222 THE DOG-FISH 



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

 vascular folds of mucous membrane, radiating out- 

 wards from the cartilaginous branchial arches, and 

 supported by the gill-rays. 



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 have cartilaginous 

 processes, which hinder the passage of food to the 

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



2. The pseudobranchise. 



Slit up the spiracle on the left side in the same way as the 

 ■gill-clefts. 



The pseudobranch is a rudimentary gill, consist- 

 ing of about half a dozen small parallel folds, on the 

 anterior wall of the spiracle. Its presence shows 

 that the spiracle is really a gill-cleft. 



VI. DISSECTION OF THE CIECULATOEY SYSTEM. 

 A. The Pericardial Cavity and Heart. 

 1. The pericardial cavity. 



Lay the dog-fish on its back on the dissecting board, and 

 vin it down throiigh the fins. Open the pericardial cavity by 

 a mid-ventral incision in front of the pectoral girdle, taking 

 ■care not to injure the heart. Gut away the ventral wall of the 

 ■cavity so as to expose it fully. Cut away also the middle 

 half-inch of the pectoral girdle, taking care not to injure the 

 sinus venosus, which lies along its dorsal border. 



The pericardial cavity is median, and lies between 

 the gills; it is triangular in shape, with the apex 

 directed forwards, and is almost completely iilled by 

 the heart, of which the globular ventricle is the most 

 prominent portion. Its ventral wall is stiffened by 

 the anterior prolongation of the pectoral girdle, and 

 its dorsal wall by the wide basi-branchial plate and 

 the fifth cerato-branchials. 



