128 THE BIOLOGY OF AN ANIMAL. 



changes of form. In fact they closely resemble certain kinds 

 of Amaibce, and we should certainly consider them to be such if 

 we found them occurring free in stagnant water. We know, 

 however, that they live only in the plasma, and for this and 

 other reasons we must regard them not as individual animals, 

 but as constituent cells of the earthworm. The coelomic fluid is 

 in fact a kind of tissue consisting of isolated colorless cells float- 

 ing in a fluid intercellular substance. 



2. Pseudhaemal. Besides the coelomic circulation there is 

 another and more complicated circulatory apparatus consist- 

 ing of tubes branching throughout the body. Through these 

 tubes is driven a red fluid formerly supposed to correspond to 

 the red Wood of higher animals. It is now known, however, 

 that this fluid is only in part comparable to true blood ; hence it 

 is called pseudJumial fluid (i.e., false Hood), and the system of 

 tubes is called the pseudhcemal system. Like the coslomic fluid, 

 the pseudhsemal fluid contains corpuscles ; but these are extremely 

 difficult to observe, since their index of refraction is very nearly 

 the same as that of the fluid.* 



The pseudhsemal tubes (which for the sake of brevity may be 

 called blood-vessels) ramify throughout the whole body, permeating 

 nearly all the tissues and organs, and forming an exceedingly 

 complicated system. Amidst all this complication we can, how- 

 ever, readily distinguish certain general features which are of 

 the greatest importance when compared w T ith the blood-vessels of 

 higher animals. These are as follows : 



a. The dorsal vessel (Fig. 75, d.v\ a long muscular tube 

 lying along the upper side of the alimentary canal. 

 In the living worm it may be distinctly seen through 

 the semi-transparent skin as a dark-red band, which 

 is tolerably straight when the worm is extended, but 

 is made zigzag during contraction. If it be closely 

 observed, a sort of wave-like contraction is often 

 seen running from behind forwards. This may be 

 very clearly observed in a worm stupefied by chloro- 

 form, especially if it has been laid open along the 

 dorsal side. The dorsal vessel then appears as a 



* We shall see hereafter that the red blood of higher animals corresponds 

 to the cvdomic and pscudhcemal fluids taken together. 



