10 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



fairly conspicuous sacs looking like large glands and apparently opening outward. 

 As seen in transverse sections they may be fairly regularly arranged, 3 or 4 of them on 

 either side; in other cases they are found only on one side, as seen in frontal sections. 

 Mortensen believes it fairly safe to conclude that these sacs originate as prominences 

 from the posterior wall of the (later) oral coelome, afterwards becoming completely 

 separated off from the coelomic wall and acquiring an opening outward through the wall. 



Histologically these sacs consist of a single layer of cells with nuclei somewhat 

 larger than those in the surrounding mesenchyme. On the inner end of each cell 

 a mass of small globules which stain yellow with picrocarmine is visible and appears 

 conspicuous against the red nuclei and the finely granulated cytoplasm. This structure 

 disappears toward the opening of the sac. 



The mesenchyme is very extensively developed, and has a distinctly fibrillar 

 structure. In the anterior portion can be distinguished an outer layer, with very few 

 nuclei and highly developed fibrillae, and an inner portion with numerous nuclei and 

 the fibrillae only slightly developed. There may be a fairly distinct boundary between 

 these two parts. In the inner portion, which surrounds the chambered organ and the 

 columnals, are found, besides the nuclei, a varying number of yolk globules, solitary, 

 or collected in small ball-like masses. Sometimes such yolk globules may be found in 

 the posterior end of the larva also, but here they are much less numerous than at the 

 anterior end. 



The skeleton is in very nearly the same stage of development in all the larvae, in 

 spite of the considerable variation in then- size. The circlets of orals and basals are 

 typically developed, and there are 4 good-sized infrabasals. In one individual there 

 are 5 iufrabasals, 2 being much smaller than the others. There are about 25 columnals, 

 and a terminal stem plate of the usual size accompanied by from 1 to 5 small supple- 

 mentary plates which lie without any definite order about its periphery. In a few 

 individuals in which the skeleton is in a slightly younger stage of development there are 

 as yet none of these supplementary plates. 



Mortensen was unable to see a reason for the occurrence of the supplementary 

 plates. 



Pentacrinoid larvae (from Dilwyn John, 1938). The Discovery Investigations 

 collected 20 pentacrinoids in the Brausfield Strait area of the Antarctic. These were 

 taken to be Notocrinus virilis, since the largest of them exhibited several characters, 

 notably the side- and covering-plates, of the adult virilis. They range in size from 1.4 

 to 9 mm. length of crown and from 6.4 to 13 mm. length of column. 



The smallest one has a column of 34 columnals and a thick roughly circular terminal 

 plate which appears to be simple. The first 7 columnals are short and discoidal. The 

 most proximal is in contact with the basal plates. From the second to the seventh 

 there is a gradual decrease in width. The eighth is about a third, the ninth about 

 a half, as long as broad; the proximal half of each is encircled by a narrow projecting 

 girdle. The remaining segments are about as long as broad except for 5 or 6 at the 

 distal end which are shorter. They are rounded off to, and narrower at, each end 

 than in the middle, where the projecting girdle occurs. The articular faces are very 

 broadly oval, the long axes of those at the opposite end of each columnal at right 

 angles to one another. 



The sides of the basal cup are strongly rounded. Its height is equal to about half 



