AN OYSTER 101 
muscle observe the pericardium. Carefully cut it away and see 
the heart, which lies in the pericardial cavity ; it will be beating 
if the animal is still alive. The ventricle is dorsal in position and 
the auricle is ventral, lying next to the gills, from which it re- 
ceives the purified blood. The four gills lie close together, no 
foot being present to separate the two right-hand from the two 
left-hand gills. Just in front of the gills, at the front end of the 
body, are the two pairs of large oral palps. The mouth is between 
these palps, two being on each side of it. Find the mouth and 
note that it lies between an upper and an under lip, each of which 
is formed by the union of a pair of palps; ze., a palp on the 
right side joins one on the left and forms the upper lip, and the 
other two palps join to form the under lp. 
Oysters feed on minute organisms contained in the water. 
These are caught in the slime which exudes from the surface of 
the gills and moved forward by the action of the cilia of the 
gills and the palps to the mouth. 
The anus and the rectum will be seen on the dorsal side of the 
adductor muscle. 
Bxercise 3. Make a drawing of the oyster as it lies in the left 
shell, representing all the organs above mentioned. Care- 
fully label all. 
The digestive tract. This consists of the short csophagus, the 
stomach and the dark-colored liver which surrounds it, and the 
long intestine. The mouth opens directly into the esophagus, 
which leads to the stomach. The position of this organ can 
easily be determined, because it is imbedded in the dark-brown 
liver. Carefully scrape or cut away the side of the visceral 
mass and expose the liver; continue the process until the 
stomach is seen. The intestine extends straight back from the 
stomach to a position ventral to the adductor muscle and between 
it and the gills. It then turns on itself and passes straight 
forward to the dorsal side of the stomach, around the forward 
