PHYLUM MOLLUSCA 



285 



Digestion 



Food is brought into the mantle cavity 

 (Fig. 174) of the clam by the water circu- 

 lating inward through the ventral incurrcnt 

 siphon. Water containing suspended minute 

 plants, animals, and debris passes over the 

 gills, and the small suspended food particles 

 adhere to the mucus which covers the gills. 

 The smaller particles now caught in the 

 mucus are carried by the beating cilia of the 

 gills to the ventral edge, where this mass of 

 material is transferred to the labial palps. 

 These flaps of tissue which surround the 

 mouth serve as a sorting mechanism that 

 selects the materials to be utilized. These 

 food materials are then carried into a deep 

 groove between the labial palps and thence 



directly into the mouth. The food passes 

 from the mouth through the short esopha- 

 gus into the bulbous stomach, which is con- 

 nected by ducts with a large digestive gland 

 ("liver"). This gland surrounds the stomach 

 and is the chief source of digestive enzymes. 

 The intestine is given off from the ventral 

 side of the stomach, descending into the 

 visceral mass, where it makes a loop. Then 

 it ascends parallel to its first portion and 

 turns sharply backward and out of the 

 visceral mass, through the pericardium and 

 through the heart itself, where it becomes 

 the rectum. It finally ends at the anus which 

 opens near the excurrent siphon, and the 

 feces are carried away in the outgoing cur- 

 rent of water. 



Renopericardial pore 



Pericardial cavity 

 Anterior aorta 

 Digestive gland 



Anterior adductor 

 muscle 



Kidney pore 

 Stomach 



Mantle 

 Mouth 



Ventricle 



Posterior aorta 



Posterior retractor muscle 



Posterior adductor 



muscle 



Excurrent 



siphon 



Anus 



Cerebropleural ganglion 

 Pedal ganglion 

 Cerebrovisceral connective 



Visceral ganglion 

 Intestine Incurrent siphon 



Nephridium 



Figure 174. The internal anatomy of a freshwater dam in the right valve as viewed from 

 the left side. 



The details of digestion in fresh-water 

 clams are not very well known. However, as 

 is true in most animals that eat fine particles 

 of food, digestion appears to be partly 

 intracellular. 



Amoeboid cells are present throughout 

 the digestive tract. From microscopic ex- 

 aminations, it is believed that such cells pass 

 through the wall of the tract, engulf food 

 particles, and digest it; then leave the in- 



