734 DE. W. B. CAKPEXTEE OX THE STllUCTUEE, PHYSIOLOGY, AND 



resembles a horseshoe finally comes to be a ring. I feel quite satisfied that there is no 

 interpolation of new segments (as some have supposed) either at the base or at any part 

 of the length of the arms, their longitudinal growth being effected in part by the addi- 

 tion of new segments to their extremities, and in part by the augmentation in the length 

 of each individual segment. Thus the average length of the basal segments of the arms 

 in this stage of Pentacrinoid life is about -007 inch, and their diameter about -003 inch ; 

 -whilst in the adult Antedon their length averages about -03 inch, and their diameter 

 about -07. As the skeleton of the arms increases in length, their vascular apparatus 

 is prolonged, and new groups of tentacles are developed from its extensions, each of 

 these consisting, as before, of a leaf-like expansion with three tentacles proceeding from 

 its base, of which one is much more extensile than the other two. The sarcodic sub- 

 stance which unites the pieces of the skeleton now begins to show a delicate fibrous 

 texture at their adjacent extremities; but no separate fibres are as yet to be distin- 

 guished. 



G8. The changes which we observe during the later stages of Pentacrinoid life 

 (Plate XXXIX. fig. 1, d, e) form in every respect a continuation of those already 

 tlescribed. The Calyx is still more opened out by the increased lateral as well as 

 longitudinal development of the first radials; but the diameter of the visceral disk 

 ■augments in e^'en larger proportion, so that it extends nearly as far as the bifurcation 

 of the arms. The oral circlet is thus separated by a much -wider interval from the 

 periphery of the disk ; and in this outer ring the anal funnel (Plate XL. fig. 2, v) is now 

 -a very conspicuous object, the anal plate [a) which it bears on its outer side being alto- 

 :gether lifted out from between the two first radials which it originally separated. Before 

 the body of the Pentacrinoid drops off its stem, an incipient absorption of the oral 

 plates is discernible ; this absorption commencing along the margins of the apical por- 

 tion, so that these plates lose their triangular form and become somewhat spear-shaped. 

 The second and third radials exhibit an increase in all their dimensions, without much 

 departure from their original form. 



69. The Arms continue to increase in length, both by the addition of new segments, 

 and by the growth of those previously formed ; and it is when they have attained the 

 length of about "08 inch, and consist of about twelve segments, that we see the first 

 indication of the development of pinna'. This shows itself, not (as might have been 

 expected) at the base or oldest part of the arm, but at its growing extremity, which now 

 presents a bifurcation (Plate XL. fig. 1), the two mmi being in the first instance almost 

 equal, and each tumefied at its extremity by an accumulation of sarcode-substance. One 

 of these rami, however, grovvs faster than the other, and soon takes a line continuous 

 with that of the axis of the arm, from which the other diverges at an acute angle ; so 

 that the former comes to be the proper extension of the Arm, whilst the latter soon 

 takes on the characters of a Pinnule. Ere long, however, the gi'owing point of the arm 

 again subdivides; two branches are formed, as previously; and whilst one of these 

 becomes a continuation of the arm, the other is soon to be distinguished as a pinnule 



