254 W. J. SOLLAS ON PHARETROSPONGIA STRAHANI. 



Thus the siliceous spicules to be sought for have been found, and 

 just that particular evidence which was thought to be needed to 

 render the demonstration of the affinities of our sponge complete has 

 been supplied. 



I take this opportunity of explaining more fully certain difficulties 

 which had only been touched upon in the body of the paper. The silica 

 of sponge-spicules is a very different substance from the mineral silica 

 of quartz. The latter is to all intents perfectly insoluble in carbonated 

 waters and most other solvent fluids ; it is crystalline with a compa- 

 ratively high refraction-index, and is composed solely of oxygen and 

 silicon. The former, on the other hand, is colloidal, with a compara- 

 tively low refraction-index, is soluable in caustic potash and in carbo- 

 nated waters, and consists of silica (probably one of the silicic acids) in 

 intimate combination with organic matter. With substances so widely 

 different it is not wonderful that a replacement by calcic carbonate 

 should be common with the one, and unknown if not impossible with 

 the other. The subject of the replacement of organic silica is a new 

 one, and requires independent working out ; next to nothing has 

 hitherto' been known about it, partly because siliceous organisms are 

 less abundant than calcareous ones, and partly because they have been 

 less studied. I need here only remark that while Sipkonia exhibits 

 the structure of a Lithistid (siliceous) sponge, Stauronema of a Hex- 

 actinellid (siliceous) sponge, and Pharetrospongia of a Thalyosian 

 (siliceous) sponge, yet the fossil skeletons of all three frequently 

 occur now in a calcareous state ; and it would be singular if these 

 three structures, so different from one another, and so similar to 

 their existing allies, should all have had their " doubles " in past 

 time resembling them in every respect except in the one im- 

 portant particular of chemical composition — and this the more 

 especially since each of them still presents cases in which it even now 

 exhibits a siliceous composition. If chemical composition and mor- 

 phological structure are invariably associated in the living organisms 

 of the present day, we may depend upon it that they were not 

 divorced in the past. 



EXPLANATION OF PLATE XI. 



Fig. 1. Pharetrospongia Strahani (Sollas), natural size. 



2. Portion of oscular surface, magnified l - 6 diam. 



3. Portion of poriferous surface, magnified 1'6 diam. 



4. Section across a tubular process, showing the character of the network, 



nat. size. 



5. Longitudinal section of fibres : c, point of anastomosis ; s, spicules in 



longitudinal section ( X 60). 



6. Mixed section of the fibre : s, spicules in longitudinal section ; t, trans- 



verse section of spicules in the transverse section of the fibre 

 (X60). 



7. Longitudinal section of fibre from the. oscular network: m, inter- 



mesh., which in many instances becomes obliterated by an enlarge- 

 ment of the surrounding fibre ( X 60). 



8. Group of spicules from the point of anastomosis of several fibres, 



similar to that shown at c, fig. 5, occurring in the section shown in 

 fig. 4 ( X 140). 



