i jo Descriptive Zoology. 



stomach, which in turn is followed by the intestine. The 

 intestine is usually twisted around so as to end in the 

 mantle chamber near the edge of the aperture of the shell. 

 Salivary glands are almost always present, and there is a 

 large digestive gland around the stomach, as in the clams. 



The Circulatory System. This is on essentially the same 

 plan as in clams. The blood comes from the gills, or lung, 

 to the heart, and is thence pumped to the other parts of the 

 body. There is usually but one auricle in place of two 

 found in the clam. 



The Excretory System. The gastropods have kidneys 

 essentially like those of clams, whose ducts open into the 

 mantle cavity. Owing to the one-sided development, usu- 

 ally only one kidney is retained. 



The Nervous System. The nervous system is primarily 

 about the same as that of the clam, consisting of several 

 pairs of ganglions connected by nerve cords. The twisting 

 of the body in many of the univalves involves the nervous 

 system so that the nerve loop becomes twisted into the 

 shape of a figure 8. 



Sense Organs of Gastropods. The eyes of the snail are 

 described below. It is doubtful how well a snail can see, 

 but it can discern light from darkness and can perceive 

 quick movements. 



A sense of touch belongs to the whole surface of the 

 body, but is more acute in the tentacles. At the base of the 

 gills are organs called "osphradia," or "smelling patches," 

 being perhaps organs for testing the quality of the water. 

 Land snails can detect odors, arid the seat of the sense of 

 smell seems to be in the tentacles. 



Respiration in Gastropods. The majority of the gastro- 

 pods breathe by means of gills. Between the mantle and 



