74 ANATOMY AND PHYSIOLOGY 



of sponge. In the production of the young fibres, the 

 projection of the new keratode and the secretion of the 

 new spicula appears to be simultaneous. In this class of 

 structure the keratose fibre is the predominant element, 

 and the spicula the subordinate one, and we accordingly 

 frequently find the fibres destitute of spicula for short dis- 

 tances ; but these occurrences are the exceptions, and not 

 the rule of the structure. Fig. 262, Plate XIII, represents a 

 portion of a longitudinal section of the skeleton of Hali- 

 c/tondria oculata, Johnston (Chalina, Bowerbank). 



The mode of the progressive development of this form of 

 fibre is interesting. In a young specimen of Chalina 

 Montayid, Bowerbank, I observed that when a new fibre 

 was projected from the skeleton it usually contained a 

 single spiculum, thinly covered by keratode at the apex, 

 and more thickly so towards the basal end. Another 



i/ 



spiculum followed the first, the terminations of each over- 

 lapping the other; and at the junction of the two the 

 keratode was accumulated in the form of a plumber's joint, 

 as represented in Fig. 263, Plate XIII, so as to give additional 

 strength to the junction of the spicula, while the middle 

 portion of the second spiculum remained very thinly 

 covered by keratode. When the distal end of the new 

 fibre has attained its proper length, or has become cemented 

 to the side of another fibre, the remaining portion of the 

 keratode is produced, and the fibre then assumes a regular 

 cylindrical form. 



3. Hetro-spicidated Keratose Fibre. 



This form of fibre has a somewhat irregular axial series 

 of spicula, with occasionally exter-axial ones disposed in 

 accordance with the axial spicula, and others at intervals at 

 right or nearly right angles to the axis of the fibre. The 

 only sponge in which I have found this form of fibrous 

 tissue is Diplodemia vesicula, Bowerbank, from deep water, 

 Shetland. Fig. 273, Plate XIV, represents a portion of a 

 skeleton-fibre. 



