BLOOD - VESSELS. 55 



skill as a dark-red band, which is tolerably straight when the 

 worm is extended, but is made zigzag by contraction of the body. 

 If it be closely observed, a sort of wavelike contraction is often 

 seen running from behind forwards. This may be very clearly 

 observed in a worm stupefied by chloroform, especially if it has 

 been laid open along the dorsal side. The dorsal vessel then 

 appears as a deep-red, somewhat twisted, tube running along the 

 upper side of the alimentary canal. AVavelike contractions 

 continually start from its hinder end and run rapidly forwards, 

 one after another, to the anterior end, where the dorsal vessel 

 finally breaks up on the pharynx into a large number of branches 

 (Fig. 24). 



The result of these orderly progressive contractions is that 

 the fluid within the tube is pushed forwards very much as the 

 fluid in a rubber tube is forced along when the tube is stripped 

 through the fingers. It is still better illustrated by the action 

 of the fingers in the operation of milking. This action of the 

 vessels is a typical example of peristaltic contraction. 



b. /Sub -intestinal vessel. This is a straight vessel which 

 runs along the middle line on the lower side of the alimentary 

 canal, parallel to the one just described, It returns to the 

 hinder part of the body the fluid which has been carried 

 forwards by the dorsal vessel. On the pharynx it breaks up 

 into many branches, which receive the fluid from corresponding 

 branches of the dorsal vessel. 



c. Circular or commissural vessels, metamerically repeated 

 trunks which run from the dorsal vessel downwards around the 

 alimentary canal and ultimately connect with the ventral vessel. 

 They are of several kinds, of which the most important are as 

 follows : 



1. The aortic arches or circumoBsophageal vessels, often 

 known as ' ' hearts, ' ' since like the dorsal vessel they are con- 

 tractile and with the latter furnish the entire propulsive force 

 for the circulation. These are five pairs of large vessels en- 

 circuling the oesophagus in somites 7 to 11 inclusive. These 

 vessels pass directly from the dorsal to the ventral vessel, giving 

 off no branches. During life they perform powerful peristaltic 

 contractions, receiving blood from the dorsal vessel and pumping 

 it into the sub-intestinal or ventral. 



