BLOOD-VESSELS. 129 



deep-red, somewhat twisted tube running along the 

 upper side of the alimentary canal. Wave-like con- 

 tractions continually start from its hinder end and 

 run rapidly forwards, one after another, to the an- 

 terior end, where the dorsal vessel finally breaks up 

 on the pharynx into a large number of branches 

 (Fig. 75). 



The result of these orderly progressive contractions is that 

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

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

 through the fingers. This action of the vessels is known as peri- 

 staltic contraction. 



The peristaltic contraction of the pseudhaemal vessel, unlike that of the 

 rubber tube, is intrinsic and automatic (p. 32), being caused by the con- 

 traction of its own muscular walls. The physiological importance of peri- 

 staltic contractions is very great. For in animals generally the blood is 

 propelled in this way ; the contents of the alimentary canal are carried on- 

 wards and finally driven out; urine is ejected, and the products of repro- 

 duction (eggs, embryos, -etc.), as well as the male elements, are similarly 

 transported or expelled. 



1}. /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. 



3. Circular vessels, which run from the dorsal vessel down- 

 wards around the alimentary canal to join the sub-intestinal ves- 

 sel. Through these vessels the fluid flows down to join the back- 

 ward-flowing current in the sub-intestinal vessel, thus completing 

 a circuit. Of these circular vessels there are usually two pairs 

 (sometimes three or four) in each somite, partly imbedded in the 

 wall of the alimentary canal and covered with ; ' chloragogue 

 cells" (p. 127), so that their red color is not apparent. In front 

 of the 13th somite, however, there is only one pair in each 

 somite. The five pairs lying between the 7th and the llth 

 somites (inclusive) are greatly enlarged, and quite free from the 

 9 



