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. 
Bxercise 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 papille 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. 
