290 REVISION OF THE MONAXONID SPONGES, 1., 



tudinally disposed spicules, which are chiefly subtylostrongyla. 

 This belt is crossed, at rather wide intervals in a radial direc- 

 tion, by single spongin-fibres, each ensheathing the basal portion 

 of a subtylostrongyle; and immediately external to the belt, the 

 already-mentioned radial fibres, composed of closely packed 

 parallel oxea, take origin, each fibre having, as its axis, one of 

 these radially directed long spicules. The remaining skeleton 

 of the subcortical layer consists of abundant oxea arranged in an 

 irregular, somewhat halichondroid, fashion, even if, for the most 

 part, more or less longitudinally directed. 



The radial fibres increase in stoutness on their way across the 

 subcortical layer, and on their arrival at the cortex sometimes 

 exceed 200 fi in diameter. On entering the cortex, each fibre 

 spreads out into a widely divergent brush, the terminal spicules 

 of which project slightly beyond the surface. Apart from the 

 occasional long spicules which project from the surface, these 

 spicule-brushes constitute the entire cortical skeleton. 



Spicules. — {a) The oxea are very slightly fusiform, mostly 

 straight, and nearly always irregularly ended, very frequently 

 having abrupt, more or less mucronate, sharp points. They vary 

 from about 340 to 760 /x, but are usually between 450 and 650//. 

 in length, and attain (rarely; to 15 or 16 /x in stoutness. (6) The 

 so-called subtylostrongyla are usually only very faintly expanded 

 at the basal end, and often are without any sign of such enlarge- 

 ment; occasionally, however, the phyma is so well developed that 

 the spicule could be called a tylostrongyle. They are nearly 

 always quite straight, taper slightly from base to apex, and vary 

 from (rarely) less than '2 mm. to upwards of 4 mm. in length, and 

 from 10 to 35 /x in diameter measured just above the base. 

 Among the slenderest spicules, there are some which are gradu- 

 ally sharp-pointed at the apex, i.e., are subtylostyli. 



Histology. Rounded cells, about 12/x in diameter, containing 

 brownish granules, occur abundantly in all parts excepting the 

 cortex. The flagellated chambers are confined to the axial 

 region of the sponge, and are of rounded shape, measuring about 

 25 /x in diameter. The cortex consists of a dense fibrous tissue, 

 resembling that of the cortex of Donatia. 



