18 LABORATORY WORK. 



parts of the tract together? Notice blood-vessels passing 

 from the mid-dorsal line of the ccelom (dorsal aorta) to 

 the intestine (anterior mesenteric artery), to the rectal 

 gland (posterior mesenteric artery), and one more anterior 

 (codice axis) to stomach, liver, and anterior part of intestine. 



Draw the alimentary tract as made out, inserting and 

 naming all parts. 



Remove the alimentary canal by cutting away most 

 of the liver, the stomach, and the rectum. On the roof 

 of the coelom are two long ridges either side of the mes- 

 entery. The lateral ones are the ' kidneys/ or Wolffian 

 bodies (mesonephros) , the much shorter medial ones are 

 the reproductive organs (gonads). Draw this system. 



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 cavity 

 in front of it (pericardial cavity). In this lies the heart, 

 consisting of a thick- walled ventricle below, and dorsal 

 to it a thinner-walled auricle. The heart is connected to 

 the posterior wall of the pericardium by a thin-walled 

 sinus venosus. In front of the heart and extending for- 

 ward to the anterior wall of the pericardium is a large 

 blood-vessel, the truncus arteriosus. Sketch the heart, 

 pericardium, etc. (ventral view), and then trace the 

 truncus forward into the flesh in front of the pericardium, 

 cutting carefully. This part is the ventral aorta. Trace 

 from it, right and left, afferent branchial arteries carrying 

 the blood into the partitions between the gill-clefts. 

 Add these vessels and the clefts to the sketch of the heart. 



Insert the scissors at the angle of the jaws and cut back- 

 wards along the lower edge of the gill-slits; repeat the 

 operation on the other side and turn back the lower jaw 



