732 DE. W. B. CAEPENTER ON THE STErCTUEE, PHYSIOLOGY, /lSB 



its sides. This spreading-out results from the increase in their own brea<lth, without a 

 corresponding increase in the diameter of the circle on which they rest ; so that they are 

 forced to extend themselves obliquely instead of vertically. The single anal plate 

 (fig. 3, a), originally interposed between two of the first radials (r', r'), being attached 

 not so much to the neighbouring plates as to the visceral mass, begins to be lifted out 

 (as it were) from between them with the development of the anal funnel, as is seen in 

 Plate XL. fig. 2 ; and the space left by it is partly fUled up by the lateral extension of 

 the two radials between which it was previously interposed, but which do not yet come 

 into mutual contact. The second and third radials (r^, r% and t^, i^) also increase in all 

 their dimensions, but particularly in breadth ; and they thus assist in supporting the 

 visceral mass, whicli, at the conclusion of this stage, extends itself as far as the bifur- 

 cation of the arms. The most remarkable change in the condition of the calcareous 

 skeleton in this stage, however, consists in the altered relative position of the five 

 oral plates (Plate XL. fig. 1, o, o). This circlet, like that of the basal plates, does 

 not partake of the enlargement so remarkably seen in the radials ; its diameter being 

 neither iticreased by the growth of its component plates, nor augmented by their sepa- 

 ration from one another. It continues to embrace the circle of oral tentacles, the dia- 

 meter of which comes to bear a smaller and yet smaller proportion to that of the ventral 

 siu'face of the disk, as the size of the latter is augmented by the development of the 

 intestinal tube around the gastric cavity ; and thus it comes to pass that the circlet of 

 oral plates detaches itself from the summits of the first radials on which it was previ- 

 ously superimposed, and is relatively carried inwards by the great enlargement of the 

 circle formed by the latter, — the space between the two series being now filled in only 

 by the membranous perisome, which is traversed by the five radial canals that pass out 

 from the oral ring between the oral valves to the bifurcation of the arms, as shown in 

 Plate XL. fig. 1. 



65. In the earlier part of this stage, a continued increase takes place in the number 

 of segments of the Stem, by the development of new rings at its summit ; whilst the 

 previously-formed segments of its middle and upper portions become progressively elon- 

 gated and solidified, as those of the lower portion have previously been. At or about 

 the period, however, at which the change already described is taking place in the rela- 

 tions of the oral and anal plates of the Calyx, the production of new calcareous segments 

 in the stem appears to cease ; and a remarkable cliange begins to show itself in the one 

 on which the calyx rests. Instead of increasing in length, its original annular disk 

 augments in diameter, becoming convex on its lower surface, and concave on its upper ; 

 and it extends itself over the bottom of the calyx, in such a manner as to receive into 

 its concavity the apices of the basal plates. This change commences whilst the calca- 

 reous segments next below are still rudimentary ; so that although no further increase 

 in the number of segments takes place subsequently, yet some increase in its length will 

 still be effected by the completion of the last-formed segments, previously immature. 

 The total number of segments in the fully-developed Pentacrinoid stem is subject to a 



