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MANUAL OF ELEMENTARY ZOOLOGY 



passes backward below it. From branches of these the 

 blood passes into spaces between the organs. From the 

 foot and viscera it is gathered into a vena cava which lies 

 below the pericardium between the kidneys. Thence it 

 passes outwards through the kidneys to the gills, where it 

 circulates in irregular spaces in the inner parts of the 

 filaments. From these it is returned to the auricles. The 

 blood from the mantle returns direct to the auricles. 



hyssus 



shell teeth. 



steeple -shaped, 

 sensory cells 



Fig. 230.— A glochidium larva, as cast out from the parent, viewed 

 from behind. — From Latter. 



The nervous system comprises three pairs of ganglia 

 with commissures uniting them. The cerebral 



system! ganglia are two small, orange-coloured bodies, 



placed one on each side behind the mouth, 



above which they are connected by a cerebral commissure. 



They supply the fore-part of the body, and each gives off 



a cerebropedal commissure to one of the two pedal ganglia 



