CIRCULATORY SYSTEM. 73 



5. The gizzard extends from the sixteenth segment back- 

 wards to about the twentieth. Its walls are very 

 thick and muscular, and it has a thick chitinous 

 lining. 



6 The intestine is a thin-walled sacculated tube running 

 straight backwards from the gizzard to the anus. 



a. The * yellow ' cells are a layer of peritoneal cells 

 surrounding the intestine and in close relation 

 with the blood-vessels. 



Cut the intestine open along one side, and wash out its 

 mtents. 



b. The typhlosole is a prominent median longitudinal 



ridge projecting into the cavity of the intestine 

 from its dorsal wall, of which it is a fold. Its 

 surface is marked by transverse folds. 



C. The Circulatory System. 



. The blood is a fluid coloured bright red by haemoglobin 

 and containing colourless corpuscles. Several of the larger 

 vessels contract rhythmically, especially the five pairs of 

 ' commissural ' vessels. The circulation is seen best in a worm 

 soon after hatching out. 



For the dissection of the circulatory system a fresh worm 

 should be taken and killed with chloroform. Owing to the 

 1 yellow cells ' overlying the alimentary canal, the dorsal vessel 

 is not easily traced. 



1. The dorsal vessel is a large median vessel running along 

 the dorsal surface of the alimentary canal through- 

 out its whole length, and dividing in front into 

 branches which ramify over the pharynx. It can be 

 seen through the skin in the living animal, and the 

 flow of blood in it is from behind forwards. 



A large lateral oesophageal vessel runs along 

 each side of the oesophagus, and communicates with 

 the dorsal vessel in the tenth segment. Branches of 

 it ramify over the pharynx and over the oesophageal 

 glands and pouches. 



