38 THE SPONGES. 



diacts, each with its own sheath of comitalia, lie side by side, but separated 

 by considerable intervals, thus forming a tract, but not a single bundle. 

 Frequently such diacts in one part of their course will be closely bound 

 too-ether, while separate elsewhere. This is the commonest arrangement, 

 although there are bundles consisting of 2 to 4 parallel diacts, which are 

 closely bound together by comitalia throughout their length. 



The principal diacts in the extreme upper part of the body average 

 a smaller size than elsewhere, the larger ones here measuring only 

 10-13 mm. in length by 200-220 fx in thickness. Some of them exhibit 

 the characteristic bend, which is frequently not at the middle, while others 

 are gently curved or nearly straight. They are in general surrounded 

 by relatively few comitalia and lie separately or in approximately longi- 

 tudinal bundles composed of a few (2-4) spicules. In one part of the 

 marginal region, about 5 mm. wide, the parietal apertures are scarcely 

 developed, being here few and small and not rounded but irregularly 

 elongated gaps. In this small region the principal diacts show what is 

 probably the general arrangement before the apertures develop. They 

 are here arranged close together in an approximately longitudinal direc- 

 tion, radiating toward the upper margin, reaching or nearly reaching or 

 projecting slightly beyond this margin, and are without proper comitalia, 

 although the slender diacts are present in abundance between them. 



The dermalia are slender hexacts, which agree with the description 

 given by Ijima (1901, p. 273) for i?. phoenix. The short distal ray is 

 cylindrical or only very slightly expanded, 6-8 fx thick, rounded at the 

 end and with " obsolete microtubercles." Measurements of a characteristic 

 spicule are : distal ray, 80 [x long ; tangential rays, 160 fx long ; proximal 

 ray, 200 /x long; tangentials and proximal, about 8 fx thick at the base. 

 Meshes of the dermal network formed by the tangential rays are in 

 general square, 160-250 jx on the side, including 3 or 4 pores which are 

 50-110 [X in diameter. 



In the upper part of the body, within a distance of 15 mm. from the 

 margin, the dermal hexacts are much less uniform in structure than else- 

 where. While spicules occur, like those which are common lower down, 

 most of the hexacts are larger, stouter forms, in which the short distal ray 

 is either rounded or pointed at the end, and the tangential rays taper 

 conspicuously to points. Many sizes are found, ranging up from spicules, 

 in which the rays are about as long but twice as stout as in the common 



