AKD PHYSIOLOGY OP THE SPONGIAD^. 
757 
about 100, we find in the centre of the fibre a series of grains of extraneous matter, 
occupying the place of the large continuous canals of the fistular forms of fibre. The series 
of extraneous matters is not always continuous, and when an interruption takes place the 
fibre becomes solid, or faint traces only of a central cavity remain. The mode of the 
inclusion appears to be due to the extreme terminations of the young fibres being viscid, 
and thus seizing on any extraneous particles that happen to come in contact with them. 
The growing keratode quickly envelopes them, and proceeding on its course of extension, 
seizes in like manner on other particles of sand or solid matter, and thus a continuous 
and regular chain of extraneous material is imbedded in the axis of the fibre, as repre- 
sented by figs. 1 & 2, Plate XXVIII. This description of fibre is found in a great 
variety of keratose sponges, and especially among the coarse rigid skeletons of the 
Australian species, as represented by fig. 1, Plate XXVIII . ; and among the flexible 
sponges, as represented by fig. 2, Plate XXVIII. 
8. 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 power in the young progressing fibre not 
being confined to its apex only, its sides also seize upon the surrounding grains of solid 
matter, and the keratode speedily passing round and enveloping them, the whole fibre 
becomes a solid 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, J ohnston, 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 growing young fibres, sometimes entirely, and at others only partially enveloped 
by the progressing keratode. Figs. 3, 4, & 5, Plate XXVIII., represent portions of 
fibre from the same individual. 
This genus of sponges appear, to the best of my knowledge, to be the only animals 
that construct an internal skeleton almost entii’ely of extraneous materials. 
Siliceous Fibre. 
This structure is widely difierent 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 the manner of the secretion of pure keratode in the 
fibres of the sponges of commerce. When cleansed from the sarcodous matter by which 
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