244 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. 241). 



The pancreatic duct runs forwards through the 

 pancreas close to its ventral border. It leaves 

 Vthe gland at the posterior angle of its ventral 

 lobe, and at once enters the ventral wall of the 

 intestine 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 the ventral border of the pancreas : 

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

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

 small papilla. 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 down the gland and duct into the rectum ; and slit open 

 the rectum to see the opening of the duct. 



V. DISSECTION OF THE RESPIRATORY SYSTEM. 



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

 of the mucous membrane of the pharynx, supported by the 

 branchial arches. Respiration is effected by rhythmical 

 compression and expansion of the muscular pharynx causing 

 water to enter in at the mouth and spiracle, to pass between the 

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



1. The gills. 



Expose thoroughly the gills on the left side. 



