MISCELLANEOUS ANIMALS 433 



without touching the tentacles, let a little more fall near 

 the lower opening. It is all drawn into the clam, and if a 

 solution or a very fine suspension of inert matter, like car- 

 bon, chalk, clay, etc., it is soon seen streaming out of the 

 upper siphon. If this prove irritating to the clam, it will 

 shut up with a snap, throwing clouds of the liquid out of 

 both siphons. We thus see that a stream of water is being 

 drawn into one siphon and thrown out at the other. Next, 

 take a suspension of yeast plants, small alga;, or bacteria, 

 let it pass into the inhalant siphon, and watch the exhal- 

 ant siphon. It goes in turbid and comes out perfectly 

 clear. This shows what the clam feeds on, — minute floating 

 particles in the water. The currents are set up by the gills 

 of the clam, the water being passed through while the food 

 material is strained out and carried up to the mouth. A 

 culture of typhoid fever bacteria, for example, flowed through 

 the gills of an oyster, which are quite similar to those of a 

 clam, came away sterile, i.e., with all the bacteria filtered 

 out. Clams are thus living filters, and in a pond well 

 stocked with them they must exert considerable influence 

 in keeping the water cleansed of floatir.g organic matter.^ 



^ The structure of the clam is explained somewhat in Fig. 175, and, if 

 the school possesses a compound microscope, the action of the gills ma\ 

 be demonstrated, but this may as well be left to later courses in zoology. 



