102 INVERTEBRATE ZOOLOGY 



and ventral sides of it, and thus back again to the dorsal side of 

 the muscle, where it ends with the anus. Most of it is surrounded 

 by the yellow reproductive gland. Lay bare the intestine. This 

 can be done best after the oyster has been hardened for a few 

 days in a 5 per cent, solution of formalin. 



Exercise 4. Make a drawing of the digestive tract in an outline 

 of the animal's body. 



The remaining systems of organs of the visceral mass will not 

 be studied in this dissection. 



The American oyster is a unisexual animal; the common 

 European oyster is hermaphroditic. The reproductive glands, 

 the ovaries or testes, are a pair of yellowish or whitish organs of 

 irregular form which occupy the larger part of the visceral mass 

 and surround the digestive tract and other organs. The kidneys 

 are also a pair of organs of irregular form which, together with 

 a portion of the intestine, occupy the lower and hinder part of 

 the visceral mass, between the muscle and the gills. The nervous 

 system has been much modified by the sessile habit of life of the 

 oyster. The cerebral ganglia are represented by a nerve ring, 

 containing ganglia, which surrounds the mouth ; it is called the 

 circumpallial nerve. Fibers from this ring go to the pigmented 

 sense papillae at the margin of the mantle. The visceral ganglia 

 lie along the antero-ventral side of the muscle and are joined 

 with the cerebral ring by longitudinal connectives. The pedal 

 ganglia are wanting. 



