278 REVISION OF THE MONAXONID SPONGES, i., 



separating the tubercles are not of the nature of specialised pore 

 grooves; immediately underlying these grooves, however, and 

 roofed over only by membrane, are narrow cleft-like spaces in the 

 cortex, so that if a thin superficial layer of the sponge were pared 

 off, the surface then would appear imperfectly divided into poly- 

 gonal areas by discontinuous narrow cracks. The characteristic 

 minute reticulation of the surface (Plate xv., fig. 4) is found, on 

 microscopical examination, to consist of polygonal or rounded 

 meshes, averaging 150/x in diameter, separated by narrow 

 partitions in which are spherasters and megascleres, the latter — 

 directed perpendicularly to, and slightly projecting beyond the 

 surface — being the terminal spicules of the branches into which 

 the radial skeletal fibres divide on entering the cortex. Super- 

 imposed upon this reticulation, and immediately external to it, 

 is a finer reticulation with meshes about 25 /x in average diameter, 

 w^hich meshes are formed by pauciserial lines of tylasters and 

 enclose each a single pore. 



The spicules composing the radial fibres are styli, which, 

 almost without exception, are more or less blunt-pointed — occa- 

 sionally to such an extent as to approximate in form to strongyla; 

 their maximum size in the several specimens varies from 1425 

 by 20 /x to 1600 by 24 /x. In the cortex, as the fibres approach 

 nearer to the surface, their megascleres gradually diminish in 

 size, and become cylindrical and abruptly sharp-pointed ; the 

 smallest of these terminal spicules are less than 240 /x in length. 

 Between the fibres, in the choanosome, a fair abundance of 

 radially directed megascleres occur, which are similar in form to 

 those of the fibres, except that a few of them are slenderer and 

 usually gradually sharp-pointed. 



The microscleres are spherasters, spheres, tylasters, and oxy- 

 asters. The spherasters are abundant throughout the choano- 

 some, and, in the cortex, occur chiefly in a broad superficial layer; 

 they have rarely less than 13, and normally not less than 9, 

 actually countable rays, and measure at most 65 /x in diameter; 

 when, as occasionally happens, the number of rays is less than 

 nine, it is because of the non-development of one or a few rays, 

 and the spicule is then no longer centro-symmetrical. The 



