360 REVIEWS. 



" In the above designations of the auxiliary spieula, it must not be understood that 

 their respective titles strictly define iheir offices, as it frf^quently occurs that un- 

 der peculiar circumstances the same form of spieulum is destined to serve two, or 

 «ven three, distinct purposes Thus, an external defensive spieulum will occa- 

 ftionally perform retentive offices for the purpose of securing prey; or internal 

 defensive spicula will combine the offices of defensive spieula against the larger 

 and more powerful of their enemies with that of wounding and securing their 

 smaller ones." 



It would lead us much too far to give particulars of the various 

 and often wonderful forms which occur under the several heads here 

 indicated. It was a very happy idea, the division of all the true 

 sponges into three gi'oups, according to the substance which forms 

 their skeleton. These Dr. Bowerbank denominates Calearea, Silicea 

 and Keratosa. In the first two the skeleton is strengthened by the 

 hard parts of which we have given some account ; in the third it is 

 composed of a peculiar substance, of the nature of which we will now 

 give our author's statement : — 



"Keratode is the substance of which the horny elastic fibres of the skeleton of 

 the officinal sponges of commerce are composed. It has, correctly spealiing, no 

 relationship either chemically or structurally with horn, and Dr. Grant has judi- 

 ciously rejected the term ' horny fibre ' as applied to the sponges of eommsrce, 

 and has substituted tliat of keratose by way of distinction; and in accordance 

 ■with that term I propose to designate the substance generally as keratode. whether 

 it occurs in the elastic fibrous skeleton of true Spongia, which are composed almost 

 entirely of this substance, or of those of the Halichondraceous tribe of Spongiadse, 

 where it is subordinate to the spieula in the construction of the skeleton, and ap- 

 pears more especially in the form of an elastic cementing medium. In a dried 

 state in is often rigid and incompressible, but in its natural condition it is more 

 or less soft, and always flexible and very elastic. It varies in colour from a very 

 light shade to an extremely deep tint of amber, and it is always more or less 

 transparent. In its fully developed condition, in the form of fibre, it appears 

 always to be deposited in concentric layers ; but in the mode of the development 

 of these layers there are some interesting variations from the normal course of 

 production. As we find in Aranea diadema, the common Garden Spider, that the 

 creature has the power of modifying the deposit of the substance of its web so 

 that the radiating fibres dry rapidly while the concentric ones remain viscid for a 

 considerable period, so we find in the production of the young fibres of the skele- 

 tons of the Spongiadae in some species, as in those of commerce, there is no adhe- 

 rent power at the apex of the young fibre, excepting with parts of its own sub- 

 stance ; while in Dysidea. and in some other genera, the apex of the newly-produced 

 fibre is remarkably viscid, adhering with great tenacity to any small extraneous 

 granules that it may happen to touch in the course of its extension (Fig. 272, Plate 

 XIV) ; but this ahhesive character appears to be confined to the earliest stages of 

 its pioduction only, as exhibited at the apices of the newly-produced fibres, tha 



