MONOGRAPH OF THE EXISTING CEINOIDS. 345 



Carpenter notes that in a large comatulid like Helwmetra glacialis these canals 

 may be found depending from the bases of the radial vessels beneath the middle 

 line of the ambulacrum in the first two or three sections beyond the edge of the 

 peristome. 



In Isocrinus and Endoxocrinus the middle line of the ambulacrum near the 

 peristome is occupied by the blood vascular plexus, which has a water vessel on 

 either side of it, and the water tubes extend outward in a radial direction as long 

 as the water vessels remain double. They may extend for a distance of more than 

 3.5 mm. from the peristome. They coil around, as it were, the subambulacral 

 plates to open below into the upper part of the circumvisceral coelome. When, 

 however, the radial extensions of the labial plexus gradually thin out and the two 

 lateral water vessels unite into a single median trunk, the water tubes become less 

 numerous and are only found in the first two or three sections beyond the point of 

 union. They are thus really in the same position as in the comatulids, if we con- 

 sider the double water vessels as expressing extensions of the water vascular ring in 

 the direction of the rays. 



BLOOD-VASCULiR SYSTEM. 



The blood-vascular system of the crinoids is very highly developed, though 

 the blood vessels themselves are nothing more than intercommunicating cavities 

 or gaps in the connective tissue of the mesenteries, bands, and cords, which in all 

 directions traverse the body cavity. Since the individual bands and cords have 

 for the most part a small diameter, while the cavities in their connective tissue 

 in which the blood circulates may be extraordinarily expanded, the former are 

 sometimes reduced to the condition of a thin sheath inclosing the blood lacuna. 



On a series of horizontal sections through the calyx a more or less regular 

 column of recurring lacunae may be made out. In Antedon mediterranea above 

 the approximate middle of the calyx there appear large lacunae, 0.1 mm. in 

 diameter, which are concentrically arranged about the axial organ, between this 

 and the outer coil of the digestive tube. In the upper part of the calyx there are 

 equal sized lacuna?, concentrically arranged, which give off large branches irregu- 

 larly on all sides. Surrounding the esophagus there lies a mass of very small 

 lacunae. 



The lacunae of the bands and cords of the body cavity are in communication 

 with others in the wall of the digestive tube. The bands and cords themselves 

 pass over into the wall of the intestine, their connective tissue layer as well as 

 their epithelial investment merging into that of the gut wall. 



The lacunae of the body cavity are not continued into the arms. The so-called 

 radial vessels in the arms, according to Hamann, are not blood lacuna? and are not 

 connected with the blood lacunar system. 



A connection between the blood lacunae and the axial (glandular) organ is 

 present in that the connective tissue wall of the latter is continued into the cords 

 ind bands in which the blood circulates. 



All these blood lacunae have the same structure. A section through a mesen- 

 tery containing a blood vessel shows externally the ccelomic epithelium which 



