks 37 



base of the gill, where it enters a tube which is without 

 obstructing interlamellar partitions, and leads backward 

 to the cloacal chamber. This has been described as a 

 space above the four gills and behind the main portion 

 of the body. 



Into this cloaca all four gills continually pour their 

 streams, which unite to form a powerful current that 

 leaves the body either directly between the unmodified 

 mantle folds, or through the dorsal chamber of the 

 siphon tube. As a large amount of water is in this way 

 being constantly forced out of the body, a correspond- 

 ing volume is being drawn in. It enters directly be- 

 tween the mantle folds into the gill chamber, or, in the 

 clams, is drawn into it through the ventral tube of the 

 siphon. So rapid is the stream in all forms, that with- 

 out doubt many gallons of water flow through the gills 

 daily. 



It is interesting to notice that this greatly perfected 

 pumping mechanism never ceases its activity during the 

 life of the individual. The cilia are not under the control 

 of the nervous system, and the direction of their beat- 

 ing is never changed or apparently slackened. There is 

 but one way to prevent the streaming of water through 

 the body, and that is to close the shell or contract the 

 siphon so as completely to close the gill chamber. 



Now it happens that this inflowing stream bears many 

 minute marine plants that these shell-fish use for food. 

 They are not so numerous but that a large amount of 

 water must be strained to enable the animal to obtain 

 enough of them for its nourishment and growth. These 

 organisms are small enough to pass through between 

 the gill filaments and so out of the body again; but on 

 coming in contact with the gill, they are instantly en- 



