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 ; i.e., 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 lip. 



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. • 



Exercise 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 oesophagus, the 

 stomach and the dark-colored liver which surrounds it, and the 

 long intestine. The mouth opens directly into the oesophagus, 

 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 

 j stomach is seen. The intestine extends straight back from the 

 I stomach to a position ventral to the adductor muscle and between 

 jit and the gills. It then turns on itself and passes straight 

 [forward to the dorsal side of the stomach, around the forward 



