Phylum MoUusca and Minor Eucoelomate Phyla 473 



the organs. In the anterior ventral region is a large muscular joot 

 which is capable of being thrust out between the valves and is used in 

 locomotion. On either side of these two structures are a pair of gills, 

 the respiratory apparatus of the clam. Outside of these gills is the 

 mantle, a sheetlike structure which secretes the shell. The mantle is 

 attached to the shell but has free margins which are capable of being 

 drawn together to produce the mantle cavity. The posterior margin of 

 the mantle forms two openings or siphons: the ventral siphon which 

 acts as the incurrent passageway permitting the water with its con- 

 tained food and oxygen to pass in ; and the dorsal siphon, the excurrent 

 passageway, which permits the escape of water. 



*NTCBIOO AOATA 



OtOCSTivt GLAND. 

 'NTESTWt 



Fig. 153. — Anatomy of the fresh-water clam, Anodonta. 



The digestive tract begins at the anterior portion in the mouth 

 which is located between the two labial palps. There is no radula. 

 Posterior to the mouth is a short esophagus and a rounded stomach, 

 located in the visceral mass. From the stomach, the long coiled in- 

 testine passes posteriorly. Its main coil is just dorsal to the foot. 

 Posteriorly it becomes the rectum which is surrounded by the heart. 

 The anus opening is in the region of the dorsal siphon. Surrounding 

 the stomach is a large digestive gland, and within the intestine is the 

 crystalline style which produces digestive enzymes. In the rectum 

 there is a typhlosole. 



The heart consists of two auricles and a ventricle within the peri- 

 cardial cavity. Leading from the ventricle is an anterior aorta which 

 carries blood to the foot and visceral mass, and a posterior aorta which 



