35 



some of them being slender, much elongated, and terminating acutely, 

 while others are short, thick, and terminating at both ends abruptly in 

 a point. The central cavity in all the forms is distinctly to be seen, 

 (PL iii. fig. 4 — 9). These varieties in form of the spicula are most 

 conveniently obtained by burning small fragments of the sponge be- 

 fore the blow-pipe, and treating the ashes with diluted muriatic acid. 

 All the authors, I believe, who have hitherto treated of this sponge, 

 have described the fibre as consisting of a series of horny tuJbes. Pro- 

 bably this error has arisen in the first place from their great transpa- 

 rency, and secondly from the want of microscopical combinations that 

 would admit of their being examined in such a manner as to allow of 

 their true nature being discovered. For it is only by a careful exa- 

 mination of their structure as opaque objects, by the aid of a Lieber- 

 kuhn, and with a power of about 500 linear, that the solidity of the 

 fibre can be exhibited with certainty. This examination is much fa- 

 cilitated if a small mass of sponge be tied firmly round with thread, so 

 as to compress it into the form of a cylinder ; and then if a section at 

 right angles to its axis be made with a sharp knife, the transverse sec- 

 tions of the fibres will be readily seen, presenting exactly the same 

 appearance that we observe in a similar section of any somewhat sof- 

 tened horny substance. The face of each section is covered with pa- 

 rallel streaks or lines, occasioned by the passage of the knife, but not 

 the slightest appearance of a central orifice can be detected, and the 

 results are precisely the same, whether the specimen under examina- 

 tion be dry or saturated with water ; (PI. iii. fig. 10). It is somewhat 

 singular that in the latter case, when the fibre was supplied with water 

 while beneath the object-glass, it did not present any sensible increase 

 of substance or the slightest variation in form. Upon examining in 

 the same manner the second species of Turkey sponge, and the West 

 Indian species from two localities, the results were precisely the same, 

 — not a single tubular fibre could be detected in any of the numerous 

 specimens subjected to examination. 



If we examine fragments of these sponges as transparent objects, 

 with the same amount of microscopic power (500 linear), we never 

 observe any partial injections of air in the interior of the fibre, which 

 would certainly be the case if they were composed of tubular instead 

 of solid threads. 



The second species of Turkey sponge, as 1 have before stated, is so 

 similar in its form and external characters to Spongia officinalis, as to 

 render it difficult, if not impossible, to distinguish one from the other 

 without the aid of high microscopic powers ; but immediately we sub- 



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