40 THE SPONGES. 



Over a part of the surface, as in the specimen of R. phoenix studied 

 by Ijima (1901, p. 269), the hydrauths of a commensal hydroid cause 

 minute elevations. The elevations are not abundant nor conspicuous, 

 although the opaque body of the hydroid catches the eye. As in Ijima's 

 specimen very large and modified dermal hexacts, mingled with the 

 common dermal hexacts, are found round the hydrozoan body. The 

 spicules differ in some details from those found by Ijima (1901, p. 274, 

 Plate X. Fio-s. 25-27). The distal ray is not club-shaped but cylindrical, 

 beginning to taper near the upper end and running out to a point. The 

 tubercles spread over its distal half. The tangential rays are sometimes 

 short, as in Ijima's spicules, again almost as long as the distal ray, and 

 they vary in length in the same spicule. The proximal ray is generally 

 shorter than, but sometimes as long as, the distal ray. A fairly character- 

 istic such spicule has the following measurements : Distal ray, 1 mm. x 85 /a ; 

 proximal ray, 500 /x x 50 /j. ; tangential rays, 500 /li x 50 /a to 200 /x x 50 /a. 

 The spicules in a single clump vary in absolute size and in proportions of 

 parts. For instance, in some cases a tangential ray is much the longest of 

 all. Many intermediate sizes between the dimensions above given and the 

 ordinary dermal hexact are to be seen. The preservation of the hydroid 

 itself is very imperfect. But it can be seen that the form is a tubularian 

 hydroid, that the hydranths have several tentacles, and are borne upon a 

 slender branching stolon. 



The sieve-plate region presents a simpler structure than in R. phoenix. 

 The sponge ends above in a thickened margin which contains 6 large 

 stauractines arranged in a ring (Fig. 7, Plate 3 ; Fig. 2, Plate 4). The 

 superior rays of the stauractines project obliquely upward and centripetally, 

 as if to form the radial beams of a sieve-plate. With the exception of 

 one stauractine the superior ray of which is bare (Fig. 7, Plate 3), these 

 rays are densely covered with the smaller parenchymalia, chiefly slender, 

 cylindrical diacts 8-20 /a thick, mingled with which are small tauacts, 

 stauracts, pentacts, and hexacts. These latter spicules, like the slender 

 diacts, have smooth cylindrical rays, usually of unequal lengths in the same 

 spicule, rounded or round-pointed at the ends, where they are frequently 

 enlarged and subterminally roughened ; rays, 34-225 /a long, 8-20 /u, thick. 

 The tip of the large stauract ray emerges from its covering. 



Unlike the adult R. phoenix, no principal diacts accompany the superior 

 rays of the large marginal stauracts. The inferior rays which extend 



