X] LUMBRICUS 145 



the internal cells of the body, (6) it conveys excreta to the excretory 

 organ, and (c) it conveys to the internal cells of the body the 

 oxygen necessary for their respiration. 



The most important blood-vessels of the earthworm are as follows : 

 (1) a dorsal blood-vessel visible through the skin as a dark streak 

 which runs along the body of the worm from head to tail in the middle 

 line (10, Fig. 61); (2) a parallel sub-intestinal vessel which 

 underlies the intestine, and (3) a third but smaller vessel, the 

 sub- neural, which lies still more ventrally under the nerve-cord. 

 There are also two latero-neural vessels lying one at each side of 

 the sub-neural vessel, but as they are connected with it at frequent 

 intervals the three together may be regarded as practically one 

 vessel. The dorsal vessel receives blood from the yellow cells 

 covering the intestine by two pairs of minute vessels in each 

 segment, and anteriorly it breaks up into a network of small vessels 

 which branch over the pharynx. Two vessels in each segment 

 connect the ventral vessel with the network of fine blood-vessels 

 which covers the surface of the alimentary canal, and the ventral 

 vessel and the dorsal vessel are connected by means of five pairs 

 of loops, called hearts, situated in the seventh, eighth, ninth, tenth, 

 and eleventh segments (10, Fig. 60). The dorsal vessel and the sub- 

 neural vessel are put into communication in each segment by two 

 parietal vessels which lie on the outer wall of the coelom and 

 which receive numerous small vessels from its substance. Each 

 nephridium is connected by one vessel with the ventral vessel and 

 by another with the parietal vessel. 



The earthworm breathes through its skin. The parietal vessel 

 sends up into the skin innumerable minute vessels or capillaries 

 which come so near the outer surface of the worm that the oxygen 

 can pass in from the air into the blood. The name capillary (Lat. 

 capillus, a hair) was suggested by a comparison of the exceedingly 

 small calibre of these vessels with the diameter of a human hair. 



The blood is red, and the red colour is due to ( the same substance 

 which colours our blood, haemoglobin, but there is this difference, 

 that whereas in Vertebrates the haemoglobin is contained in certain 

 cells which float in an almost colourless fluid, in the earthworm it 

 is dissolved in the fluid itself. This substance has a strong attrac- 

 tion for oxygen, which it takes up from the air that comes into 

 the neighbourhood of the skin-capillaries, forming a bright red 

 compound called oxy-haemoglobin. This compound is unstable, 

 and when the blood in its course round the body encounters a cell 



S. & M. 10 



