MONOGRAPH OF THE EXISTING CRINOIDS. 489 



between them are much less extensive. At their bases the nuclei lie at different 

 heights. In the more developed an axial fiber can always be clearly seen, though 

 the presence of bristles on their outer ends has not been determined. In the 

 proximal basal portion, in the region of the fold, a low epithelium passes over 

 by an abrupt curve, into that of the inner wall of tlie mouth flaps. The sides of 

 the two tentacles which face each other have the same structure as the connecting 

 fold. 



INTESTINAL CANAL. 



Although the intestinal canal is without a special muscular investment its 

 component parts are nevertheless able to alter their relationships with each other 

 somewhat, and the esophagus especially can change its form to a considerable 

 degree. 



This is rendered possible through the musculature of the hydrocoele, and is 

 thus connected with the contraction of the entire calyx and of the oral disk. 



The mouth opening is usually oval, like the cross section of the esophagus, 

 and lies a little excentrically in the interradial area delimited by radii I and V, 

 rather closer to the former. Correlatively the esophagus occupies a position which, 

 in reference to the major axis of the bilateral larva, is toward the left. In the 

 vicinity of the mouth the esophagus is broader than elsewhere, and here its right 

 side extends for some distance over the median plane into the originally right 

 antimere. 



The esophagus is more or less sacklike in form. Its longer axis is not quite 

 parallel to the transverse axis of the animal, but is approximately parallel with 

 a line joining the distal ends of radii I and III, or IV and V. It is compressed 

 in the direction of radius II. 



Thus it comes about that the wall adjoining the stomach is somewhat concave 

 and the outer wall is convex, so that a cross section appears somewhat crescentic, 

 this being especially emphasized in the lumen. When contracted the lumen may 

 become narrowed to a slitlike canal, but on the other hand it sometimes appears of 

 considerable size. 



Sometimes, as a result of changes in the calyx, the aboral end of the esophagus 

 becomes more or less closed off from the stomach, while at other times they 

 appear broadly connected. 



Histologically the wall of the esophagus resembles the epithelium of the oral 

 disk. In the older larvae the cells become relatively hi^h, appearing almost wand- 

 shaped, and traverse the entire thickness of the wall. This one layered condition 

 often can be very clearly recognized in the outer part of the wall toward the 

 posterior portion of the gut when it is extended more than usual. Under these 

 conditions the nuclei are arranged in a single regular row, in all the cells being at 

 the same height. The inner wall toward the stomach is often strongly folded and 

 thickened, so that the nuclei come to lie in several layers. Under normal conditions 

 of contraction the nuclei are not always at the same height, but lie indifferently 

 in the middle or in the outer ends of the cells. On very thin sections or in teased 



