110 I. IJIMA : HEXACTINELLIDA. I. 



(lauce at first. It occur?, for instance, in the specimen of fig. 9, 

 in some numbers. Tlie oscularia also begin at about tbe same 

 time to differentiate around tlie oldest formed parietal oscula. 

 Most of them are at first scarcely distinguishable from the 

 gastralia. Essentially tbe same spicular elements as in adults 

 occur in the delicate sieve-plate, though much more sparingly 

 and in looser arrangement. 



Concerning the separate beams of the skeletal tube, I may 

 mention that throughout the body of quite young specimens, as 

 in the growing upper end of older specimens, the oblique systems 

 are wanting or at most are represented by isolated fibers. Of 

 the two other systems, the beams of the longitudinal are always 

 somewhat thicker than those of the transverse (fig. 7). This is 

 at any rate partially due to the presence of basal needles in 

 apposition with the former. 



In order to obtain insight into the mode of development of 

 the skeletal tube, I have counted the number of the transverse 

 and longitudinal beams in a series of not only variously sized 

 young specimens but also of those which might well be consi- 

 dered to be nearly or quite full-grown. The list is given on 

 the following page ; in it the specimens are arranged in the as- 

 cending order of their body-length, beginning with the shoitest. 



I must say that, notwithstanding my efibrts to be as exact 

 as possible in counting, the figures in the columns I and II of 

 the annexed table, can represent only ajiproximatel}' the numbers 

 of the beams they stand for. /ibsolute precision in this matter can 

 not be expected owing to the frequent occurrence, especially in 

 the longitudinal system, of beams of such a character as makes 

 it impossible to decide whether they are to be reckoned in any 

 case as single beams or not. This ambiguity evidently stands in 



