Sponge-fauna of Norway. 147 



The stellates of the mark possess a comparatively small 

 number of rays, a character in curious consistency with the 

 substitution of biradiate bacilli forv multiradiate dermal stel- 

 lates in this genus ; six or eight rays is a common number ; 

 as many as twelve may occur; but reduction to four, three, or 

 even two, is frequent. With only two rays in the same 

 straight line, the spicule presents a central globular enlarge- 

 ment and looks like a magnified copy of a young bacillus. 



A study of the various forms of these stellates is a study of 

 nearly all the characteristic forms of spicules which distin- 

 guish the various groups of sponges : hexactinellid, tetracti- 

 nellid, triradiate are all here, and a great number of other 

 forms besides. We seem in these spicules to have the results 

 of unhindered variation, freed from the conditions imposed by 

 a selective environment. 



The long-shafted spicules in the specimen under examina- 

 tion exhibit a great variety of monstrous growths : in many a 

 number of siliceous globules cover as excrescences one end of 

 the spicule (fig. 9) ; in others one end becomes bifid, trifid, 

 or even quadrifid (fig. 6), the last deviation being met with 

 in the usually simple proximal end of the shaft of a tetracti- 

 nellid form ; some, again, possess simple ends, but a double 

 body (fig. 7) ; and, finally, in one instance the end of a spicule 

 has sprouted out rays which are arranged in a combination of 

 prong and anchor endings in one (fig. 8). Since anchors 

 do not occur normally in Pachymatisma^ this variation is of 

 particular interest. Bowerbank has already remarked, in his 

 description of the species, on the great variety of these 

 spicules, and particularly says that their radii are frequently 

 bifurcated or contorted to a great extent. This, and the 

 irregular disposition of the trifid spicules, is worthy of notice 

 in connexion with the possibility of a transition from the 

 trifid to the quadriradiate Tetractinellids and the Lithistids. 



Many of the spicules appear to be subject to some disease, 

 by which the central canal has been enlarged till it occupies 

 one third of the entire diameter, the axial thread remaining as 

 a straight sharply defined rod of the usual size ; sometimes it 

 projects quite freely at the end of the spicule. 



By manipulating the cover-glass over a teased fragment of 

 the sponge the axial thread could be " wriggled" out nearly 

 entire from the spicule. It is faintly bluish, transparent, 

 structureless, very flexible, like a piece of sewing-thread, and 

 takes a decided stain with magenta. 



Some of the spicules are united where they touch in crossing 

 each other by some tough brown-coloured matter, which stains 

 with magenta and looks like spongin. 



