THE SPONGES. 81 



latter into four triangular areas (Fig. 6, Plate 11). The covering of 

 pentacts comes to an end at the margin of the attached surface of the 

 sponge, and at the margin of the oscula, although m the lining membrane 

 of the cloaca-like main canals a very few scattered pentacts are found. 



The scopulae (Fig. 4, Plate 10, Fig. 2, Plate 11) vary in size, but in 

 other respects are alike. The shaft tapers to a fine point. Above, nearer 

 the rays, it is nearly smooth, while below it is distinctly roughened with 

 sharp denticulations. Just below the rays is a terminal enlargement on 

 which the rays are set. The rays are four in number, only very slightly 

 divergent, roughened with minute though distinct denticulations which die 

 away at the base. The upper ends of the rays are rounded and slightly 

 enlarged. The extreme upper end is smooth. In the larger scopulae the 

 shaft is about 580 fi long and 8 /x, thick just below the upper enlargement; 

 rays 80 /x long x 7 /x, thick at the base. The smallest forms seen were about 



two-thirds the size of the largest. The scopulae are in part scattered 



through the interior of the sponge, in part arranged radially or obliquely 

 to the dermal membrane and to the lining membrane of the main canals. 

 Scopulae also lie in or project into the rim of membrane which surrounds 

 the oscula. A dermal scopula is usually found together with a few oxy- 

 diacts, all forming a loose sheaf associated with the proximal ray of a 

 pentact. The scopulae, like the accompanying oxydiacts, may not reach, 

 or may project slightly beyond, the dermal membrane. 



The dermal oxydiacts are arranged radially or obliquely to the sur- 

 face, singly or in small loose sheaves, often associated with the pentacts. 

 The spicules are roughened, taper to fine points, and vary in size, the 

 larger measuring about 600 /x x 5 /x. Smaller forms down to 200 /a x 2 /a 

 are found. The larger are abundant. On these it may be seen that the 

 roughening is of a definite character. Fig. 4, Plate 11. The minute 

 denticulations project obliquely from the shaft and in one direction, 

 pointing when the spicule is in place away from the dermal membrane. 

 Toward each end of the spicule, the denticulations lose their definite 

 character, and it is not always equally distinct on all parts of the shaft. 



In the parenchyma are abundant oxydiacts, similar to the dermal 



forms, although the larger sizes are relatively less common. 



In Hexactinella ventilahriim F. E. Sch., Schulze (1887, p. 331, Plate XCVI.) 

 has described roughened oxydiacts, which he regards as possibly represent- 

 ing uncinates. The denticulations have not the oblique character I have 



11 



