REPORT ON THE TETRACTINELLTDA. 
167 
On entering the cloacal tube the oxeas and cladoxeas extend longitudinally through 
the walls of the component canals in bundles, which chiefly occur in the angles between 
the canals. The long cladi of the cladoxeas, extending from them at right angles, are 
curved so as to encircle the canal of which they form the skeleton for about one-half of its 
circumference. As the history of the cladi of these spicules can be traced with tolerable 
clearness, it may be as well to introduce an account of it in this place. Let a, h, c, 
(Fig. 1a) be the three cladi of a normal orthotrisene seen in plan. In the cloacal tube 
the spicule is so placed that the cladus a has opposed to it the rhabdome of the 
next succeeding spicule of the bundle ; it therefore becomes suppressed ; the cladus 
h projects into the coUenchyma of the canal, which is present only in thin lamellse ; 
as soon as it reaches the limits of a lamella its growth is arrested by default 
of supporting tissue, and it remains a mere tubercle ; the cladus c, on the other 
hand, lies in the circumference of the wall of the canal r, r, its growth can take place 
unimpeded, and it does not cease increasing in size till it has half encircled the canal 
(Fig. 1b). 
Again the spicule may be so situated (Fig. Ic) that both cladi h and c lie in the direction 
of the circumference of the canal, both these may enlarge (Fig. Id), though as one is usually 
more favourably situated than the other their growth is unequal. Dichodisenes are produced 
as follows : — They occur in the spicular bundles which lie closest to the exterior of the 
cloacal tube ; the long cladus c follows the wall of the canal to which it belongs (Fig. 1e), 
but as this suddenly changes from a curve of less radius to one of much greater radius 
on leaving the interior to form part of the external boundary of the cloacal tube, so the 
spicular cladus changes direction with it and thus becomes bent. In this way spicules 
