88 MANUAL OF ANIMAL BIOLOGY 



to all regions of the body. Such a system forms a necessary part 

 of the structure of all the higher forms of animals. It consists 

 of (a) a circulating medium, the blood, which is adapted for trans- 

 porting the various materials, and composed of a fluid portion, the 

 plasma, containing an enormous number of isolated living cells, 

 the blood cobpuscles. (b) The blood is contained in a system 

 of connected, closed, muscular-walled tubes, the blood vessels. 

 (c) Contractile portions of the blood vessels, heabts, drive the 

 blood through the vessels by means of rhythmical muscular con- 

 tractions. There is also present in such types of circulatory sys- 

 tems, (d) numerous blood spaces permeating the tissues, by means 

 of which the fluid portion of the blood can come into direct 

 contact with the living cells of the tissues. The vascular system 

 exhibits a high degree of development and complexity, as will be 

 evident from the following description. 



(1) Longitudinal Vessels. There are five blood vessels 

 which run the length of the body. The largest of these is (a) the 

 dobsal vessel which lies on the dorsal side of the alimentary 

 canal. Ventral to the alimentary canal in the same relative 

 position is (6) the slightly smaller ventbal vessel. These two 

 vessels are directly connected by five pairs of heavy walled con- 

 tractile heart-like vessels (c) the aobtic loops, which lie in the 

 anterior end of the body in segments viii to xii. The other three 

 longitudinal vessels are considerably smaller and lie near the 

 ventral body wall in close association with the nerve cord. These 

 are (d) the important subneubal vessel, lying ventral to the 

 nerve cord, and (e) the two smaller latebal neubals, situated to 

 the right and left of the nerve cord. 



The Dorsal Vessel is the chief collecting vessel and, posterior to 

 segment xii, receives blood in every segment from the various 

 segmental vessels noted below. Anterior to segment xiii it is a 

 distributing vessel and gives off the collected blood to the aortic 

 loops and to the tissues of the pharynx where it ends. 



The Ventral Vessel is the chief distributing unit of the vascular 

 system and throughout its length gives off the blood received from 

 the aortic loops to the connecting segmental vessels. 



The Subneural Vessel receives blood at the anterior end of the 

 body from small branches and distributes it throughout the length 

 of the body by means of segmental vessels which connect with the 

 dorsal vessel, as noted below. 



