THE FISH 



215 



vessels give off branches, and while the exact course of 

 the blood flow from the ventral vessel is at present un- 

 known, it is supposed that in some way it is ultimately 

 brought back into the dorsal vessel and thus completes 

 its circuit. The branches of the two main vessels sub- 

 divide into capillaries, and by means of these the blood 

 is brought into contact with the various tissues of the 

 body, and thus food is distributed and wastes collected. 



The red color of the blood is due to the hsemoglobin 

 dissolved in the fluid, the only corpuscles 

 present being a few white ones. ^., 



Besides this inclosed fluid, we find the 

 body cavity filled with a similar liquid to 

 that found in the grasshopper, and it is 

 known that part of the absorbed food 

 enters the inclosed system and part is 

 distributed by the cavity fluid, so that we 

 actually have two methods of circulation in 

 this one animal. 



The fish. — It is only in the vertebrate 

 animals that we find a true blood with red 

 corpuscles. The simplest vertebrate system 

 is found in the fish. In this animal the 

 heart is a simple structure consisting of a 

 single ventricle and auricle, and is located 

 in a pericardial cavity separated from the 

 rest of the body organs by a thin mem- 

 brane. The blood is driven from the 

 ventricle through an artery (the aorta) 

 to the gills. Here the artery branches into capillaries 

 and through the thin walls of these is effected the inter- 

 change of oxygen and carbon dioxide between the water 



Fig. 79— Plan of 

 circulation in .i 

 fish ; a, auricle ; 

 6, ventricle; c, 

 branchial arte- 

 ry; e, branchial 

 veins ; d, gills ; 

 /, aorta; (/, vena 

 cava. 



