206 GENERAL ZOOLOGY 



of both species and individuals. Though they move about, 

 the majority of individuals spend most of their time with 

 the posterior end bearing the siphon directed upstream. 

 Thus the flow of water downstream brings a continuously 

 fresh supply of food to the animal. Fishermen (Fig. 109) 

 make use of this habit. They always drag their clam bars 

 downstream, so that the hooks may catch into the open 

 shells. Mussels cannot survive in water that is polluted 

 with sewage. 



Definition of Acephala. The four animals described in this 

 chapter are representatives of a class called Aceph'ala. 

 Acephala means "without a head." They are called bi- 

 valves because the shell is in two pieces. Acephala are 

 usually bilaterally symmetrical animals with an external 

 skeleton composed of two nearly equal valves. They have 

 no internal skeleton. The body is without a head and is not 

 divided into somites. There are no jointed appendages. 

 The mantle folds surround the body proper and secrete the 

 shell substance. Locomotion is most frequently accom- 

 plished 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 in- 

 ward. The esophagus is short. The stomach receives the se- 

 cretion 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 pairs of glands of the reproductive system 

 in male and female individuals seem 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. 



