264 ART. 7. 1. IJIMA : HEXACTINELLIDA, IV. 



in or near the middle, 9-15/^. The spicular center is generally 

 plain ; otherwise it is externally marked, sometimes by an annular 

 swelling and sometimes by knobs which may occur either in a 

 single pair or in two pairs. The rough-surfaced rays generally 

 taper slightly tow^ards the tip which is rounded off or conically 

 pointed. 



The gastralia (PI. XX., fig. 13) are hexactins with rays 

 appearing exactly like those of the dermalia. Length of rays, 

 165-245/^ (about 200/-« on the average). Breadth at base, 11- 

 15/^. All the six rays in the same spicule may in general be 

 said to be nearly equally long, though in some cases the distal 

 ray has been found to be somewhat shorter, and the free proximal 

 ray to be somewhat longer, tlinn the paratangential rays. The 

 microtubercles may be slightly more pronounced on the proximal 

 ray than on any other, but in any case the differentiation of that 

 ray is never carried out to any considerable degree. The unusually 

 small and slender-rayed gastralia, which are occasionally met with, 

 are apparently those that have not yet attained full development. 



The oxyliexasters occur much more abundantly in the deeper 

 parts of the wall than in the periphery, as has been pointed out 

 by F. E. Schulze. Diameter, 102-160 i"; usually about 120 /i. 

 The microtubercles on the basal parts of terminals show the 

 tendency to develop into barbs in an unusual degree. 



One point concerning the spicules, which has not been 

 mentioned by F. E. Schulze, is the fact that they occur in two 

 slightly differing forms, the one chiefly in the subdermal space and 

 the other in more deeply situated parts. It must however be said 

 at once that the two varietal forms intersirade. 



