MONOGRAPH OF THE EXISTING CRINOIDS. 475 



A lumen is alwaj's demonstrable in the hydrocoele, though it is frequently 

 much narrowed dorso-ventrally. 



During the turning the five broad evaginations of the hydroccsle ring alter 

 their form, becoming narrower radially, and appearing as elongate lappetlike 

 structures. At the same time their distal ends, embedded in the floor of the 

 vestibule, begin to divide into three points, so that now 15 processes are present. 

 These are the beginnings of the 15 hydroccele tentacles, but at first they are only 

 of inconsiderable length, and are entirely concealed in the vestibular wall. 



The way in which the left ccelome inserts itself between the gut and hydro- 

 ccele ring, bordering the anterior wall of the latter, which earlier was directed 

 dorsally, has already been noticed. 



Histologically the wall of the hydroccele is a single-layered cylindrical or 

 cubical epithelium of considerable thickness. Except in the ring, where the 

 lumen is most extensive, the walls are usually thicker than the diameter of the 

 central canal. 



As a result of the strong curvature of the walls the latter appear very 

 commonly in sections to be made up of several layers, chiefly for the reason that 

 the nuclei are not everywhere at the same height. 



After the hydroccele ring has attained a horizontal position it remains open 

 for a long time between radii I and V, the interradius in which the horseshoe- 

 shaped oral coelome is also open. 



Between the five primarj' evaginations the ring is almost circular in section, 

 but near the latter it passes to an oval form, elongated parallel to the major axis. 



The originally high cylindrical cells of the wall become lower and lower, 

 and shortly before the rupture of the vestibule they have transformed into low, 

 in part pavement, cells which are somewhat swollen in the middle, where they 

 carry the nuclei. This process continuing, a fine pavement epithelium is formed, 

 which becomes somewhat thicker on the inner side of the ring. Very noticeable 

 are elongated, often thread-shaped cells, which are always to be found in larvae 

 of this age traversing the lumen of the canal and running radially between the 

 outer and inner walls, to which they stand usually, though not always, at right 

 angles. 



Rather early, usually on the third day, individual cells in the hydroccele canal 

 and its processes raise themselves up and project freely into the lumen, in the 

 same way as similar cells in tlie coelome. Usually these appear on the thicker 

 inner wall, but occasionally on the outer. 



At first the cell rests entirely in the epithelium of the wall and is only dis- 

 tinguishable from the neighboring cells through its spherical or pear-shaped 

 form ; but very quickly the free end elongates and passes entirely across the lumen 

 to the opposite wall. 



The nuclei of these cells often remain at the base at the same height as the 

 nuclei of the neighboring cells, although the cell itself has passed entirely across the 

 lumen. But very commonly the original nuclei at the base divide and the daughter 

 nuclei assume a position more or less near the middle of the lumen. Sometimes 



