Potash on the Siliceous Skeletons of Sponges. 287 



bedded spicules are again revealed, Carter says no more than 

 that it is a chemical question, and that in consequence he 

 leaves it to the investigations of the chemist. It was partly 

 Avitli the hope that I might be able to throw a little light on 

 this subject that the following investigation was undertaken. 



The spicular silica of sponges has for a long time been 

 known to be somewhat soluble in caustic potash, so that spon- 

 gologists seldom, if ever, employ this reagent to remove from 

 sponge-skeletons the organic matter associated Avitli them — -a 

 use for which otherwise it would be well fitted. Considering 

 this, it appeared to me that in caustic potash we possess a 

 valuable means for the analytical treatment of sponge-spicules, 

 so as to be able to dissect them into their structural elements, 

 and to reproduce in them those characters wldch they exhibit 

 in the deciduous state both recent and fossil. 



Specimens of various vitreo-hexactinellids were accordingly 

 procured, and subjected to a microscopical examination to 

 make sure that they were in the fresh state, or, in other words, 

 liad not begun to be excavated by internal spicular casts. 

 Til is ascertained, they were next exposed to the action of strong 

 boiling caustic potash for varying intervals ol" time. The 

 changes thus produced will now be described. 



Dactylocalyx subglohosus. 



The solid libres of this sponge soon exhibit delicate canals, 

 which start from the broken ends of the fibres, and radiate, 

 .as a rule, from the nodes of the skeletal network, each stellate 

 group of canals forming a single sexradiate system with its 

 centre generally situated in or about the centre of a node. 

 The canals are at first very small and may be regarded, I think, 

 as representing simply the enlarged axial canals of the im- 

 bedded sexradiate spicules. This stage of solution we may 

 call stage 1. As the boiling continues, the canals progressively 

 enlarge and come to occupy a very considerable portion of the 

 interior of the fibre, till at length they appear to coincide with 

 the walls of the imbedded spicule on which the fibre is con- 

 structed (stage 2). That the canals do at length really repre- 

 sent the original spicule is shown by the reappearance of the 

 besom-shaped spicule in Aphrocall isles. Finally the canals 

 extend so far and Avidely as to touch and open into each other 

 and thus channel the fibre continuously or render it truly " fis- 

 tulose " (stage 8) . This is the condition in which the skeletons 

 of the vitreo-hexactinellids are usually found when fossil, though 

 it is by no means rare to find the preceding stage well repre- 

 sented also. It is moreover the state in which the onlv known 



