OF THE SPONGIALLE. 69 



straight, and most frequently they curl considerably in 

 different directions. They appear to be perfectly solid ; I 

 could not by the aid of polarization discover the slightest 

 indication of a central cavity. They vary in diameter 

 in different species of sponge, and frequently so even 

 in the same individual. In a species of Stematumenia from 

 the Mediterranean, I measured an average-sized fibre which 

 was 3^5 inch in diameter, while a smaller one, closely 

 adjoining, measured ~ inch. In this genus these fibres 

 are more fully developed and larger in size than in any 

 other sponges with which I am acquainted. In the 

 sponges of commerce, in the membranes of which they are 

 exceedingly numerous, they are much more slender. In one 

 of the excurrent canals of the common honeycomb sponge, 

 one of the largest measured Tr ,\^, inch in diameter, and one of 

 the smallest p,^ inch. In the dermal membrane of the 

 best Turkey sponge they were still less, not exceeding 



u;,oou men. 



This description of fibre is not an absolutely necessary 

 constituent of a sponge, and in many of the Halichondrace- 

 ous tribes it is exceedingly difficult to find even a single 

 straggling fibre on the interstitial or dermal tissues, while 

 in other genera, as in JSpotiffia, Stematumenia, and Alcyoii- 

 cellum, they form an important element in the structure of 

 the compound membranous tissues, in which they are 

 closely disposed in parallel lines, occasionally giving off 

 branches, but never appearing to anastomose with each 

 other like the larger fibres of the skeleton. 



These fibro-membranous tissues were described by me 

 in the ' Annals and Magazine of Natural History,' vol. 

 xvi, page 406, plate xiv, figs. 1, 3, 4, and 5, in my 

 description of the genus Stematumenia. 



If a small portion of the dermal membrane of a young 

 Stematumenia be carefullv removed from the surface of the 



t/ 



sponge, the primitive fibres will be seen projecting from 

 the edges of the membrane in considerable numbers ; and 

 occasionally they may be seen to be furnished with a 

 terminal bulb, the greatest diameter of which is about three 

 times that of the fibre. The bulbs are variable in form ; 



