218 COMPARATIVE ANATOMY. 



into a circular canal, which surrounds the mouth. The radial 

 vascular trunk corresponds to the ventral vessel of the Vermes, 

 which has a similar relation to the ventral medulla. A tube 

 which passes from the oral ring- to the stone-canal (for which see 

 below) was formerly regarded as the heart, but this organ cannot be 

 regarded as such. The same remark applies to the similar structure 

 iu the Echinoi'da. We have therefore still to search for a heart as 

 the central organ of the blood- vascular system. The enteric vessels 

 form a second division of the blood-vascular system. 



In the Echinoi'da the nerves lie within the radial blood-vascular 

 trunks ; in the Crinoi'da and Holothuroi'da they lie on their outer 

 side, and the same is the case in the Asterida and Ophiurida. The 

 circular vessel surrounding the mouth in the Asterida and Crinoi'da 

 and in the Spatangidse among the Echinoi'da, where it has the form 

 of a wide sinus, is described as having the same relations to the nerve- 

 tract ; although in Echinus there is said to be a blood-vessel placed 

 farther from the mouth, and above the masticatory apparatus which 

 surrounds the oesophagus. It is probable that this separation of the 

 blood-vessel from the nerve-ring is due to the development of the 

 masticatory apparatus. In the Holothuroi'da the adoral blood- 

 vascular ring is connected with the nerve-ring, but is placed inside 

 it, and nearer the mouth. It may break up into a plexus. The 

 aboral vascular ring found in the Asterida and Echinida does not 

 appear to have so much morphological importance, as it is confined 

 to a few divisions. Other vessels, which surround the generative 

 glands, and there form wide sinus-like spaces, pass into it, in addition 

 to the vessels from the perisome. In Comatula also a vessel, 

 which forms a covering around the genital chord, is continued into 

 the arms and pinnulae. In the Asteroida and Crinoi'da the vessels 

 of the enteric canal are not independent. In Comatula they form a 

 network, with wide meshes, in the coelom ; this is connected with 

 the oral vascular ring*. A bundle of vessels passes from this net- 

 work, along the axis of the cup to the centrodorsal plate, forming 

 a special organ widened out into five chambers, the importance 

 of which is not known. 



The enteric vessels in the Echinoi'da and Holothuroi'da are more 

 independent. A dorsal and a ventral vessel can be distinguished, 

 which have just the same characters, as the same vessels in the 

 Vermes (cf. § 138). In Echinus the dorsal vessel is double, for in 

 addition to the one which runs directly on the enteron there is one a 

 little way from it, which gives off branches to the former one, as 

 well as to the enteron. In the Spatangidas the ventral vessel has 

 been observed to communicate with the water- vascular ring. The 

 enteric vascular trunks of the Holothuroi'da are enlarged in the 

 middle of their course, and the dorsal vessel passes into retia 

 mirabilia. 



