BRITISH SPONGIAD^. 337 



opacity of the sarcode ; and this observation applies equally 

 well to the interstitial structures of the interior of the 

 sponge. The minute spicula rarely exceed 555th of an inch 

 in length, and require a linear power of about 600 to render 

 them distinctly visible. They are profusely spinous, and 

 the spines towards the middle of the spiculum are frequently 

 as long as the greatest diameter of the shaft of the spiculum 

 on which they are based. 



The spicula of the skeleton of this sponge are very re- 

 markable. The spiculated triradiate ones have the three 

 radii applied evenly to the under surface of the dermal 

 membrane, w^hile the shaft or spiculate ray is projected 

 amid the tissues beneath, at right angles to the surface of 

 the sponge. In the biangulated spicula the two points of 

 one angle are applied to the inner surface of the denual 

 membrane, and the opposite pair of points are immersed in 

 the body of the sponge; and this appears to be the in- 

 variable position of these spicula at the surface of the 

 sponge; but in the interior I could not detect any such 

 definite arrangement, but the spicula appear to be dis- 

 persed on the tissues at such distances from each other as 

 just to allow of their apices touching, or slightly crossing 

 each other, and the two forms appear to be indiscriminately 

 mixed together. Throughout the whole of the tissues we 

 find minute entirely spined cylindrical spicula, like those 

 of the dermis, dispersed in great abundance. 



The tricurvate tension spicula of the interstitial mem- 

 branes are few in number and very slender ; the central 

 curve is distinctly produced, while the terminal ones are 

 but very slightly so. 



The two forms of skeleton spicula in this species are very 

 similar in degree of size and stoutness, and in some positions 

 they are not readily to be distinguished from each other, 

 but they may always be determined by the fact, that in the 

 spiculated triradiate form the central canals, at their junc- 

 tion in the middle of the spiculum, form three angles of 

 each about 120°, and three others, which are right angles; 

 while those of the biangulated spiculum form four right 

 angles only at their crossing at the centre of the spiculum. 



