BRITISH SPONGIADiE. 27 



In form and size it is very like G. ciliata, but the colour 

 and the strikingly tessellated character of its surface at 

 once distinguishes it from that species. The tessellated 

 appearance is produced by the corymboid fasciculi of short 

 acerate spicula, which are based one on the centre of each 

 of the distal ends of the interstitial cells, and the spicula of 

 which diverge slightly from each other until, meeting the 

 apices of the adjoining fasciculi, they mutually compress 

 each other into four-sided figures ; but although these 

 bundles present so dense an aspect at their distal termina- 

 tions, there appears always a free space around each of 

 their proximal ones, by the means of which free access to 

 the external water is afforded to the pores after all large 

 extraneous matter has been separated by the distal termi- 

 nations of the fasciculi. The difference that exists in the 

 defences of the porous systems of this species and that of 

 Grantia ciliata is very remarkable and interesting. The 

 same end is attained in each, but by completely opposite 

 contrivances. 



The internal defences of the cloaca are remarkablv stout 

 and abundant. The external defensive organ also, although 

 apparently like that of G. ciliata, differs from it in structure 

 in several essential points. Thus, in G. ciliata the ciliary 

 spicula around the mouth of the cloaca are remarkably 

 long, slender, and flexible, and are, therefore, supported" at 

 their bases by short but very strong acerate spicula ; while 

 in G. tessellata we find the ciliary spicula stouter and very 

 much more rigid than in the kindred species, and there is, 

 accordingly, a total absence of the large acerate subsidiary 

 spicula ; but as some support of the proximal ends of the 

 ciliary spicula appears to be still necessary, we have them 

 bound together at their bases, for about one third or one 

 fourth of their length, by a beautiful regular interlacement 

 of rectangulated triradiate spicula. Again, the same end 

 being attained in each species by an admirable variety of 

 construction. 



The whole of the spicula of this sponge are stouter in 

 their proportions than those of G. ciliata. I have seen 

 but two specimens of this species ; they are both of about 



