MONOGRAPH OF THE EXISTING CRINOIDS. 



457 



immediate vicinity of the vestibular invagination. Its foremost end extends out 

 somewhat over the gut and reaches the transverse branches of the parietal canal. 



The walls are single layered throughout; that appressed to the gut is some- 

 what thicker than the opposite, though always markedly thinner than the gut 

 wall, and is composed of cubical or low-prismatic cells, which for the most part 

 carry their nuclei near the ccelome lumen. In the dorsal wall there are already 

 found in many places pavement cells, which, however, are always sharply differ- 

 entiated from the mesenchyme cells. 



In cross section the lumen appears almost sickle-shaped, since the dorsal and 

 ventral walls have moved considerably nearer to each other, and both are strongly 

 concave toward the gut. 



The course followed by the left ccelomic sack is somewhat more difficult to 

 determine than that followed by the right. Anteriorly, it never reaches so far 

 as the other, extending only slightly beyond the fourth ciliated band, while it 

 terminates on the obliquely running end pieces of the parietal canal. From the 

 left it grows out over the whole broadened posterior side of the intestinal canal 

 and water tube, and stretches over their right side, where it extends itself anteriorly. 

 This encroachment on the right side in larvae just liberated is emphasized in very 

 different ways. In the posterior ventral portion of the gut and water vessel it of 

 necessity must remain closely circumscribed, but its anterior extension varies. 

 Sometimes it is merely a small tip which is found at the right of the gut ; again, 

 it extends farther forward, especially on the right side of the water vessel, and 

 sometimes it reaches anteriorly as far ventrally as on the left side; that is to say, 

 far over the region of the fourth ciliated ring. 



But the ventral side of the water vascular system remains uncovered by the 

 left cffilomic sack almost throughout its entire extent. Only at the hindmost end 

 can the latter be seen extending over a small area of its ventral side. 



The lumen of the left ccelomic. sack, like that of the right, is narrowed to a slit. 



Histologically its walls differ very strikingly, for they are throughout thinner, 

 and are composed of a single layered pavement epithelium; only in the portion 

 joining the right ccelomic sack, the mesentery, are the cells somewhat higher and 

 prismatic. 



In larvae just liberated the plane of contact between the two ccelomic sacks 

 appears horseshoe-shaped in section, the horseshoe being open anteriorly. 



During the several days of free-swimming larval life important changes occur 

 in the ccelomic sacks. 



The right, after entirely growing about the gut dorsally in the anterior part, 

 has passed over a little onto its left side : but the border of the right sack always 

 lies nearer the ventral side than that of the left. The anterior end no longer 

 covers anteriorly the entire intestinal canal, while the parietal canal moves more 

 ventrally. 



On the anterior side of the gut it grows in toward the ventral side, and 

 laterally anteriorly in two pockets separated from each other. These pockets come 



