MONOGRAPH OF THE EXISTING CRINOIDS. 



495 



The trabeculse traversing the ring canal and connecting the inner and outer 

 walls have increased considerably in numbers. In each a nucleus is found, usually 

 in the middle ; frequently two or more occur in a single trabecula. In the younger 

 stages these have no special fibrillar structure, and in cross section are rounded 

 or quadrilateral, somewhat flattened, or bandlike. The last is the most common 

 in the older stages, in which the broad side is usually, but not always, parallel 

 to the oral wall. 



When the trabeculse contract they become more cylindrical. On the border the 

 cross section shows a prominent stripe which on strong magnification resolves 

 itself into a number of fibrillse running closely side by side. These fibrillse are 

 usually from two to four in number, often only one, in a few cases five. They 

 appear usually entirely structureless, but sometimes finely granulated; cross 

 striping has not been proved. At the end of the trabeculse the fibrillse appear 

 to be somewhat thicker than in the middle. They enter the hydroccele wall and 

 end with it. 



In the larvae in which the septum in the hydrocrele ring still persists the origin 

 of the stone canal from the blind end lying to the right in interradius V-I can 

 be very clearly made out. It forms a V-shaped loop, of which the two arms 

 are bent in a different way, so that the whole canal can never be seen at one time 

 on a thin section. It runs in the vertical mesentery of the oral ccelome and opens 

 on its border against the horizontal mesentery in the parietal canal. This branch 

 often runs quite horizontally and is directed outwardly and toward radius I, 

 while the part joining the hydroccele ring runs more vertically. 



In its structure the primary stone canal of the larva entirely resembles the 

 numerous canals of the adults. It is a thick-walled tube, of which the diameter 

 of the lumen in the older larvae somewhat exceeds the thickness of the walls, and 

 is composed of a single layer of moderately high prismatic or truncated pyramid 

 cells, the nuclei of which are large and stain strongly. In the terminal section 

 the cells in the wall toward the parietal canal are somewhat lower, often cubical. 

 From the mesenchyme tissue the canal is sharply differentiated. In the lumen 

 a striped border zone can often be demonstrated, suggesting ciliation. 



From the preceding description it is evident that the stone canal is formed 

 rather late as an independent evagination of the hydroccele ring which leads 

 into the parietal canal and at no time opens freely to the exterior. 



The hydroccele cavities of the tentacles consist of single-layered sacks, with 

 broad lumina, which taper toward the distal end, lying directly beneath the 

 ectoderm. In the 15 large tentacles the separation of the individual sacks has 

 advanced to the hydrocoele ring, so that each of these tentacles now arises from 

 that ring quite independently of the other two. 



In the hvdroccele of the tentacles there are now no muscular trabeculae, 

 though these were present in the original evagination. The walls are composed 

 of a flat epithelium in which the nuclei, lying on the inner side, are somewhat 

 larger but less easily stained than those of the ectoderm cells. At the distal end 

 the cells are somewhat thicker and richer in plasma, and further extension of 

 the sack, correlated with the lengthening of the tentacle, appears to take place here. 



