THE OYSTER 83 



Oysters feed on minute organisms contained in the water. 

 These are caught in the sUme 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. Carefully 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 embedded 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 posi- 

 tion 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 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 re- 

 productive 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 



