OF THE SPONGIAD^. 71 



among the groups of cells, that the bulbs in connexion with 

 the fibres were to be discovered. 



This form of fibrous tissue is not essentially a sponge 

 structure; it enters largely into the composition of the 

 membrana putaminis, and the shell of the egg of the 

 domestic fowl, and I have also found it in the foliated por- 

 tion of a coral, Pavonia lactuca, when deprived of its 

 earthy matter by dilute hydrochloric acid ; and it occurs 

 also in the membranes of some species of Ascidians. Prof. 

 Bowman, in his treatise on mucous membrane, in the 

 ' Cyclopaedia of Anatomy and Physiology,' in his descrip- 

 tion of the white fibrous element of areolar tissue, says, 

 " Beside these bands, commonly called fasciculi, there are 

 some finer filaments of the utmost tenuity, which seem to 

 take an uncertain course among the rest." These fila- 

 ments, it is very probable, are the homologues of the 

 primitive fibrous tissue which I have thus described. 



2. Keratose Fibrous Tissue. 



General character of the keratose fibres of the horny 

 skeleton. — The essential character of the fibres of the horny 

 skeleton is, that their normal form is always that of a cylinder, 

 while the network of the skeletons of the Halichondroid 

 sponges, which approach nearest in structure to that of 

 spiculated keratose fibre, is always more or less irregular 

 in shape ; and in the fully developed state generally com- 

 pressed to a very considerable extent ; but a careful exami- 

 nation of the youngest portions of the two forms of skeleton- 

 tissue wall always render the difference in the two structures 

 apparent. In the spicixlated keratose fibre the keratode is 

 always the predominant element, and the spicula the sub- 

 ordinate one ; while in the skeletons of the Halichondroid 

 sponges the spicula always predominate, and the keratode 

 is merely the secondary or surrounding medium. In the 

 former structure, in the extension of the terminations of 

 the skeleton, the keratode is the leading element, while in 

 the latter the spicula take the lead. 



The fibre is formed of a succession of concentric layers, 



