HEXACTINELLIDA 1 97 



CLASS II. HEXACTINELLIDA 1 



Silicispongiae, defined by their spicules, of which the rays lie 

 along three rectangular axes. The canal system is simple, with 

 thimble-shaped chambers. The body-wall is divided into endosome, 

 ectosome. and choanosome. 



Some authors would elevate the Hexactinellida to the position 

 of a third main sub-group of Porifera, thus separating them from 

 other siliceous sponges. In considering this view it is important 

 to realise at the outset that they are deep-water forms. They 

 bear evident traces of the influence of their habitat, and like 

 others of the colonists of the deep sea, are impressed with 

 marked archaic features. Yet they are still bound to other 

 Micromastictora, first by the small size of their choanocytes, and 

 secondly by the presence of siliceous spicules. This second 

 character is really a double link, for it involves not merely the 

 presence of silica in the skeleton, but also the presence in each 

 spicule of a well-marked axial filament. Now this axial filament 

 is a structure which is gaining in importance, for purposes of 

 classification, in proportion as its absence in Calcarea is becom- 

 ing more probable. The Hexactinellida are the only sponges, 

 other than the bath spoDge, which are at all generally known. 

 They have won recognition by their beauty, as the bath sponge 

 by its utility, and, like it, one of their number the Yenus's 

 Flower-Basket forms an important article of commerce, the chief 

 fishery being in the Philippine Islands. This wonderful beauty 

 belongs to the skeleton, and is greatly concealed when the soft 

 parts are present. 



AYe have said that the Hexactinellids are deep-sea forms ; 

 they are either directly fixed to the bottom or more often 

 moored in the ooze by long tufts of rooting spicules. In the 

 "glass-rope sponge," the rooting tuft of long spicules, looking 

 like a bundle of spun glass, is valued by the Japanese, who 

 export it to us. In Monorhaphis the rooting tuft is replaced by 

 a single giant spicule, 2 three metres in length, and described as 

 " of the thickness of a little finger " ! Probably it is as a result 

 of their fixed life in the calm waters of the deep sea 3 that 



1 F. E. Sclmlze, Challenger Monograph, xxi. 



2 Chun, " Aus den Tiefen des Weltmeeres," 1900, p. 481. 



3 Shipley, "Fauna of the Antarctic Regions." See also p. 216. 



