106 THE VOYAGE OF H.M.S. CHALLENGER. 
aboral ring to terminate in the perisome of the central part of the disk. It would be 
very interesting to determine the relation of this dorsal extension of the plexiform gland 
in those Asterids such as Zoroaster fulgens, which retain well-developed basal and 
radial plates in the centre of the disk, so as to resemble the calyx of a Crinoid.? 
It will be remembered that Prof. Perrier has noted the identity in structure between 
the axial organ of a Crinoid and the so-called ovoid gland of the Echinozoa. This organ 
is considered by Ludwig and myself to be in a close relation with the blood-vascular 
system, and intimately united to the oral blood-vascular ring, just as the axial organ is 
in the Crinoids. But Perrier believes it to be an excretory gland, unconnected with the 
blood-vascular system and opening to the exterior through the madreporite. Koehler’s 
observations on the Urchins, however, tend to disprove this theory, as I have shown 
elsewhere.” Perrier has recently asserted that the axial organ of a Crinoid communicates 
with the exterior (see Appendix, Note D); but although he has described its structure 
as identical with that of the ovoid gland of Starfishes and Urchins, he nevertheless 
compares it with the madreporic or .stone-canal of these types.’ He thus considers an 
organ which is related to the blood-vascular system of a Crinoid to be represented by a 
part of the water-vascular system of other Echinoderms; and he denies that the latter 
corresponds to the water-tubes and water-pores of a Crinoid, as is generally supposed. 
The only reasons which he brings forward for this conclusion are that the walls of the 
axial organ in the young Crinoid are folded like those of the stone-canal in Starfishes ; 
while it has the same position with regard to the digestive apparatus as the stone-canal 
of Urchins. The first reason appears to me to be of very little value, as I have pointed 
out elsewhere; * while Perrier seems to have overlooked the fact that the second one is 
equally applicable to the doctrine of a general homology between the axial organ of a 
Crinoid and the so-called heart of an Urchin. For this structure lies in the immediate 
neighbourhood of the stone-canal, and Perrier himself admits that it is identical in 
structure with the axial organ of a Crinoid, which is certainly not the case with the 
stone-canal. 
It is difficult to see what rational grounds Perrier has for his suggestion that a part 
of the water-vascular system of a Starfish or Urchin is represented by an organ which 
belongs to the blood-vascular system of a Crinoid, as his own observations show, though 
he nowhere admits that such is the case. (See Appendix, Note F.) 
The chambered organ of Comatula is contained within the cavity of the centro-dorsal, 
and is covered in above by the rosette of metamorphosed basals ; it is a larger structure, 
both relatively and absolutely, than that of a stalked Crinoid, owing to the concentration 
of the cirri at the top of the larval stem. 
1 See Sladen, On the Homologies of the Primary Larval Plates in the test of Brachiate Echinoderms, Quart, 
Journ. Mier, Sct., 1884, vol. xxiv., N. S., pp. 32-34, pl. i. fig. 16. 
* Quart. Journ. Mier. Sct., 1883, vol. xxiii., N. S., pp. 599-609. 5 Comptes rendus, t. xeviii, pp. 445, 446. 
4 Quart. Journ. Micr, Sci., 1884, vol. xxiv., N.S., p. 323. 
