REPORT ON THE CRINOIDEA. 105 



organ, first into two (PI. XXIV. fig. 6, v), and tlien into four or more; while the 

 peripheral vessels around it expand into the chambers (PI. XXIV. figs, 6, 8 ; PI. LVIII, 

 figs. 1-3 — ch), just as they do on a smaller scale within the nodal stem-joints (PI. XXIV. 

 fig. 4, clin). 



As the ascending vascular axis of Pentacrinus decorus passes out of the basal ring, and 

 enters the central plug within the radial pentagon, the chambers become elongated and pear- 

 shaped in the direction of the rays ; and at the same time their cavities are traversed by 

 trabeculae, which break them up into numerous smaller cavities that contain masses of 

 yellowish-brown pigment-granules (PI. LVIII. fig. 2, ch). Some of these spaces have a 

 definite epithelial lining, like the chambers with which they are connected, while others 

 are deficient in this respect. Before reaching the level of the circular commissure, 

 however, these radial extensions of the outer vessels of the plexiform gland terminate 

 somewhat abruptl}' ; and the gland is reduced to a small but compact bundle of vessels 

 in the middle of the central plug.^ As it rises it becomes extended laterally, and its 

 shape when it enters the visceral mass (as seen in section) is that of a more or less 

 irregular L ; while this soon passes into a lobular form of the usual variable 

 character. 



The appearance of these radial extensions of the lowest part of the plexiform gland in 

 Pentacrinus decorus forcibly recalls Koehler's description of the minute structure of the 

 ventral end of the ovoid (plexiform) glaud in the Urchins. This is connected with the 

 oral ring by a single vessel, which is called by Koehler the glandular canal. The other 

 end of the gland is said by the French anatomists to open externally through the 

 madreporite in all the Echinozoa, together with the water-tube ; though both in Asterids 

 and in Ophiurids Ludwig has described it as connected with an aboral vascular ring in 

 which the genital vessels arise, and there is strong reason to l^elieve that the same is the 

 case in the Urchins too. 



It wiU be a matter of no little difficulty to 'determine exactly the portions of the 

 blood- vascular system of a Crinoid which are represented in a Starfish or Urchin ; and the 

 question will probably only be satisfactorily settled by careful studies in organogeny. 

 There is no doubt about the oral ring upon the one side of the disk, and the chambered 

 organ on the other. But where is the line to be drawn between the two ? 



It seems to me not unlikely, as I have suggested elsewhere,^ that the labial plexus of 

 Crinoids may represent the aboral ring of the Echinozoa, the plexiform glaud being much 

 shortened, but expanded laterally instead. Both the intervisceral and the genital vessels 

 are in communication witli it, just as they are -with the aboral ring of Asterids, according 

 to Ludwig ; and in this class too the plexiform gland is continued upwards beyond the 



1 The upward passage of the chambers into the peripheral cavities of the lower part of the gland is better showTi 

 in the section next to that represented in PI. LVIII. fig. 2. 



2 Q«arf. Joum. Micr. Scl, N. S., vol. xxi., 1881, p. 185 ; and also Notes on EchinodermMorphology, Xo. V. ;^Oii 

 the Homologies of the Apical System, with some Remarks upon the Blood-vessels, Ibid., N. S., vol. xxii., 1882, p. 375. 



(ZOOL. CHALL. EXP. PART XSXII. ISSl.) 11 1 4 



