xiv THE VOYAGE OF H.M.S. CHALLENGER. 



and we point them out here because no one else seems to have taken the trouble to do so, 

 and lest the student should be led to waste valuable time in endeavouring to reconcile 

 the observed facts with Dr. Bowerbank's descriptions and figures. 



It might be urged that Mr. Carter has abeady given good descriptions and figures of 

 the leading forms of spicules, including even the most complex ; this is true, but 

 unfortunately these occur scattered through a number of separate papers which arc 

 not likely ever to come into the hands of any but sj)ecialists. Vosmaer has also 

 given some general account of the spicules, but his figures of the more complex 

 and minute forms are far from perfect, being for the most part copied from other 

 authors. 



Perha2:)s we need say no more in excuse of giving here a general account of the minute 

 anatomy of the spicules, and, more especially, of discarding the old and cumbersome 

 nomenclature, and endeavouring to bring into use a more scientific and rational terminology. 

 The terminology here adopted is the result, in so far as that result afiects the Monaxonida, 

 of the mutual agreement of several spongologists, who consulted together with a view to 

 formulating a set of terms which might have some chance of being generally accepted, as 

 an experience of over twenty years has shown that this cannot be hoped of the 

 Bowerbankian nomenclature, and there exists no other system of scientific terms to fall 

 back upon. We give in the form of an appendix a list of the terms here employed, 

 with the more important synonyms of other authors. 



Concerning the chemical composition and the development of the spicules very little is 

 as yet known. It is, however, a generally recognised fact that each (at any rate of the 

 larger forms, and probably, judging from analogy, of the smaller forms also) consists of a 

 central, axial, organic thread (the " axial thread," commonly, though incorrectly, referred 

 to as the "central canal"), surrounded by concentric layers of a peculiar siliceous 

 substance, vaguely spoken of as " organic silica." That the spicules are the products of 

 the activity of special cells (silicoblasts),^ within which they originate, is now also an 

 admitted fact; but it is also pretty certain that the larger forms at any rate become 

 free from the parent cell (silicoblast) before attaining their full size. Whether it be 

 within or without the cell, the spicule grows by apposition. The axial thread appears to 

 Ije the portion first developed, and upon the form assumed by this in the course of its 

 growth, depends the form of the fully developed spicule. Not infrequently, in the case 

 of the minuter forms of spicules, several originate in one and the same silicoblast, and 

 they then remain together in bundles for a longer or shorter period ; to such bundles 

 the general term dragmata^ is here applied. 



This brief account of the nature and growth of siliceous spicules is necessary for the 



1 Vide PI. XXI. fig. 13., and cf. Vosmaer, Bronn's Klasseu u. Ordnungen d. Tluerreiclis, Porifera, p. 436, where the 

 name " Silicoblast " is first introduced. 

 - Greek, o^a'/y.x, a sheaf. 



