292 THE VOYAGE OF H.M.S8. CHALLENGER. 
cavity of the centro-dorsal (Pl. LXI. fig. 2), and the same is the case with the flattened 
basals of Atelecrinus. 
The chambered organ of Pentacrinus is both relatively and absolutely smaller than 
that of Comatula, and the reason for this is obvious. The Comatulid centro-dorsal 
represents, as it were, a coalesced series of the nodal stem-joints of Pentacrinus, and the 
five cavities of the chambered organ lodged in its upper part are each in connection 
with one cirrus-vessel only. But the remaining cirri which are borne by the centro- 
dorsal in greater or less abundance are supplied by a number of vessels that come off 
below the chambers. Within the dorsal portion of the chambered organ, “lying at the 
bottom of the centro-dorsal basin, there is a succession of verticils of five triangular 
leaflets, increasing in size from below upwards, from the extremities of some of the upper 
of which leaflets issue groups of three diverging cords that proceed to the cirri. I can 
scarcely doubt that these verticils mark the origins of the earlier cirral cords from the 
Crinoidal axis; and this obviously suggests that the five-chambered organ is itself only 
another and larger verticil, which has come by the formation of ventricular cavities in 
its substance (analogous to the lateral ventricles of the brain), to occupy the whole 
cavity of the enlarged centro-dorsal basin.”* In Pentacrinus, however, the cirri all 
come off from the nodal joints of the stem, where the five downward prolongations of 
the cavities of the chambered organ in the calyx enlarge and each gives off a cirrus- 
vessel (Pl. XXIV. figs. 3, 4, chn). No cirrus-vessels come off from the enlargement 
of the vascular axis within the calyx, which represents the chambered organ of Comatula 
without the verticils of cirrus-vessels below it. The chambers, however (Pl. XXIV. 
figs. 6-8, ch), ‘are scarcely larger than the nodal cavities in the stem from which the 
cirrus-vessels arise,” and are much smaller than the corresponding chambers within the 
upper part of the centro-dorsal of Comatula (Pl. LXI. fig. 2), which give off the vessels 
to the younger cirri. In fact they are sometimes almost equalled in size by the central 
vessels within the ring of chambers, as seen in Pl. XXIV. fig. 6. They are not closed 
below as is practically the case in the Comatulze, nor do they present as sudden enlarge- 
ments of the stem-vessels as in Fhizocrinus and Bathycrinus; but these vessels are, as it 
were, permanently enlarged in the upper part of the stem, owing to the closeness of 
the nodal joints which are successively formed beneath the calyx, and are only gradually 
separated by the intercalation of internodal joints between them. The chambers of 
Pentacrinus therefore taper very gradually downwards into the stem-vessels, and it is 
difficult to say where the latter begin and the former end (Pl. LVIII. fig. 3). 
The primary Y-shaped interradial cords which come off from the fibrillar envelope 
surrounding the chambered organ of Pentacrinus (Pl. XXIV. fig. 7; Pl. LVIIL 
figs. 1, 3—a.7.) sometimes bifurcate before entering the basals. On the inner face of the 
See W. B. Carpenter, Proc. Roy. Soc., vol. xxiv., 1876, p. 219. 
Compare Pl. XXIV. figs. 3, 4, 6, which are all equally magnified. 
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2 
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