THE MUSSEL OR CLAM AND OTHER BIVALVES 151 



ferent from that of the crayfish. The food passes through the 

 short oesophagus into the saclike stomach (Fig. 84, s), where it 

 is acted upon by digestive juices from the liver (Fig. 84, I). 

 That part not absorbed by the walls of the stomach enters the 

 intestine (Fig. 84, i) which is coiled about in the foot. The in- 

 testine passes through a cavity (the pericardial cavity, Fig. 84, pc) 

 just beneath the hinge of the shell and terminates in the anal 

 opening just above the posterior adductor muscle (Fig. 84, a). 



Circulation. — As in the crayfish, the digested food is ab- 

 sorbed by the walls of the intestine and passes into the blood. 

 There is a heart in the pericardial cavity consisting of a muscu- 

 lar portion, the ventricle (Fig. 84, v), which forces the blood 

 through the anterior and posterior aortas (aa and pa), and a 

 pair of auricles (ra) which receive the blood after it has circu- 

 lated throughout the body, and deliver it to the ventricle. Dur- 

 ing this circulation, part of the blood passes through the gills, 

 where it receives a fresh supply of oxygen and is relieved of its 

 carbon dioxide, and part enters the walls of the excretory 

 organ, the kidney (k), just beneath the pericardial cavity, 

 where the waste materials it bears are excreted. Thus are 

 the functions of digestion, circulation, respiration, and excre- 

 tion carried on. 



Reproduction. — Reproduction in mussels is quite a remark- 

 able process because of the peculiar habits of the young. The 

 adults are either male or female, and the ovaries of the female 

 and testes of the male are situated in the foot (Fig. 84, gg). 

 The male fertilizing elements, the spermatozoa, arise in the testes, 

 pass out through the genital opening (ep), and are carried from 

 the animal's body in the current of water flowing out of the dor- 

 sal siphon. If a female mussel is near, the water containing sper- 

 matozoa is drawn into her mantle cavity through the ventral 

 siphon, and the eggs which have dropped from the female geni- 

 tal opening into the gills become fertilized. The developing 

 eggs remain in the gills for a long time, finally changing into a 

 young stage known as a glochidium. 



