66 ■ THE EAKTHWORM. 



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 haBmoglobiQ 

 and containing colourless corpuscles. Several of the larger 

 vessels contract rhythmically, and especially the five pairs of 



' hearts.' 



For the dissection of the circulatory system a fresh worm 

 should he taken. Owing to the dark red colour of the con- 

 tained blood the vessels can be easily traced. The animal 

 should be hilled luith chloroform. 



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



the dorsal surface of the alimentary canal throughout 

 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 oesophag'eal 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. 



Prom the alimentary canal the blood is returned 

 to the dorsal vessel by intestinal vessels, of which 

 there are in the region of the intestine two pairs to 

 each segment, and which are surrounded by the 

 ' hepatic cells..' 



2. The ventral vessel is a median vessel running backwards 



below the alimentary canal the whole length of the 

 animal : in it the blood flows backwards. 



3. The ' hearts ' are five pairs of large vessels lying in seg- 



ments VII. to XI., and connecting the dorsal with the 

 ventral vessel. During life they are rhythmically 

 contractile, the contractions passing along them from 

 above downwards, commencing with the hindmost 



