J. G. WALLER ON THE BURROWING SPONGE. 273 



upon it as abnormal. But in Dysidea fragilis we have the whole 

 structure of the skeleton made up of silicious particles, enveloped 

 or agglutinated by the keratose fibres. Had this " boring sponge " 

 possessed such a structure, one might, perhaps, be persuaded it had 

 the power attributed to it ; unfortunately, it is one of the simplest 

 and feeblest of its class, consisting chiefly of thin structureless 

 membranes upon which spicula are indiscriminately thrown. But the 

 " silicious particles," which Mr. Hancock said he found after he 

 application of nitric acid, and which he has figured, have a pris- 

 matic form, which suggests that he has m some way or other de- 

 ceived himself. That this was the case seems evident, for he sent 

 his specimen, which contained these erosive organs, to Dr. Bower- 

 bank, who discovered that they were only disintegrated cells of 

 carbonate of lime — the debris, in fact, left by the annelid, by whom 

 the perforations were made. Such I have myself often seen, the 

 sponge having covered them by its membrane during its growth, 

 showing that they must have been the product of a previous 

 " borer." And it suggests to us this question— What did the 

 " boring sponge " do with the materials it removed ? They must 

 have passed through the excurrent canals, and it is inevitable that 

 in doing so these canals would be found with numerous particles 

 adherent to their surfaces. None, however, are found. Then, again, 

 if. the sponge was a miner, he would have his tools at the most 

 distant part of his mine, where he was engaged in enlarging his 

 dwelling. Now what do we find here ? A thin, pellucid mem- 

 brane, so transparent that it is scarcely to be distinguished from the 

 colour of the shell it covers, having more the appearance of a 

 varnish than anything else. Indeed, we should not be conscious 

 of any structure but for a few scattered spicula which lie upon its 

 surface. Nothing can be more consistent than this appearance in 

 a growing sponge ; nothing less so in a sponge which is said to 

 make the dwelling it occupies. We cannot possibly imagine a 

 structure, so feeble capable even of conveying the power of excava- 

 ting at all without an entire subversion of the mechanical law, viz., 

 that an effect cannot be greater than its cause. The feeble, simple 

 character of this organism seems indeed to give us the reason of 

 its seeking for a protection in holes and corners from external 

 attacks. It is remarkable, however, that it does not get into any 

 opening that lies in its way. It does not build in the sand-con- 

 structed tubes made by some of the annelids, nor in mere crevices ; 



