DOGFISH. 19 



and floor of the throat as a flap. This will lay open the 

 cavity of the mouth and the pharynx. 



See that the gill-clefts are slits in the wall of the tube, 

 each slit bearing delicate filaments on its walls, while a 

 bar of cartilage (branchial arch} lies in each partition 

 between two successive gill-clefts. See also the internal 

 opening of the spiracle. Is there a bar of cartilage (hyoid 

 arch) between it and the first gill-cleft? Could the spiracle 

 be regarded as a modified gill-cleft? Draw these parts X4. 



Slit the skin on the roof of the mouth and carefully 

 remove it with the forceps. This will expose the efferent 

 branchial arteries, which can readily be traced from the 

 septa between the gills to their union to form the dorsal 

 aorta, which runs backward above the alimentary canal. 

 Taking the two aortse and the branchial arteries into 

 consideration, could you consider the circulatory system 

 as consisting of two tubes, one on either side of the alimen- 

 tary canal, connected by pairs of semi-circular vessels? 

 Draw a diagram illustrating the relations of the blood- 

 vessels to the alimentary canal and the gill-slits. From 

 the anterior efferent branchial trace the common carotid 

 arteries of either side forward and toward the middle 

 line to their union where the internal carotid is given off, 

 noting the external carotid about half way between the 

 branchials and the internal carotid. Add these arteries 

 to the sketch of the roof of the mouth and gill region. 



Cut off the tail with a sharp scalpel just in front of the 

 posterior dorsal fin and in the cut surface make out the 

 following points: In the centre a gelatinous rod, the 

 notochord, with a tough notochordal sheath around it. 

 Dorsal to the notochord is the spinal cord (nervous), while 

 below it are two blood-vessels, the caudal artery and rein. 

 Extending from the notochordal sheath, around these 



