176 GENERAL ZOOLOGY 



most frequently accomplished by a single muscular organ, 

 the hatchet-shaped foot. The valves are held together by a 

 hinge-ligament, aided sometimes by ridge-like processes fitting 

 into grooves. The valves are closed by one, or two, adductor 

 muscles. 



Four palps surround the mouth and carry the food inward. 

 The oesophagus is short. The stomach receives the secre- 

 tion from a pair of digestive glands. The intestine coils 

 several times and ends posteriorly. The circulatory system 

 is nearly complete. There are two auricles and one ventricle 

 in the heart. The pair of glands of the reproductive system 

 in male and female individuals seems to differ only in micro- 

 scopic structure. In some species the sexes are separate. In 

 others the individuals are hermaphroditic ; that is, the male 

 and female sexual glands are present in the same animal. 



