REPORT ON THE CRINOIDEA. 107 
In Bathycrinus, Rhizocrinus, and Pentacrinus the central vascular axis of the stem 
consists of five peripheral vessels’ surrounding a core of smaller ones (PI. VIa. fig. 2; 
Pls XXIV. fies 557) Peeve fic. 3—ch’); and from these peripheral vessels are 
derived the central vessels of the cirri borne by the stem. In Pentacrinws these cirri 
are borne by special nodal segments which occur at more or less regular intervals all 
down the stem. The five large peripheral vessels expand slightly in each nodal joint, 
and each gives off one cirrus-vessel (Pl XXIV. figs. 3, 4, chn, ev; Pl. LXII.). Hence, 
every nodal joint contains, as it were, a small edition of the chambered organ situated 
in the calyx (Pl. XXIV. figs. 6, 8, ch; Pl. LXII.). 
In Comatula, however, the centro-dorsal represents physiologically “a coalesced 
series of the nodal stem-joints in the stalked Crinoids,”* and the downward prolonga- 
tions of the chambers into the stem are ruptured when this organ is discarded. A 
minute opening in the floor of each chamber close to the central axis remains to indicate 
their former existence, while a small aperture in the middle of the peripheral wall of 
the chamber leads into a cirrus-vessel. The vessels of the remaining cirri are derived 
from those forming the central axis of the chambered organ. They pass outwards 
horizontally beneath the chambers in five groups which are thus radial in position, as are 
the earlier cirri and those on the stem of Pentacrinus (Pl. XXIV. fig. 4, cv; Pl. LXII.). 
In Actinometra parvicirra, the only species of the genus in which I have made 
horizontal sections through the calyx, the central axis of the chambered organ contains 
only two vessels, instead of the larger number present in Antedon rosacea; and there 
are fewer cirrus-verticils beneath the chambers. This is only what might have been 
expected, from the reduced size of the centro-dorsal in this type and the small number 
of cirri which it bears. In the Pentacrinoid again, with an undeveloped centro-dorsal 
bearing only five cirri, the vessels of these organs are derived directly from the cavities 
of the chambers, just as in the nodal joints of the stem of Pentacrinus. 
Perrier has described the cirrus-rudiments as originating from the “ cordon central” of 
the larval stem, and as alternating with the rays, 7.e., as interradial in position.” He has 
given no figures in support of his statements, which are far from being in accordance 
with the observations of M. Sars, Dr. Carpenter, and myself, as I have explained else- 
where.’ 
The smaller size of the cavities of the chambered organ in the stalked Crinoids than 
in the Comatule, and the greater simplicity of its central axis, are obviously related to 
the absence of a cirrus-bearing centro-dorsal. 
Both in Rhizocrinus and Bathycrinus, so far as my experience goes, the axis of the 
chambered organ is formed throughout of a single vessel (PI. VUb. fig. 2, V). In Penta- 
crinus there is only a single vessel in the upper part of the stem (PI. XXIV. figs. 2-5, V). 
1 Wyville Thomson, Phil. Trans., 1865, p. 536. 2 Comptes rendus, t. xcvill. pp. 445, 446. 
3 Quart. Journ. Micr. Sci., 1884, vol. xxiv., N.S., pp. 325, 326. 
