344 



HYALONEMA (OONEJMA) DENSUM. 



ally the thickest part of the ray does not lie at its proximal end but farther out, 

 some distance from the centre of the spicule. The largest hexactin^ observed 

 was of this kind. In this spicule the two longer opposite rays measured 1.1 mm. 

 and 2.4 mm. in length respectively. The longer of the two is 70 /i thick at the 

 base. Its point of maximum thickness is 0.4 mm. from the centre of the spicule, 

 and here the ray measures 80 m in transverse diameter. At the rounded end 

 it is 15 M thick. In the proximal part of the ray the axial thread is simple and 

 quite thin (0.5 n in diameter) ; distally it gradually increases in thickness to 

 5 ju at the end of the ray. In its distal and middle-parts it is not simple but 

 provided, at frequent intervals, with verticillate groups of strongly inclined 

 branches with a maximum length of 15 m- 



The superficial and ordinary choanosomal amphioxes are centrotyle, usually 

 0.6-2.2 mm. long, and 7-27 n thick near the middle. The central tyle, which not 

 infrequently protrudes much more on one side of the spicule than on the other, 

 is 13-37 /J in transverse diameter, that is 1-18 m more than the adjacent parts 

 of the spicule. 



The large axial amphioxes and rod-shaped fragments found in the central 

 part of the sponge are 25-130 n thick. The largest intact one observed is a 

 fusiform amphiox, blunt at both ends, 5 mm. long, and 28 m thick. 



The acanthophores (Plate 94, figs. 34-36) have from two to six, most fre- 

 quently two or four rays. The dimensions of these spicules are tabulated 

 below. 



ACANTHOPHORES. 



