504 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



earlier stages of its growth within the free-swimming larva. They increase 

 constantly in volume during its later stages, and with but little change of 

 character make up a large portion of the visceral mass in the adult. 



In the pentacrinoid toward the end of the "prebrachial stage" the rapid 

 enlargement of the body causes five diamond-shaped spaces to appear at the 

 points where the upturned angles of two oral plates adjoin the truncated upper 

 angles of two adjacent basals. In these spaces cylindrical spicules appear which 

 soon become club shaped, dichotomize, branch, and anastomose into delicate net- 

 like superficial plates, irregularly oval in shape, slightly produced superiorly, 

 their upper narrower portions resting beneath and supporting the gradually 

 extending sarcode projections which are terminated by the azygous tentacles. 

 The equatorial portion of the body, the band between the upper edges of the basals 

 and the lower edges of the orals, now rapidly expands. The five young arms 

 extend outward, their bases carrying out with them a zone of sarcode which gives 

 the central portion of the body a great additional width. The oral plates maintain 

 their original position, so that they are now completely separated from the basals 

 by this intervening equatorial band, and are left, a circlet of five separate plates 

 each inclosed in its sarcode lobe, on the center of the upper surface surrounding 

 the mouth and inclosing the 10 interradial ("nonextensile") tentacles only 

 The radial plates begin to thicken, especially toward the upper margin, and this 

 thickening is produced by the growth beneath the cribriform superficial calcareous 

 film of a longitudinal mass of tissue of the same character as that which forms 

 the cylindrical axis of the columnals. On the lower surface of each arm, in 

 linear series immediately above the radials, two spicules, horseshoe shaped with the 

 opening above, appear almost simultaneously and become quickly filled up with 

 elongating sheaves of longitudinal trellis work. These extend along beneath the 

 extending arms, and indicate the IB^ and the IBr 2 (axillaries). 



The upper surface of the arms now becomes grooved by the development 

 on either side of the central vessel of a series of delicate crescentic leaves. These 

 leaves are hollow, communicating by special apertures with the radial vessel 

 and filled with fluid from it. At the base of each of the leaves there is a pair of 

 tentacles forming a group with the leaf, and along with it communicating with 

 the vessel. One of these tentacles (the distal) is somewhat larger than the other. 

 They are both slightly club shaped, the club-shaped extremity fringed on either 

 side with conical papillae. They are nonextensile, and resemble in every way 

 the 10 nonextensile (interradial) tentacles early developed from the oral ring. 

 A group consisting of a crescentic leaf and two nonextensile tentacles lies imme- 

 diately at the base of each extensile tentacle, and a little lower down the arm. 

 Minute spicules, some of them simple or key shaped and others expanding into a 

 cribriform film, appear in the superficial sarcode layer along the back or edges 

 of the arms; and usually at the base of each of the tentacles, irregularly embedded 

 in the sarcode substance, there is a sacculus. 



A little later the end of the arm shows a tendency to bifurcate, and two half 

 rings, with their inclosed sheaves of calcified tissue, give the first indication of 

 the first two brachials. 



