IX] OF THE RADIOLARIAN SKELETON 459 



laria, for instance the genus Collosphaera*. Johannes Miiller 

 (whose knowledge and insight never fail to astonish us) remarked 

 that these were identical in form wdtli crystals of celestine, a 

 sulphate of strontium and barium ; and Biitschli's discovery of 

 sulphates of strontium and of barium in kindred forms render it 

 all but certain that they are actually true crystals of celestinef. 



In its typical form, the Radiolarian body consists of a spherical 

 mass of protoplasm, around which, and separated from it by some 

 sort of porous "capsule/' lies a frothy mass, composed of proto- 

 plasm honeycombed into a multitude of alveoli or vacuoles, filled 

 with a fluid which can scarcely differ much from sea-water J. 

 According to their surface-tension conditions, these vacuoles may 

 appear more or less isolated and spherical, or joining together in 

 a "froth" of polygonal cells; and in the latter, which is the 

 commoner condition, the cells tend to be of equal size, and the 

 resulting polygonal meshwork beautifully regular. In many cases, 

 a large number of such simple individual organisms are associated 

 together, forming a floating colony, and it is highly probable that 

 many other forms, with whose scattered skeletons we are alone 

 acquainted, had in life formed part likewise of a colonial organism. 



In contradistinction to the sponges, in which the skeleton 

 always begins as a loose mass of isolated spicules, which only in 

 a few exceptional cases (such as Euplectella and Farrea) fuse into 

 a continuous network, the characteristic feature of the Radiolarians 

 lies in the possession of a continuous skeleton, in the form of a 

 netted mesh or perforated lacework, sometimes however replaced 

 by and often associated with minute independent spicules. Before 

 we proceed to treat of the more complex skeletons, we may begin, 

 then, by dealing with these comparatively simple cases where 

 either the entire skeleton or a considerable part of it is represented, 

 not by a continuous fabric, but by a quantity of loose, separate 

 spicules, or aciculae, which seem, like the spicules of Alcyonium, 



* For figures of these crystals see Brandt, F. u. Fl. d. Golfes von NeajKl, xrrr. 

 Radiolaria, 1885, pi. v. Cf. J. Miiller, Ueber die Thalassieollen, etc. Ahh. K. 

 Akad. Wiss. Berlin, 1858. 



■j- Celestine. or celestite, is SrS04 with some BaO replacing SrO. 



% With the colloid chemists, we may adopt (as Rhumbler has done) the terms 

 spumoid or emulsoid to denote an agglomeration of fluid-filled vesicles, restricting 

 the name froth to such vesicles when filled with air or some other gas. 



