400 Mr. W. J. Sollas on the 



protoplasm, from which in many cases very fine fibrils are 

 prolonged irregularly in various directions. Here and there 

 greyish granular oval cells (PL XVII. fig. 16), which have a 

 very distinct outline, and stain deeply with carmine, occur in 

 the matrix. Each one generally occupies a corresponding 

 cavity in the matrix, from which it is completely separated 

 except at one or two points of contact ; this separation is 

 probably the result of contraction after placing in spirit. 

 These cells are often pointed at one end, which differs from 

 the rest of the cell in being hyaline and more refringent. 

 The pointed end is sometimes produced into a fine structure- 

 less fibre (PI. XVII. fig. 15). 



2. The Math. — The substance of the mark consists of 

 minute granules abundantly dispersed throughout a struc- 

 tureless colourless matrix, forming a greyish tissue, in which 

 small oval nuclei occur at intervals. It stains generally with 

 carmine, but not so deeply as the corresponding tissue of 

 Stelletta Normani and Geodia Barretti. 



It never presents any appearance which might suggest that 

 it consists of a number of separate but closely apposed cells, 

 although, from the remarkably perfect manner in which other 

 delicate histological features of the sponge are pi'eserved, one 

 would expect evident signs of such a constitution if it existed ; 

 and as, on the other hand, it is not a mere gelatinous connec- 

 tive tissue like the mark of Thenea Wallichii and many other 

 sponges, we may at least provisionally regard it as a genuine 

 syncytium. 



3. The Skeleton. Long-shafted Spicules. — The long ace- 

 rates lie longitudinally side by side, forming spicular fibres, 

 which take chiefly a radiate direction from the centre of the 

 sponge towards the rind. On approaching the rind the con- 

 stituent spicules of each fibre diverge from each other and 

 pass out of the sponge in the form of a fascicle ; at the same 

 time trifid spicules put in an appearance, the coarser forms 

 having their heads within, below, and outside the rind, the 

 finer, grapnel-like and slender fork-like forms bearing their 

 heads exclusively outside and at some distance from the rind. 

 In Geodia Barretti, it will be recollected, all forms of trifid 

 spicules were exclusively confined to the interior of the sponge, 

 their heads occurring just beneath the rind. The frequent 

 irregularity in the form of the bifurcated ternate spicule of 

 hops is caused by the obstruction of the globates in which 

 it is imbedded, these obstacles hindering its free growth. 

 With each spicular bundle or fibre is associated a tract of 

 tissue very similar to, and, indeed, almost identical with, 

 that of the subcortical layer ; it consists of (i) finely granu- 



