GENERAL ZOOLOGY 



Anterior 

 mantle artery 



Mantle 



(oxygen uptake; 



loss of CO2 ) 



Mantle vein 



Anterior 



Stomach, intestine, 



and digestive gland 



(absorption) 



Gills 



(oxygen uptake; 

 loss of CO 2) 



Nephridium 



(removal of 



nitrogenous wastes) 



Fig. 13.3. Schematic dias;ram of the course of circulation in a pelec\pod. In biiateraiiv 

 paired organs, only one memt^er of the pair is shown. .Arterial vessels are clear, venous 

 channels stippled. 



Mollusks are clearly coelomate animals, but in the adult pelecypod the 

 coelom is very much reduced. It is represented chiefly by the cavity in which 

 the heart lies, the so-called pericardial cavity. This chamber is also related 

 to the excretory organs, which are paired tubes or nephrldia leading from the 

 anterior end of the coelom to external openings in the suprabranchial cham- 

 bers of the inner gills. Each nephridium is folded upon itself and is differen- 

 tiated into glandular and bladder-like portions. The structure and relation- 

 ships are such that excreta in solution within the coelomic, or pericardial, 

 cavity may enter the tubule; or excreta carried by the circulating blood may 

 be extracted by the cells in the glandular portion of the nephridium. The 

 details of the process of excretion are not fully known. 



The nervous system consists of a pair of cerebropleural ganglia, one on each 

 side of the mouth; a pair of pedal ganglia in the foot; and a pair of visceral 

 ganglia lying on the ventral surface of the posterior adductor muscle. Com- 

 missures unite the ganglia of each pair, and cerebropedal and cerebrovisceral 

 connectives course between the brain and the other chief ganglion pairs. 

 Nerves extend from the ganglia to the sensory surfaces of the body and 

 to the muscles. The chief sense organs in clams and mussels include tactile 

 organs, such as the papillae of the siphons, which are also sensitive to light; 

 the osphradia, two areas of sensory epithelium near the visceral ganglia. which 

 function as chemoreceptors; and a pair of statocysts, organs sensitive to 

 changes in position, located in the foot near the pedal ganglia. Among the 



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