CIRCULATORY SYSTEM. 245 



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. 



Grills are borne on the posterior surface of the 

 hyoid arch, and on both the anterior and posterior 

 surfaces of each of the four following arches. The 

 inner borders of the branchial arches have carti- 

 laginous processes, which hinder the passage of 

 food to the gill-clefts. The last branchial arch 

 bears no gill. 



2. The pseudobranchs. 



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

 gill-clefts. 



The pseudobranch is a rudimentary gill, con- 

 sisting 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 CIRCULATORY SYSTEM. 



A. The Pericardial Cavity and Heart. 



1. The pericardial cavity. 



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

 pin it down through the fins. Open the pericardial cavity by 

 removing the middle portion of the pectoral girdle, taking care 

 not to injure the heart, which lies in close contact with it. Cut 

 away the ventral wall of the cavity so as to expose it fully. 



The pericardial cavity is median, and lies be- 

 tween the gills : it is triangular in shape, with the 

 apex directed forwards, and is almost completely 

 filled 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 



