18 ELEMENTS OF COMPARATIVE ZOOLOGY. 



body-cavity is the two-lobed liver, and between its lobes 

 find the u-shaped stomach. The intestine begins at the 

 end of the u. Cut off a bit of its wall with the scissors 

 and see the spiral valve inside. What function can you 

 suggest for it ? Is there a mesentery ? 



Remove the alimentary canal, and on the roof of the ab- 

 dominal cavity see two long ridges on either side of the 

 mesentery. The outer ones are the kidneys, the inner pair, 

 much shorter, the reproductive organs. 



Cut off the skin between the pectoral fins and clean the 

 muscles from the support of the fins (pectoral girdle) 

 which crosses the median line. Is this composed of bone ? 



Cut through the pectoral girdle and lay open the peri- 

 cardial cavity. Is the heart like that of the bony fish ? In 

 front of the ventricle is an arterial cone. How does this 

 differ from the arterial bulb (p. 13) ? Trace the ventral 

 aorta into the afferent branchial arteries by carefully pick- 

 ing away the muscles. 



Now cut back from either angle of the mouth along the 

 lower margin of the gill-slits, and turn back the lower jaw 

 as a flap. Now the gills can be studied, and in the cut 

 arches the gill-cartilages can be seen. How do the gills 

 differ from those of the bony fish ? 



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 to their 

 union into the dorsal aorta. 



Cut off the tail, and in the cut surface make out 

 the following points : In place of bony structures in the 

 position of the centrum (p. 14) a gelatinous notochord 

 forming the axis of the column, surrounded by a tough 

 notochordal sheath. Above and below neural and haemal 

 arches surrounding spinal cord and blood-vessels. On the 



