CHAUNOPLECTELLA CAVERNOSA. 59 



appears to be a structure which begins to be formed at a com- 

 paratively late stage of life. 



The excurrent canalar apertures as compared with the incurreiit, 

 are on the whole somewhat larger but less numerous. This is 

 evidently in correlation with the difference in extent between the 

 inner and the outer choanosoraal surfaces respectively occupied by 

 the two sorts of apertures. The fact just mentioned may be laid 

 down as a rule holding good for a large number of Hexactinellids 

 of similar shape. 



The fineness of all the spicules, the wide subdermal space 

 and the broad canals — both incurrent and excurrent — separated 

 by thin septal walls, all combine to give to the sponge a light, 

 delicate and cavernous character, which is especially pronounced 

 in specimens of larger size (fig. 4, PI. IV). 



Spiculation. 



The following account of the spiculation refers, unless other- 

 wise stated, to large and full-grown individuals as represented 

 chiefly by the fragment shown in PI. IV., fig. 1. It may be 

 regarded as holding good also for young specimens, but these are 

 not wdthout noteworthy points of difference, of which special 

 mention is required. 



The parenchymalia are, it may be said, mainly hexactins 

 and diactins, although those with 3-5 rays are not uncommon. 

 They are also very variable in dimensions. The rays are smooth, 

 gradually tapering and terminate either simply pointed or with a 



