130 Prof. W. Thomson on the '' Vitreous'''' Sjponges. 



are so characteristic of Euplectella. The long hexradiate 

 clubbed spicules and the irregular hexradiate spicules make up 

 the mass of the skeleton. 



The investing film of sarcode is thickly studded with ex- 

 tremely minute spicules, which in the dried sponge seem to be 

 adhering to the larger spicules and irregularly massed among 

 them. On shaking the sponge, quantities of these minute 

 spicules of the sarcode fall out. They are of two marked 

 types. The most abundant are small hexradiate stellate 

 spicules (PI. IV. tig. 1 /;, x 250), with the rays nearly equal, 

 formed exactly upon the same plan as the larger spicules of 

 the skeleton, Avith a well-marked but extremely minute six- 

 rayed tube occupying the centre of the rays. The rays when 

 broken show the same lamellar structure as the larger spicules. 

 The second type is also very abundant ; but it is rare to find 

 these spicules at all perfect : it is the " floricomo-hexradiate 

 stellate " of Bow erbank. The centre of the spicule is hex- 

 radiate stellate, like the other spicules ; but each ray, not more 

 than four times its own width from the point of divergence, 

 spreads out into several, probably six or eight, expanded plates : 

 these plates curve outwards and form a cup ; they then bend 

 upwards and slightly inwards, and become so extremely deli- 

 cate that they are little more than visible mider a high magni- 

 fying-power ; finally they sweep upwards and outwards, end- 

 ing in a trifid expansion. These trifid ends turn gracefully 

 over, so that the processes from each of the six radii unite in 

 producing a beautiful vase-like form. A perfect spicule with 

 all its vases complete is an exquisite microscopic object. The 

 star-like centres of these spicules are not very common ; but 

 the sarcode is full of the ultimate branches. (PL IV. fig. If.) 

 The vast number of these separate hooks may be exjjlained 

 by the extreme tenuity of their attachments to the central 

 stars. The strain of the contraction of the sarcode in drying 

 may probably be sufficient to break them off. These spicules 

 appear to be most abundant near the edges of the openings in 

 the network. 



The conical sponge is abruptly truncated above, the wall 

 •ending in a well-defined ridge or lip, somewhat more dense in 

 structure than the rest of tfie wall. The tube is then closed 

 by a very irregular, horizontal, netted lid, composed of fibres 

 which are much more thread-like and closer in their texture 

 than those of the wall. The ultimate elements of the fibres of 

 the lid, however, are exactly the same as those of the wall- 

 network ; only the large stellate spicules are less numerous, and 

 the fusiform spicules are usually much shorter. The spicules 

 of the sarcode are equally numerous and of the same character 

 throughout. 



