314 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



vestibulum has not yet opened. There is only a depression showing the position of 

 the future opening. 



On the fully formed young pentacrinoid the orals have separated and the tentacles 

 are protruding. As yet no new columnals have appeared. The terminal stem plate 

 has developed into a fairly large, irregular, roundish disk consisting of a thick reticu- 

 late network. The orals and basals are beginning to form reticulations. It is a 

 conspicuous feature that the upper edge of the basals embraces the lower edge of 

 the orals. 



In the oldest stage studied by Mortensen the anal has been formed, but as yet 

 there are no traces of the radials. The basals are still seen to have at their upper 

 edge a wider circumference than that formed by the orals, so that there is a charac- 

 teristic offset between the two circlets of calyx plates, a feature that does not appear 

 to be due to the preservation. The plates are already considerably thickened, 

 strongly reticulated, their surface appearing coarsely spinous when seen in profile. 

 Mortensen said that on the whole this pentacrinoid is by no means such a beautiful, 

 delicate looking object as are several other pentacrinoids — for instance that of Hafhro- 

 metra prolixa. The orals are but very slightly concave. There are now thirteen 

 columnals, three new ones having formed at the upper end of the column. These 

 uppermost columnals are conspicuously thickened in the middle, a feature that 

 gradually disappears on the lower columnals. From the seventh to eighth there is 

 no thickening in the middle of the columnals, which are now simply cylindrical. A 

 dark line across the middle still indicates the originally formed plate from which the 

 columnal develops by means of vertically growing processes which unite by cross 

 beams, as described by Seeliger in Aniedon. The final shape of the columnals, as 

 well as their ultimate number in the fully grown pentacrinoid remains unknown. 

 The terminal stem plate is an irregular disk with some short rounded prominences. 

 The infrabasals are now considerably thickened, and are destined to form, together 

 with the uppermost columnal, the centrodorsal. 



A young specimen with the arms 10 mm. long was found by Dr. H. L. Clark, 

 who gave it to Dr. Mortensen. The diameter of the disk was 2 mm. There were 

 ten pinnules on each arm, and the cirri were XIII. The tentacles are studded with 

 small simple spicules, generally lying in a series along one side. The orals are still 

 fairly large, and are of a very peculiar shape, with a prominent point bent inward 

 toward the mouth. The skin of the disk contains numerous fenestrated plates, 

 among which is a large anal. The primary hydropore is seen in the anal interradius. 

 Each of the other interradii has some five to ten hydropores, though in the anal 

 inteiTadius there is stUl only the primary pore. A larger plate is found in the lower 

 part of each interradius, in the angle between the IBr series. The basals have already 

 completely disappeared from the exterior of the calyx and formed the rosette, as 

 may be seen from the inside. The first cirri are seen to be radial in position. 



In fully grown specimens the orals, as well as all the other fenestrated plates have 

 disappeared, and the skin has become studded with small more or less bone-shaped 

 spicules inextricably entangled. 



Parasites. — Mortensen wrote that an interesting fact in connection with Tropio- 

 metra carinata as it occurs in Tobago is that none of the specimens proved to be 

 infested with myzostomes. He remarked that this would seem to indicate that the 



