292 THE VOYAGE OF H.M.S. CHALLENGER. 



cavity of the centro-dorsal (PI. LXI. fig. 2), and the same is the case with the flattenetl 

 hasals 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 ai-e 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 (PI. 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 (PI. 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 Comatulse, nor do they present as sudden enlarge- 

 ments of the stem- vessels as in Rhizocrinus and Bathycriniis ; but these vessels are, as it 

 were, permanently enlarged in tjie upjDer 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 thenx 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 LVIII. 

 figs. 1, 3 — a.i.) sometimes bifurcate before entering the basals. On the inner face of the 



' See AV. B. Carpenter, Proc. Roy. Soc, vol. xxiv., lS7fi, p. 219. 

 - Compare PI. XXIV. figs. 3, 4, 6, which are all equally magnified. 



