212 I. IJIMA : HEXACTINELLIDA. I. 



The large oxysta,uractin-principalia of the circular and the 

 longitudinal skeletal beams may measure 45 mm. in length of the 

 lonaitiidinal axis and 00 n in breadth of ravs near the center. 

 Other spicules of the beams are almost exclusively thetactins of 

 the usual shape, rarely diactins and paratetractins. Tlie 

 thetactins and diactins are occasionally sufficiently large and 

 strong to be classed with the principalia ; all the rest are thin, 

 ranging from 7 to 20 n in thickness near the center. 



The parenchymalia of the loose tissues are again predomi- 

 nantly thetactins, quite variable in size, running partly in strands 

 and partly in more or less diffuse arrangement. Occasionally 

 there occur paratetractins, rarely stauractins and pentactins, 

 especially among the larger parenchymalia. Many of the comi- 

 talia in the strands are thin and rather short diactins, provided 

 with four tubercles at the center. 



The oscularia (PI. VI, tig. 10) are predominantly diactins, 

 which have been very aptly called by Marshall compass-needle- 

 like. Length 200-600 a and over ; breadth near the middle 

 6-17 a. The center usually with two or four oppositely placed 

 tubercles. Tlie oscularia are densely crowded, Avith their long 

 axis disposed paratangentially to the edge of the oscular mem- 

 brane. In the ring-like zone occupied by them, the innermost 

 are generally the smallest. The outermost are the largest, and 

 some of these may have one or more of the central tubercles 

 produced into shorter or longer lateral rays, thus assuming the 

 form of thetactins, tetractins or pentactins and even hexactins. 

 These lead over the oscularia on the one hand into the paren- 

 chymalia, and on the other, into the gastralia. 



