CIRCULATORY SYSTEM. 237 



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 each of the four following arches. The 

 inner borders of the branchial arches have cartilagi- 

 nous 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, 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 OP THE CIECULATOEY SYSTEM. 



A. The Pericardial Cavity and Heart. 



1. The pericardial cavity. 



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

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

 removing the middle portion of the pectoral girdle, taking 

 cofre not to injure the heart, which lies in close contact with 

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

 fully. 



The pericardial cavity is median, and lies between 

 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 girdle, 



