CHAP, xiii.] 



CIRCULATION. 



195 



I 



always contractile, and it drives the blood from behind forward. 

 The liquid of the r perienteric cavity is colourless, and we find in 

 it figurate elements. 



In the hirudinates the perienteric cavity exists only among the 

 young. Habitually this cavity communicates with 

 the exterior, and consequently the liquid which 

 it contains is more or less mixed with water. 

 In many of the hirudinates and of the annelids 

 the blood is yellewish or more or less red. 

 (Figure 16.) 



In the tunicate* there is constantly an impul- 

 sive organ, a heart, situated on the passage of the 

 ventral trunk. 



The circulatory system of the echinoderms id 

 constituted by two vascular rings surrounding 

 the orifice of the digestive tube. These rings 

 are connected with each other, they emit radi- 

 ating ramifications, and one of. them receives 

 vessels coming from the intestine. 



In the arthropods (crustaceans, arachnida, 



insects) there is still, as in all the preceding 



groups, a general cavitv filled with blood. In all 



there exists an impulsive or cardiacal organ, 



whence proceed efferent vessels, called for that 



reason arterial. The blood returns to the heart 



by the lacunar spaces situated between the 



organs. These conduits without special walls 



debouch into a pericardiacal reservoir, and the 



blood penetrates afterwards into the heart by 



] cardiacal clefts. In the decapod* (Figure 17) 



I the blood before returning to the heart is 



oxydised in passing through the branchiae. In 



j insects the vascular system with well determined walls is almo&t 



' limited to the contractile dorsal vessel. 



All the mollusks have a sanguiferous organ contractile or car- 



02 



Anterior portion of 

 the vascular san- 

 guineous system 

 of a young Sce- 

 nuris varUgata ; 

 d, dorsal vessel; 

 v, ventral vessel ; 

 c, transversal an- 

 astomosis enlarg- 

 ed ; heart. The 

 arrows indicate 

 the direction of 

 the blood. 



