THE APODOUS HOLOTHTJRIANS 55 



VI, fig. 6; Plate VII, fig. 29), the size and number of which are very diverse. 

 They are more or less elongated, blind sacs, the diameter of which varies 

 greatly with the amount of fluid contained within. In Synapta and some- 

 times in Euapta they may be branched; that is, a number of vessels arise from a 

 common stalk-like outgrowth of the circular ring. Comparatively few Synap- 

 tids have only a single polian vessel, the great majority having more than 

 two. Many have from 8 to 16, and a few species have 50 or even more. When 

 more than one, the polian vessels are seldom of the same size, and when 

 numerous there is the greatest variety in length. When only one is present, 

 it is generally of very moderate size, usually less than a tenth of the body 

 length. This single polian vessel lies on the left side of the body, usually in 

 the left dorsal interradius ; but when more are present, they arise from the 

 ventral portion of the circular ring, and when very numerous they spring 

 from all parts of that ring. The wall of the polian vessel has essentially the 

 same histological structure as the circular ring; in the connective tissue of 

 either there may occur small calcareous particles, but they are never conspic- 

 uous or numerous. Arising from the circular ring is still another outgrowth, 

 the so-called "stone-canal," which may be single, or there may be a number. 

 In the great majority of cases the "stone-canal" is a single, unbranched 

 tube, arising in the mid-dorsal interradius and more or less completely in- 

 closed in the mesentery. In Synapta the stone-canal is usually single, but 

 branched, while sometimes in that genus, and commonly in Opheodesoma, 

 a large number of stone-canals are present, arising from almost any part of 

 the circular ring. While the inner end of the stone-canal is in imme- 

 diate connection with the ring, the outer end is almost always unattached, 

 and bears a more or less prominent and irregular enlargement which corre- 

 sponds to the madrepore plate of other echinoderms, and is known as the mad- 

 repore body. Through the openings in the madrepore body the stone-canal, 

 and hence the entire water-vascular system, is in direct communication with 

 the body-cavity. In Synaptula hydriformis, moreover, the madrepore body 

 lies in the mesentery and is attached to the dorsal wall in such a way that 

 there is present at least one opening between the stone-canal and the water out- 

 side. This opening is on the dorsal side, just in front of the genital duct. 

 In all Synaptids the wall of the stone-canal consists of only three layers, the 

 outer epithelium, a connective-tissue layer, and an inner epithelium of high, 

 more or less cylindrical cells, which are conspicuously ciliated. There are no 

 muscle fibers anywhere in the tube wall, but the connective-tissue layer con- 

 tains a large quantity of irregular particles of carbonate of lime, giving that 

 firmness and brittleness to the tube which has led to its being called a stone- 

 canal. The madrepore body is mainly a mass of such calciferous connective 

 tissue, more or less extensively pierced by the openings between stone-canal 



