236 THE DOG-FISH. 



blue injection: follow it forwards to the liver and gall-bladder, 

 and backwards to its opening into the intestine. 



2. The pancreas has been described above (p. 233). 



a. The pancreatic duct runs forwards through the pan- 

 creas close to its ventral border. It leaves the 

 gland at the posterior angle of its ventral lobe, 

 and at once enters the ventral wall of the intes- 

 tine about an inch beyond the pylorus : it runs 

 in the wall of the intestine for about half an 

 inch, and opens just to the outer side of the line 

 of attachment of the spiral valve. 



Find the duct along tlie ventral border of the pancreas : 

 trace it to the intestine : slit open the intestine, and trace the 

 duct backivards in its wall as a white ridge ending in a very 

 small jpapilla. It is easier to follow after injection with a 

 coloured fluid. 



3. The rectal gland is a thick-walled tube, about three- 



quarters of an inch long, lying in the abdominal 



cavity dorsal to the rectum. From its hinder end a 



duct runs forwards along the edge of the mesentery 



to open into the dorsal wall of the rectum. 



Cut the gland across about the middle of its length. Note 



the thickness of its walls and the small size of its cavity. 



Pass a bristle doivn the gland and duct into the rectum ; and 



slit open the rectum to see the opening of the duct. 



V. DISSECTION OF THE RESPIRATOBY SYSTEM. 



The dog-fish breathes by gills, which are vascular folds 

 of the mucous membrane of the -pharynx, supported by the 

 branchial arches. Eespiration is effected by streams of water, 

 which, entering at the mouth, pass between the branchial 

 arches, over the gills, and out through the gill-clefts. 



1. The gills. 



Expose the gills on the left side by prolonging the external 

 apertures upwards and downwards with scissors. 



