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 mj'self 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 Uttle 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-caual, and Perrier himself admits that it is identical in 

 structure mth 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, tlum that of a stalked Crinoid, owing to the concentration 

 of the cirri at the toji of the larval stem. 



1 See Sladen, On the Homologies of the Primary Larval Plates in the test of Brachiate Echinoderms, Quaii. 

 Joiirn. Micr. Scl, 1884, vol. xxiv., N. S., pp. 32-34, pi. i. t^R. 16. 



- Quart. Journ. Micr. ScL, 1883, vol. xxiii., J^. S., pp. 599-609. ^ Comptes rendas, t. xcviii. pp. 445, 446. 



* Quart. Journ. Micr. Sci., 1884, vol. xxiv., X. S., p. 323. 



