RESPIRATORY SYSTEM 217 



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 body-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 the 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. 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 Vy prolonging the external 

 apertures upwards and downwards with scissors. 



The gills are a series of closely set, soft, and 

 highly vascular folds of mucous membrane, radiating 

 outwards from the cartilaginous branchial arches. 



Gills are borne on the posterior surface of the 

 hyoid arch, and on both the anterior and posterior 

 surfaces of the four following arches. The inner 

 borders of the branchial arches bear cartilaginous 

 processes, which hinder the admission of food to the 

 gill-clefts. The last branchial arch bears no gills. 



