MONOGRAPH OF THE EXISTING CRINOIDS. 491 



fluid secreted from the cells themselves. Ver^' often this clear zone is of consider- 

 able width. 



When the stomach is filled with food the inner ends of the cells lose their 

 distinct borders, and the cell plasma appears to merge into the included mass. 



The hind gut, representing the hind gut on the originally right side of the 

 stomach, arises gradually from the posterior end of the latter, so that no sharp 

 dividing line between te two can be drawn. Such a dividing line would lie in 

 the interradius bounded by radii III and IV. 



The hind gut tapers quickly to a tube, which, as in yoimger stages, runs in 

 the horizontal mesentery close to the body wall, passes through the interradius 

 IV-V in a broad curve, and,' extending somewhat beyond the original median 

 Mne, opens to the exterior through the anus, which is situated more or less near 

 radius V, though always in the interradial space V-I. Seen from the oral side 

 of the larva the turning of the gut is clockwise. 



The wall of the hind gut is composed of a single layer of cells. In the proxi- 

 mal part the cells resemble those of the stomach wall. Farther posteriorly they 

 usually become lower, and the differentiation of the inner ends described for the 

 cells of the stomach wall can no longer be made out. Thus in the younger stages 

 the walls here may be closely pressed against each other, so that the lumen entirely 

 disappears. Usually no cell borders can be distinguished, though the nuclei lie 

 fairly regularly distributed near the outer walls. In a few cases, however, cross 

 sections through the end gut show clearly high prismatic cells, the nuclei of which 

 lie in the outer ends, while from the inner ends pseudopodialike processes radiate 

 which form with those of the adjacent cells a reticular structure. In the cases 

 where no cell borders can be demonstrated similar pseudopodialike structures are 

 found, but here the network appears to be more coarsely developed, often filling 

 the entire lumen. It is possible that this is the effect of preservation, and that in 

 the li\Tng object the cells are ciliated, like those of the stomach. 



According to Perrier the anus is bordered with large cilia — an observation 

 partially confirmed by Seeliger. 



The distalmost end of the gut is of exclusively entodermal origin. The ecto- 

 dermal invagination observed in numerous cases in the formation of the anus, 

 by which an ectodermal rectum is formed, appears to be of later origin, occurring 

 after the anus has moved from its lateral position on the side of the calyx to the 

 definitive position. 



CCEI.O.ME AND ITS DERIVATIVKS. 



At the end of the last developmental period the two ccelome pockets had 

 already reached their definitive position, and their further alterations are relatively 

 unimportant. 



These alterations consist in a progressive advance in the dissolution of the 

 mesentery, so that the originally separate sections become more and more broadly 

 united, and finally can no longer be clearly differentiated. 



The wall of the oral ccelome is now composed of an extremely delicate pave- 

 ment epithelium which can everywhere be clearly demonstrated as such, though 



