THE DOGFISH 7 



vessel with a small central cavity. Note the muscle ridges on 

 its inner surface. In its dorsal wall is a large opening by which 

 the auricle communicates with it. Blow into this auriculo- 

 ventricular opening with a blowpipe and note the lips of the valve 

 which guard it. 



Attached to the wall of the conus arteriosus are small pocket- 

 like valves, the semilunar valves, arranged in two or three rows, 

 which permit the blood to flow only away from the heart, the 

 anterior row of valves being much larger than the others. In the 

 smooth dogfish three rows are present ; in the spiny dogfish there 

 is some variability, although two rows are usually present. Bring 

 them into view by means of the blowpipe ; note the direction in 

 which they act. 



Fill the auricle with air by blowing into it through the auriculo- 

 ventricular opening. Open the auricle by a lateral incision and 

 wash it out. Note its thin walls and its large cavity. With the 

 blowpipe find the opening into the sinus venosus. 



Exercise 6. Draw a view of the opened ventricle and conus arteriosus, 

 showing the muscle ridges, the valves, and the openings. 



The Pharynx and the Mouth. Insert one blade of the scissors 

 in the corner of the mouth on the left side, and carry a cut straight, 

 back across the gills as far as the pectoral girdle. From the end of 

 this incision carry another across the floor of the pharynx, just 

 back of the heart, to the opposite side of the body. Turn the flap 

 thus formed, which is the entire ventral side of the head, over to 

 the animal's right and pin it there, exposing the cavity of the 

 mouth and pharynx. 



These two cavities will be seen to form a single large space, 

 bounded in front by the opening of the mouth and behind by 

 the beginning of the gullet or oesophagus. The whole is lined 

 by a slightly folded mucous membrane. On each side the body 

 wall is pierced by six large clefts, of which the five posterior 

 are the gill slits and the anterior one is the spiracle. Probe these 

 clefts. Examine the gills in the gill slits, and the cartilaginous 

 gill arches which support them. Note that the hinder wall of the 

 last gill slit is without a gill. The spiracle is homologous to a gill 



