chap. v.i HEART OF ARTHROPODA. 189 



supply the front part of the head, and the hepatic, 

 which go to the chief viscera. The posterior artery 

 rims backward along the dorsal surface of the tail, and 

 gives off on its course a downwardly-directed vessel 

 (sternal artery), which on reaching the ventral region 

 divides into an anterior and a posterior abdominal 

 artery. As these several vessels ramify, they break 

 up into smaller vessels, and then finally open into 

 spaces among the various organs of the body. The 

 largest and most important of these is the great 

 sternal sinus, which lies in the region of the 

 entrance to the gills, into the spaces in which the 

 blood passes to receive a fresh supply of oxygen (see 

 page 224) ; thence the blood returns by the branchio- 

 cardiac veins or canals to the pericardial sinus, to 

 again pass into the heart, and resume its journey 

 through the body. 



It will be observed that the blood goes to the 

 organs of the body from the heart before it goes to 

 the gills. Such a heart is known as a systemic 

 heart, in contradistinction to the branchial heart of 

 fishes, for example, in which the blood pumped from 

 the heart goes firstly to the gills, and secondly to the 

 other organs of the body. 



The vascular system of Peripatus is described 

 by Balfour as consisting of a dorsal vessel shut off 

 from the body cavity by a continuation of the endo- 

 thelial lining of the latter. It has definite walls, 

 but it is not clear whether they are muscular. It ex- 

 tends from near the hinder end of the body to the 

 head, and is largest behind. Between the skin and 

 the outer layer of muscles there is a very delicate 

 ventral vessel. 



In the Myriopoda the heart extends through 

 the whole of the body, and is made up of a number of 

 chambers separated from one another by valves pro- 

 vided with orifices for the entrance of the venous 



