130 THE BIOLOGY OF AN ANIMAL. 



wall of the oesophagus (Fig. 75, ad). These vessels pulsate regu- 

 larly like the dorsal vessel, powerful peristaltic contractions con- 

 tinually passing along them from above downwards. Owing to 

 their great size and powerful pulsations they must be a very effi- 

 cient means of maintaining the flow of the pseudhsemal fluid ; in 

 fact they are often called the " hearts" of the earthworm. It is 

 misleading, however, to call them hearts, because they do not 

 correspond exactly to the hearts of higher animals. The earth- 

 worm has no true heart, but only a system of pulsating blood- 

 vessels. Instead of " hearts," these five pairs of circular vessels 

 should be called the aortic arches. 



The pseudhsemal fluid makes in its flow a circuit of the whole 

 body, passing forwards in the dorsal vessel, downwards in the 

 anterior circular vessels (especially in the aortic arches), back- 

 wards in the sub-intestinal vessel, and then up into the dorsal 

 again by vessels in the hinder part of the body. Inasmuch as 

 the flow makes this complete circuit, it is spoken of as a circu- 

 lation m the strict sense of the word. From the main vessels 

 numerous branches are given off which ramify through every 

 part on the walls of the body and alimentary canal, and over 

 and through all of the organs contained in the coelom. 



In the foregoing account only the more obvious features of the pseud- 

 haemal'vessels have been mentioned, and many important- details have been 

 passed over. The circular vessels of the stomach-intestine can be followed 

 for only a short distance out from the dorsal vessel, where they seem to 

 break up into a large number of small parallel vessels lying close together 

 and running around to the lower side. Here they do not directly join the 

 sub-intestinal" but empty into a sinus or vessel which runs parallel to the 

 latter, closely imbedded in the wall of the stomach-intestine. The sub- 

 intestinal vessel proper is quite separate from the stomach-intestine, and 

 communicates by short branches (usually two in each somite) with the 

 vessel lying above it. This may be clearly seen in the region of the giz- 

 zard. On this there is a variable- number of small lateral vessels, which 

 break up partly into a branching network, and are partly resolved into 

 extremely fine parallel vessels surrounding the organ. On the crop are 

 three or four pairs of lateral branches from the dorsal vessel which 

 branch out into a fine network, but do not break up into parallel vessels 

 as on the gizzard. in the two somites (13th and 14th) in front of the 

 crop there are usually two pairs of vessels running around the oesoph- 

 agus. In the llth and 12th somites a small branch is given off to 

 each calciferous gland. The most anterior pair of circular vessels are in 

 the 6th somite, and are very small, lu front of this the dorsal A'essel 



