450 COMPARATIVE ANATOMY CHAP, vm 



the following may be considered as the chief constituents of the blood 

 vascular system in the Echinodermata: (1) a blood vascular network 

 in the intestinal wall, whose evident function is to absorb the digested 

 and dissolved nourishment as albuminoid out of the intestinal wall ; 

 (2) two large vascular trunks, which, arranged on opposite sides of 

 it, accompany the intestine in its course ; these conduct the blood, 

 which has become enriched with albuminoid in the vascular network 

 of the intestine, to other parts of the vascular system ; (3) a blood 

 vascular ring", which surrounds the mouth or oesophagus, and into 

 which the two intestinal vascular trunks open ; (4) five radial blood 

 vessels, which run, like the water vascular and the nerve trunks, in 

 the radii ; (5) a vascular network on the surface of the gonads 

 (genital glands) ; (6) a vascular network on the surface of the axial 

 organ. 



The connective tissue surrounding the vessels may, at various 

 points of the vascular system, become modified for yielding corpuscles 

 to the blood (lymph glands). 



Contractions, very irregular and indistinct, have been observed 

 only in the vascular trunks of the intestine of the Holothurioidea. 



1. Holothurioidea (Fig. 371). The simplest arrangement of the 

 blood vascular system in this class is found in the Paractinopoda 

 (Synaptida). The intestinal lacunse pour their contents first into two 

 longitudinal trunks, one of which runs along the dorsal, and the other 

 along the ventral side of the intestine. The ventral trunk opens 

 anteriorly into the dorsal, which then runs along within the dorsal 

 mesentery direct to the genital gland. It is here continued into a 

 spacious lacunar system, which develops in the wall of the gland in 

 such a wa} r as to split into two diverging lamellae, an outer and an 

 inner, the latter carrying the germinal epithelium. The dorsal vessel 

 further gives off a small lateral branch near the point at which the 

 stone canal enters the circular canal. According to recent researches, 

 neither a circular canal of the blood vascular system, nor radial 

 vessels, nor tentacle vessels are to be found. In the Adiimjiuilt (Fig. 

 371) the blood vascular system is more completely developed. Here, 

 again, the blood from the lacunar network of the intestinal wall (which 

 lies on the inner side of the muscle layer) is collected by two vascular 

 trunks which accompany the intestine along its whole length, i.e. to 

 the hind-gut ; one of these trunks is the dorsal or mesenterial, and 

 the other the ventral or anti-mesenterial. They both open anteriorly, 

 immediately behind the circular canal of the water vascular system, 

 into a circular vessel which surrounds the oesophagus, from which five 

 radial vessels run in the radii. Each radial blood vessel lies between 

 the radial nerve trunk on the outer and the radial water vessel on 

 the inner side (Fig. 352, p. 409). It gives off lateral branches to 

 the oral tentacles, the ambulacral feet, and the papillae. The wall 

 of the genital glands is everywhere richly " vascularised," either by a 

 lacunar network or by a more simple splitting of its wall of connective 



