SOFT-BODIED ANIMALS— THE MOLLUSKS 



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ach and absorption through the intestine. Egestion is through the anus 

 which opens just dorsal to the excurrent siphon, so that the waste is car- 

 ried out without mixing with the water in the mantle cavity. 



Excretion is taken care of by a pair of nephridia which lie in the 

 dorsal portion of the visceral hump and empty into the water that is on 

 its way out of the dorsal siphon. 



Umbo 



Visceral hump 



Muscle 



Muscle 



Excurrent siphon 



Anus 



In current 

 siphon 



Gills 



Mantle 



Fig. 19.4. A fresh-water clam with the shell and mantle removed from the left side 

 showing the position of the body organs within the shell. 



The circulatory system includes a single heart and connecting arteries 

 and veins which transport the blood over the body. Some of the ar- 

 teries, instead of forming small capillaries which connect with the veins, 

 empty into blood sinuses, which are cavities that connect directly with 

 the veins. The blood has no hemoglobin but does have white corpuscles. 



The nervous system consists of three paired ganglia with connecting 

 nerves, smaller nerve branches, and rather poorly developed sense or- 

 gans. There is a pair of ganglia above the mouth, the cerebropleural 

 ganglia, that are the equivalent of the brain, with paired nerves running 

 posteriorly to the visceral ganglia, and another pair of nerves running 

 ventrally to the pedal ganglia in the foot. 



Reproduction in the clam is quite interesting. The sexes are separate 

 and, during the reproductive season, the male releases the sperms through 

 the excurrent siphon, some of which will be taken into the incurrent 

 siphon of the female. In the meantime the female has released her eggs 

 which are held in the gills where they are fertilized by the sperm passing 

 through. The resulting zygotes are held here until small larvae, called 

 glochidia, are produced. These glochidia are then expelled from the 



