Chap, xiii.] 



CIRCULATION. 



195 



of- 



\ 



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

 The liquid of the "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. 

 Tn many of the hirudinates and of the annelids 

 the blood is yellewish or more or less red. 

 (Figure 16.) 



In the tunicates there is constantly an impul- 

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

 ventral trunk. 



The circulatory system of the echirwderms is 

 constituted by two vascular idngs 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 cavity 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 decapods (Figure 17) 

 the blood before returning to the heart is 

 oxydised in passing through the branchiae. In 

 insects the vascular system with well determined walls is almost 

 limited to the contractile dorsal vessel. 



All the molliisJcs have a sanguiferous organ contractile or car- 



o2 



Anterior portion of 

 the vascular san- 

 guineous system 

 of a young 5as- 

 nwris variegaia ; 

 d, dorsal vessel; 

 V, ventral vessel ; 

 c, transversal an- 

 astomosis enlarg- 

 ed ; heart. The_ 

 arrows indicate* 

 the direction of 

 the blood. 



