148 HEXACTINELLA MONTICULARIS. 



These end-rays are, at the base, 1-2 n thick, very rarely 2.6 n, and attenuated 

 towards the distal end to 0.6-1 m, rarely 1.4 n. They are densely covered with 

 minute, backwardly directed spines, and usually bear a terminal verticil of 

 larger, recurved spines which together form a kind of convex terminal disc with 

 deeply serrated margin, 1.2-2.5 n in transverse diameter. Sometimes the termi- 

 nal spines are so small that no disc-shaped terminal thickening at all can be 

 detected. 



The exceptionally occurring lateral end-rays are more divergent, more 

 curved, and shorter than the terminal ones above described, which they resemble 

 in all other respects. 



From a point in the middle of the centrum six axial threads extend in three 

 straight lines vertical to each other. One of these is long and well-developed. 

 This one is continued in the axis of the shaft, which can be traced quite to the 

 end of the latter. The other five axial threads are short, rudimentary, and 

 terminate within the centrum. The one in line with and opposite the axis of 

 the shaft is directed towards the terminal end-ray verticil, and ends before 

 reaching it without giving off branches. The end-rays are destitute of axial 

 threads. The other four axial threads terminate in the four lateral protuber- 

 ances of the centrum. 



The shape of the scopules and the arrangement of their axial threads indi- 

 cate: — that the upper part of the centrum, from which the end-ray verticil 

 arises, is, as far as it is traversed by the axial thread, an end-ray bearing main- 

 ray; that the shaft is a well-developed, simple ray; that the four lateral pro- 

 tuberances of the centrum are rudimentary simple rays, and that the end-rays 

 are homologous to hexaster end-rays. Thus the whole scopule appears as a 

 hemihexaster. Since its end-rays bear the terminal verticils of recurved spines 

 characteristic of the discohexasters and hemidiscohexasters, these scopule- 

 hcmihexasters are discohemihexasters. 



In view of this I think it not unlikely that the scopules of the Hexactinellida 

 generally are to be considered as apically highly differentiated hemihexasters, 

 the scopules of the sponge here described being not quite so far advanced in this 

 development and not so far removed from the ancestral form as the scopules 

 destitute of lateral protuberances of the centrum of other hexactinellids. 



The comparability of the scopules with hexasters was first noticed by F. E. 

 Schulze who says' concerning their end-rays, "I should be more inclined to 

 compare them with the terminal rays of the rosettes." But this author does not 



1 F. E. Schulze. Kept. Voy. Challenger, 1887, 21, p. 34. 



