78 ANATOMY AND PHTSIOLOGY 



9. Irregular Arenated Keratose Fibre. 



I have described this form of fibre in a paper descriptive 

 of two species of Bysidea, read at the Microscopical Society 

 of London, Nov. 24, 1841, and subsequently published in 

 vol. i, p. 63, of their ' Transactions.' 



The adult and fully produced fibre is frequently half a 

 line or more in diameter. It is built up in all parts of its 

 substance, of grains of extraneous matter, each one being 

 separately enveloped in keratode. The adhesive povrer in 

 the young progressing fibre not being confined to its apex 

 only, its sides also seize upon the surrounding grains of 

 soUd matter, and the keratode speedily passing round and 

 enveloping them, the whole fibre becomes a sohd cylinder 

 of irregularly imbedded molecules. There is a great variety 

 of substances imbedded in these fibres, dependent, as a 

 matter of course, on the amount of material surrounding 

 them at the period of their development. The skeleton of 

 Bysidea fragilis, Johnston, a British species very common 

 on the south coast of England, presents one of the best 

 types of this form of fibre. And single grains of sand are 

 frequently to be found among the fibres of the surface of 

 the sponge, elevated on short pedicels of the rapidly grow- 

 ing young fibres, sometimes entirely, and at others only 

 partially, enveloped by the progressing keratode. Figs. 270, 

 271 and 272, Plate XIV, represent portions of fibre from 

 the same individual. 



This genus of sponges appears, to the best of my 

 knowledge, to be the only animals that construct an inter- 

 nal skeleton almost entirely of extraneous matter. 



Siliceous Fibre. 



This structure is widely dilferent from any of the keratose 

 fibres which contain either secreted silex in the state of 

 spicula, or extraneous silex in the form of sand. The 

 whole substance of the skeleton-fibre consists of solid silex 

 secreted and deposited in concentric layers, exactly after 



