OF THE SPONGIADJ3. 63 



or structurally with horn, and Dr. Grant has judiciously 

 rejected the term " horny fibre" as applied to the sponges 

 of commerce, and has substituted that 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 Spongiadae, where 

 it is subordinate to the spicula in the construction of the 

 skeleton, and appears more especially in the form of an 

 elastic cementing medium. In a dried state it is often 

 extremely 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 con- 

 centric 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 

 skeletons of the Spongiadse in some species, as in those of 

 commerce, there is no adherent power at the apex of the 

 young fibre, excepting with parts of its own substance; 

 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 adhesive character appears to be 

 confined to the earliest stages of its production only, as 

 exhibited at the apices of the newly-produced fibres, the 

 external surface immediately below the apex exhibiting no 

 subsequent adhesive property. 



Lehman, in his ' Physiological Chemistry,' Cavendish 

 Society's edition, vol. i, p. 401, states that Spongia officinalis 



