PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 663 



Pentacrinoid young. On the single broken specimen secured by the Antarctic 

 there was a fairly complete series of pentacrinoids from the youngest stage to the stage 

 at which they are ready for detachment. 



A few eggs were found in the marsupia, but none of these were fertilized so that no 

 information was afforded concerning the cleavage and the larval development. 



In the youngest larva the vestibule is not 3'et open and the orals and basals are 

 still in a rather embryonic condition, their central portion retaining the original char- 

 acter of a branching spicule; this feature, however, remains distinct, especially in the 

 oral plates, until a much later stage. There is a distinct naked space left between each 

 of the basals and the oral above it, and it appears as if these plates never join com- 

 pletely in the midradial line. Infrabasals cannot be made out, and apparently do not 

 exist in this species. There is no trace of radials, or of the radianal. The column 

 consists of 11 segments which are in a very undeveloped stage, the central annulus in 

 all being still very conspicuous. There is no terminal stem plate. This specimen is 

 attached to the upper edge of the marsupial wall. 



In the next stage the vestibule has opened and the primary tentacles are protrud- 

 ing. The orals and basals have increased considerably, the zones of growth being 

 indicated by the regular arrangement of the holes. The orals have begun to assume 

 the usual form, with the edges turned outward. The radials have appeared, and are 

 peculiar in having an unfenestrated central portion. The columnals have increased 

 somewhat and are now very difficult to distinguish from one another so that their 

 exact number cannot be given. 



In the next stage the radials have increased somewhat and have assumed a tri- 

 angular form. The IBrj has appeared and is visible as a branching spicule. The 

 radianal lies wholly outside the midradial line, and has scarcely any influence on the 

 shape of the adjoining radial. The columnals in the upper part of the stem cannot be 

 made out; those in the lower part are assuming their definitive shape. 



In the following stage the arms have begun to form. The radials have come into 

 contact, separating the basals from the orals. The radianal is still small, but the 

 radial to the right of it is now considerably influenced by it; a naked space only partially 

 filled by the radianal separates this radial from the one to the left. The posterior oral 

 has undergone some resorption. The IBrj are long and narrow with the original 

 spicule still discernible in the center. The IBr 2 are somewhat shorter, and bej-ond 

 them the first brachials are making their appearance as transverse spicules sending out 

 processes proximally and distally. The proximal columnals are still indistinctly sepa- 

 rated from one another. 



In a slightly more advanced stage the arms have grown considerably, about 

 6 brachials being developed. The IBr 2 has assumed its definitive shape with two 

 oblique distal edges, though the primary spicule is still distinct both in the IBrj and in 

 the IBr 2 . The columnals are also taking on their definitive form in the upper portion 

 of the column; they are still only 11 in number. 



In the most developed stage represented a pinnule has appeared on the twelfth 

 brachial. The orals, which are of somewhat larger size than usual, are widely sepa- 

 rated from the calyx plates. The anal cone is present, and the radianal remains dis- 

 tinct close to the adjoining oral. The cirri are very far advanced in development; those 

 of the first whorl, radial in position, consist of 12 segments with a conspicuous opposing 

 spine and terminal claw; those of the second whorl, alternating with those of the first, 



