MONOGRAPH OF THE EXISTING CRINOIDS. 459 



the ventral side and entirely covers this side of the hydroccele, which is only 

 partially done by the left. 



As in the preceding stages the wall of the ccelome is composed throughout of 

 one layer of cells, which becomes very thin on the portions adjacent to the gut, 

 where it is sometimes reduced to a fine pavement epithelium. Of all the cells 

 those in the mesentery are the highest. 



Eydroc&le and Parietal Canal. 



Toward the end of the embryonal period the commencement of the hydroccele 

 closes itself off from the parietal canal and bends itself about in such a way that 

 in cross section it appears horshoe-shaped with the right branch somewhat larger 

 than the left. This right branch develops rapidly at the expense of the left, 

 which finally reaches much less far posteriorly than previously. So much does 

 the right branch enlarge that posteriorly it reaches as far toward the left as the 

 other, causing the opening of the horseshoe to appear displaced toward the 

 left, where it is covered in part by the anterior end of the left ccelome. 



At this stage five knob-like evaginations directed toward the ventral side 

 appear on the hydroccele, from each of which later three tentacle vessels arise. 

 It has not been definitely determined whether these evaginations are simultaneous, 

 or whether, if not, there is any definite regularity in their appearance. In any 

 case they appear very suddenly and almost simultaneously, for of larvae of the 

 same age a short time after their liberation some show none and some show all 

 five already formed. 



Numbers 1 and 5, at the two extremities of the sack, are commonly more 

 advanced than the others, and it is not improbable that they are formed first. 

 Number 4 appears to be in early stages the least developed. 



The peculiar horseshoe-shaped form taken by the hydroccele indicates that 

 it can readily become closed into a ringlike form: but the formation of a closed 

 water vascular ring first occurs at a considerably later stage. 



The walls are everywhere composed of a single layer of cells which are 

 prismatic or pyramidal in form and of approximately equal height, as in the 

 gut. Their nuclei lie throughout in the inner ends of the cells, near the lumen. 



In the older free-swimming larvae the opening of the horseshoe becomes 

 directed toward the anterior and left, turning about an angle of more than 90 

 anticlockwise. The two ends of the liydroccele have now closely approached each 

 other, and the five primary evaginations have become considerably more extensive. 

 The walls are still of a single layer of cells, but on the side toward the gut they 

 are markedly thinner and are there for the most part composed of almost cubical 

 cells. 



It is in the older free-swimming larvae that the first beginnings of the primary 

 stone canal can be recognized. It shows itself as a short process from the (origi- 

 nally) left blind end of the horseshoe-shaped hydroccele running dorsally and 

 toward the left and inserting itself between the anterior end of the gut and the 

 parietal canal, ending blindly on the right ccelomic sack. At this stage there is 

 no longer an opening into the parietal canal. 



