698 ON CONCRETIONS, SPICULES. [ch. 



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

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

 sulphates of strontium and of barium in kindred forms renders it 

 all but certain that they are actually true crystals of celestine*. 



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 protoplasm, bubbled up into 

 a multitude of alveoU or vacuoles, filled with a fluid which can 

 scarcely differ much from sea-waterf. According to their surface- 

 tension conditions, these vacuoles may appear more or less isolated 

 and spherical, or joined together in a "froth" of polyhedral 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 some cases a large number of such simple individual 

 organisms are associated together, forming a floating colony; and 

 it is probable that many others, with whose scattered skeletons we 

 are alone acquainted, had likewise formed part 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 production of a continuous skeleton, of netted mesh or 

 perforated lacework, sometimes replaced by and oftener associated 

 with minute independent spicules. Before we proceed to treat of 

 the more complex skeletons, we may begin by dealing with those 

 comparatively few simple cases where the skeleton is represented 

 by loose, separate spicules or aciculae, which seem, like the spicules 

 of Alcyonium, to be isolated formations or deposits, precipitated in 

 the colloid matrix, with no relation to cellular or vesicular boun- 

 daries. These simple acicular spicules occupy a definite position 

 in the organism. Sometimes, as for instance among the fresh- water 

 Heliozoa (e.g. Raphidiophrys), they lie on the outer surface of the 

 organism, and not infrequently (when few in number) they tend to 



* Celestine, or celestite, is SrS04 with some BaO replacing SrO. 



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

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

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



