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On the Occurrence of Gelatinous Spicules, and their Mode of 

 Origin, in a Neiv Genus of Siliceous Sponges. 



By Arthur Dendy, D.Sc, F.K.S., Professor of Zoology in the University 

 of London (King's College). 



(Received March 25, 1916.) 

 [Plate 11.] 



The siliceous microscleres, or flesh-spicules, of the tetraxonid sponges 

 have long been regarded by those who have studied them as amongst the 

 most beautiful and at the same time most inexplicable structural phenomena 

 met with in the organic world. Their exquisite symmetry, their great 

 diversity in form in different genera and species, and their remarkable 

 •constancy in details of shape within the Limits of the same species, taken 

 in conjunction with the fact that it is impossible to account for this shape 

 by reference to any function that they may perform in the vital economy 

 of the sponge, constitute a problem of no little interest to the philosophical 

 biologist. 



These spicules are generally stated to be composed of hydra ted silica, or 

 opal. So far as has been known hitherto, they are perfectly transparent, 

 hard and brittle, and are unaffected by prolonged boiling with strong acids. 

 They are universally believed to be intracellular in origin, and on several 

 •occasions have been figured within nucleated mother-cells. The evidence on 

 this point is, however, rather scanty and by no means conclusive, and though 

 several observers have studied the mature form of these spicules in great 

 detail, singularly little is actually known about their mode of origin. 



The chief object of the present communication is to describe an entirely 

 new type of spicule, differing, not so much in form as in chemical composition, 

 from any previously known, the study of which, it is hoped, may throw 

 considerable light on the nature and origin of siliceous microscleres in general. 



The most striking and novel feature about the spicules in question is that, 

 although still composed, so far as can be ascertained, of colloidal silica, they 

 are gelatinous ; contracting greatly in alcohol and swelling up again on 

 addition of water. I first observed them in Australia many years ago, when 

 engaged in cataloguing the great collection of Victorian sponges made by the 

 late Mr. Bracebridge Wilson. They were found in only a single specimen, 

 and their nature was so problematical and the sponge in which they occurred 

 so ill-characterised in other respects that I set it aside as " indeterminable at 

 present," and did not include it in my published catalogue. 



