214 THE DOG-FISH 



In the male the testes are a pair of elongated 

 solid bodies of a whitish colour, lying along the 

 dorsal surface of the anterior two- thirds of the body- 

 cavity. 



6. The internal openings of the abdominal pores are a pair 



of funnel-like pouches at the sides of the cloaca. 



Pass a seeker along one of these from the abdominal cavity 

 to the exterior. 



7. The peritoneum is the smooth membrane lining the 



body-cavity, and reflected downwards from the mid- 

 dorsal line as a double sheet the mesentery 

 which covers the alimentary canal and other viscera. 

 The mesentery is well developed at the hinder 

 end of the body-cavity, but is interrupted by one or 

 more large fenestrse opposite the greater part of the 

 length of the intestine. 



8. The kidneys are covered by the peritoneum, but can be 



seen as a pair of slightly elevated longitudinal ridges, 

 one on each side of the mid-dorsal line of the body- 

 cavity, and extending nearly its whole length. 



IV. DISSECTION OF THE DIGESTIVE SYSTEM. 



In dissecting the alimentary canal, be careful not to injure 

 the blood-vessels in relation with it ; these lie for the most part 

 in the mesentery. 



A. The Alimentary Canal. 



1. The mouth and pharynx. The detailed examination of 



these parts is best deferred until the circulatory 

 system is dissected. 



2. The oesophagus is a short wide tube, which enters the 



abdominal cavity at its anterior end and expands 

 almost at once into the -stomach. 



Pass the handle of a scalpel or seeker through the mouth, 

 and down the oesophagus into the stomach. 



