138 INTRODUCTION TO ZOOLOGY. 



Additional Facts About the Clam. 



The clam belongs to a branch of animals called 

 mollusks. The name is applied to them because of 

 their soft body, without any skeleton. The class to 

 which the clam belongs is called Lamellibranchiata, 

 and this name refers to the kind of gills it has. The 

 gill is thin, or flat, like a plate or the leaf of a 

 book. Formerly, animals of this class were called 

 bivalves, because of the two parts of the shell. Still 

 another name was Acephala, which means without a 

 head. 



The fresh-water clam moves very slowly in the 

 sand at the bottom of a pond or river. It stands on 

 edge, half imbedded in the sand. The foot, which is 

 very muscular, is pushed slowly out of the shell and 

 downward and forward into the sand. After it has 

 been pushed out into the sand until it is securely 

 fastened, a sudden retraction of the foot itself, aided 

 by two muscles (retractors) inside of the shell, shortens 

 the foot. The tip of the foot being securely fastened 

 in the sand, the body enclosed in the shell must be 

 drawn toward it. In this way the clam, by a sin- 

 gle impulse, may move itself one-half or one-third 

 its length. This movement leaves a furrow in the 

 sand, and fresh-water clams can often be discovered 

 by following along this furrow. In the same way, 

 when the animal chooses to do so, it can burrow down 

 into the sand, completely concealing itself. 



In order to breathe, the animal must get a current 

 of water to flow over the gills. Since the gills are 

 never extruded, it is necessary that the water be taken 

 into the shell. The water enters the shell through the 

 lower, or incurrent, siphon. In the fresh-water clam, 

 the siphons are not permanent, but are formed merely 

 by the lapjDing together of the two edges of the man- 

 tle. The edge of the lower siphon is very sensitive to 



