REPORT ON THE HEXACTINELLIDA. 27 



death, to the influence of sea-water, one always notices a more or less marked widening 

 of the axial canal. This is due to the fact that the central siliceous layers are slightly 

 more soluble than the outer. The time that has elapsed since death can thus be 

 approximately estimated, and the differences have, of course, no systematic importance 

 as was formerly supposed. 



THE SKELETON. 



Under the title skeleton I include all the solid parts of the Sponge, whether these 

 are bound together into a united framework, or lie isolated in the soft tissues. I 

 would avoid the expressions, "skeletal" and "flesh" spicules, which are much used by 

 Carter, Zittcl, and others to distinguish the hard parts into two leading categories ; 

 since, on the one hand, these expressions are ambiguously used by diff'erent authors, 

 and even by one and the same author, and, on the other hand, since they in no way 

 indicate any essential distinction. By "spicules of the skeleton" Carter originally under- 

 stood "large spicules, which are only concerned in the formation of the supporting 

 structure or skeleton," but he subsequently applied the term only to the larger free 

 spicules, in contrast to the smaller free spicules or "flesh spicules," and to the vitreous 

 fibre of the continuous lattice framework. Zittel used the phrase " skeletal spicules," 

 for the most part in reference to the skeletal parts concerned in the formation of a 

 united lattice-like framework, but he occasionally applies it also to the larger free 

 spicules which form the chief support of the soft parts. 



Most spicules present a structure similar to. those which Max Schultze and Claus 

 first discovered in the larger spicules of Hyalonema sieboldii and Euplectella asp)er- 

 gillum. A fine central canal, corresponding to the typical axes of the skeletal 

 body, is surrounded by numerous concentrically arranged layers of a solid substance. 

 In its appearance and other physical jaroperties the latter resembles glass so closely 

 that it has been often, without the least hesitation, spoken of as vitreous fibre. For the 

 same reason the entire Hexactinellid group has been designated " vitreous sponges," or 

 more shortly "Vitrea." As to the nature of the. soft, finely granular mass composing 

 the axial thread or cord which fills the central canal, but little is as yet known. 

 During the growth of the spicule it appears to be connected with the surrounding soft 

 parts through an opening which is present at the end of every ray. After the ray has 

 ceased to grow in length, the terminal opening is closed by an expansion of the layers 

 of the glassy substance. That all spicules have a central canal is very probable, but not 

 quite certain, since it has not been possible to recognise it in all cases. Especially in 

 very thin and small spicules it is often very difticult or impossible to detect it. I have 

 never found it in lateral prickles, prongs, scales, and the like, nor in the secondary 

 terminal rays of the rosettes and scopulas, nor in the short transverse connecting beams. 



