REPORT ON THE CRINOIDEA. 107 



lu Bathijcnnuff, Rhizocrinus, and Pcntacrinus the central vascular axis of the stem 

 consists of five peripheral vessels surroimding a core of smaller ones (PI. Vila. fig. 2 ; 

 PI. XXIV. figs. 2, 5 ; PL LVIII. fig. 3— c/t') ; and from these peripheral vessels are 

 derived the central vessels of the cirri borue hj the stem. In PentacHnus 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 ofi" one cirrus-vessel (PL XXIV. figs. 3, 4, dm, cv; PL LXIL). Hence, 

 every nodal joint contains, as it were, a small edition of the chambered oi-o-an situated 

 in the calyx (PL XXIV. figs. 6, 8, ch ; PL LXIL). 



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, whde a small aperture in the middle of the peripheral wall of 

 the chamber leads into a cirrus-vessel. The vessels of the remaining cin-i 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 LXIL). 



In Actmometra imrvicirra, 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 Pentacriuoid 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 originatiua; from the " cordon central " of 

 the larval stem, and as alternating with the rays, i.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 Comatulse, 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 (PL Vllb. fig. 2, V). In Penta- 

 crt'mw there is only a single vessel in the upper part of the stem (PL XXIV. figs. 2-5, T'^. 



• Wyville Thomson, Phil. Trans., 18G5, p. 536. * Comptes rendus, t. xcviii. pp. 445, 446. 



2 Quart. Journ. Micr. Sci., 1884, vol. xxiv., N. S., pp. 325, 326. 



