MONOGRAPH OF THE EXISTING CKINOIDS. 493 



rudiments of the chambered organ lie close down upon the ccelomic wall, though 

 there seems never to be a connection between the two cavities. Often the five cham- 

 bers do not lie in the same transverse plane, but it is not difficult to find individuals 

 in which this is the case. 



Since in this region a section through the calyx now shows the pentamerous 

 structure strongly marked, it is easy to see that the chambers are radial in position, 

 a fact not previously clearly brought out. 



Regarding the question in what way the original evaginations which appeared 

 in the free-swimming larva have moved into radial positions, it is evident in 

 sections cut farther orally that the vertical mesentery is in interradius IV-V, 

 and correspondingly the chamber lying in radius V can be traced back to that 

 evagination which originally appeared on the left side ventrally and was designated 

 as evagination 1. From this it is easy to see that the original evaginations 2, 3, 

 4, and 5 are now the chambers lying in radii I, II, III, and IV. 



The extensions of the chambered organ traverse the stem as cords of cells in 

 which the cross section shows no lumen, and indeed the individuality of the cords 

 themselves is lost a short distance from the calyx. 



In longitudinal view the component cells are seen to be elongate and spindle- 

 shaped, with distinct nuclei, joined together end to end. The ends of adjacent 

 cells are so intimately united that the cell borders are not recognizable, and the 

 impression given is that of a continuous plasmatic cord bearing more or less 

 regularly spaced nuclei. A similar developmental stage can be commonly observed 

 in the formation of the muscle fiber lines, but whether the further developmental 

 course of the two is the same is uncertain. 



AXIAL ORGAN. 



Soon after the opening of the vestibule the axial organ becomes detached from 

 the whole oral part of the fold of the visceral layer through the thickening of 

 which it has arisen, and appears as an isolated cord of cells. 



This cord very quickly becomes hollowed out into a tube of which the lumen 

 at first is always narrower than the thickness of the walls. The latter consist of 

 a single-layered epithelium, though cell borders can not always be differentiated in 

 the thick mass of plasma. The structure appears to be in course of active growth, 

 for the nuclei are commonly seen in process of division. The resting nuclei are 

 larger than those of the ccelomic wall. 



The fold of the visceral layer enveloping the axial organ is composed of a 

 more or less strongly flattened epithelium, which in the middle section always lies 

 closely on the tube. In the oldest larvae of this period the axial organ on the side 

 toward the gut is accompanied by a number of longitudinal muscle fibers which 

 are formed from the fold of the visceral layer on the side toward the axial organ. 

 These fibers also are foimd in the esophageal section of the fold, and in a short 

 section lying on the gut. 



Toward the oral side the axial organ has become extended as far as the oral 

 disk and into the vicinity of the oral coelome, where it approaches the border of 



