THE SPONGES. 55 



The prostalia are the large oxjdiacts just referred to and pentacts, 

 which properly are hypodermal, but which may protrude. The oxydiacts 

 measure 2 cm. x 150 /x to 3.5 cm. x 225 /x, are slightly curved, smooth, 

 tapering evenly toward each end. The smaller ones run out to very fine 

 points. In the larger ones the extreme ends are broken off. 



The prostal and hypodermal pentacts have paratropal, paratangen- 

 tial rays. Fig. 5, Plate 6. The shaft is 6 mm. or more in length, 100 /a 

 thick at the upper end, tapers evenly to a point, and is very faintly 

 tuberculate, appearing smooth. The paratangential rays measure 2.5 mm. 

 X 75 /I to 3.75 mm. x 85 n, and are nearly straight or slightly curved ; 

 tapering evenly to points ; minutely tuberculate. Small forms of the 



same spicule occur, with paratangential rays as short as 550 /m. The 



tubercles on the paratangential rays are fine, closely set prickles, which 

 in general project toward the apex of the ray, Fig. 7, Plate 6. They 

 diminish in number toward the proximal end of the ray, and here may 

 be nearly or quite absent (Fig. 5, Plate 6). In some spicules the 

 tubercles are so fine that the whole ray appears nearly smooth. The 

 tubercles are outgrowths of the superficial silicious layer, which is fre- 

 quently cracked, and may peel off, leaving the ray smooth. 



The parenchymal macroscleres are chiefly long slender diacts, more 

 or less curved, extending in all directions through the body ; many 

 running parallel to the surface ; others more or less radially to the 

 surface and protruding slightly. Length extremely various, 4 mm. to 

 about 1 cm. ; diameter, 8-24 fx. The two ends of a spicule are unlike : 

 one end sharp-pointed ; the other end blunt-pointed, or rounded without 

 enlargement, or dilated. Both ends are minutely spinose, the shaft smooth. 

 Smaller oxydiacts, about 0.5 ram. x 8 [m, are also common in the paren- 

 chyma ; wMth median enlargement and axial cross; both ends running out 

 to fine points ; feebly spinose along the whole length. 



In the parenchyma the following additional macroscleres occur rarely: 

 hexacts, with rays about 0.5 mm. x 30 fi, smooth, tapering evenly to 

 points ; tauactines and stauractines, rays smooth, tapering evenly to points, 

 about 250 /i x 8 /a. There are no discoverable Iiypogastralia. 



Discoctasters are abundant in the parenchyma. Many conform to 

 the type, having 8 principal rays, each of which bears usually 3 slender 

 terminals; terminals minutel}' enlarged at the end; centrum with a 

 rounded protuberance in the centre of each set of 4 iwya. The principal 



