PHYLUM ANNELIDA 221 



special system of organs must be provided to enable the proper 

 distribution of nutriment. 



Circulation. — The blood of the earthworm is contained in 

 a complicated system of tubes which ramify to all parts of the 

 body. A number of these tubes are large and centrally located; 

 these give off branches which likewise branch, finally ending in 

 exceedingly thin tubules, the capillaries. The functions of this 

 system of tubes are to carry nourishment from the alimentary 

 canal to all parts of the body, to transport waste products, and 

 to convey the blood to a point near the surface of the body where 

 oxygen may be obtained and supplied to the tissues. 



The blood of the earthworm consists of a plasma in which are 

 suspended a great number of colorless cells, called corpuscles. 

 Its red color is due to a pigment termed haemoglobin which is 

 dissolved in the plasma. In vertebrates the haemoglobin is 

 located in the blood corpuscles. 



There are five longitudinal blood-vessels connected with one 

 another and with various organs by branches, more or less regu- 

 larly arranged. These are shown in Figure 155, and are as 

 follows: (1) the dorsal or supra-intestinal vessel (sp), (2) the 

 ventral or subintestinal trunk (sb.), (3) the subneural trunk (sn), 

 (4) two lateral neural trunks (nl), (5) five pairs of hearts (Jit) / 

 in segments VII to XI, (6) two intestino-tegumentary ves- 

 sels (it in A and B) arising in segment X and extending to the 

 oesophagus, integument, and nephridia in segments X to VI, 

 (7) branches from the ventral trunk to the nephridia and body- 

 wall (D), (8) parietal vessels connecting the dorsal and sub- 

 neural trunks in the intestinal region, (9) branches from the 

 dorsal trunk to the intestine, (efi. in C), (10) a typhlosolar ves- 

 sel connected by branches with the intestine and dorsal trunk, 

 and (n) branches from the ventral vessel to the nephridia and 

 body-wall (sb. in D). 



The dorsal trunk and hearts determine the direction of the 

 blood flow, since they furnish the power by means of their 

 muscular walls. Blood is forced forward by wave-like contrac- 



