148 I. IJIMA : HEXACTINELLIDA. 1. 



well as the oscular membrane, the marguial membrane and the 

 membrana reuniens, all of which have been spoken of before as 

 adaptations of the general trabecular system. I will repeat that 

 all these membranes are not different from the thread-like 

 trabeculiiE' either in histological character or in the manner of 

 their occurrence. I am even under the impression that, during 

 life, there obtains in the tissue concerned a certain degree of 

 instability in the form, whether membranous or filamentous, which 

 is assumed at different times. As the result of a certain stimu- 

 lus, causiug protoplasmic contraction, a membranous area may 

 thin out and finally break apart in the middle ; then, by en- 

 largement of the gap or gaps thus produced, the area may 

 readily convert itself into a mesh or a series of meshes bounded 

 by filamentous beams (see PI. VIII, fig. 30). Contrariwise, the 

 trabecular cobweb may become so close meshed as to finally fuse 

 together into a continuous sheet, or a part of it may be so drawn 

 out as to form a filmy expansion. Indications of such trans- 

 formations are indeed very frequently to be observed. This 

 theory presupposes a viscous semi-fluid nature of the trabecular 

 substance, which assumption seems also to explain the apparent 

 facility with which floricomes traverse a dense cobweb of trabe- 

 culse in order to reach the tips of the dermal hilt-rays. The 

 above nature appears to me all the more assumable, since, as 

 will soon be dwelt upon at length, I am inclined to ascribe no 

 pinacocytal covering to the entire trabecular system, but to regard 

 this as a network of bare-surfaced syncytial protoplasm. 



A spicular sheath, consisting of a continuous layer of the 

 soft tissue, has been assumed or mentioned by some writers 

 (Thomson '70, p. 710 ; Schulze '87, p. 24). Although I have 

 never been able to prove the fact, yet I can not but hold it very 



