166 PHYLUM ECHINODERM ATA. 



only in Crinoid and Echino-pluteus larvae, no Echinoderm larva has 

 a right hydrocoel equal in size to the left, and in Crinoids and Holo- 

 thurians there is no trace of a right hydrocoel at any stage of existence. 

 Lastly, in no normal* Echinoderm does the right hydrocoel ever possess 

 a water-pore. We do not wish to be unduly critical, but we think it not 

 unreasonable to point out that, in the absence of any test which enables 

 us to decide which characters are ancestral and which secondarily acquired, 

 an ancestor constructed by this somewhat one-sided application of the 

 recapitulation theory can have very little value in advancing zoological 

 knowledge. We do not say that this kind of speculation has no value, 

 for it is a source of delight and stimulus to many minds ; but we think 

 that it is most important that its value should not be overrated and that 

 it should not be allowed to divert attention from more important and 

 more practicable problems. 



To continue the imaginary history of the dipleurula ancestor. The 

 next change is due to its fixation, which is supposed by Bather to have 

 taken place by the right side of its preoral lobe, though the fixation actual 

 occurs in the middle line. This led to the passage of the mouth to the 

 left side and to the establishment of the radiate structure of most organs 

 except the genital and to the shifting and asymmetry of the coelomic 

 sacs. This brings us to the so-called Pentactaea.f Now came the diver- 

 gence into types. The Holothurian type in which the generative organs 

 never acquire a radial arrangement, was the first to separate. In this type 

 the attachment was entirely lost from the whole life history. Next, 

 after the acquisition of radial structure by the gonads, the Asteroids and 

 Echinoids separated off ; cf these the Echinoids entirely lost their attach- 

 ment, while the Asteroids appear to have retained it in some if not in all 

 cases (larval attachment, p. 149). Lastly, or perhaps as a continuation of 

 the main stem but little modified, came the Crinoids, in which the attach- 

 ment is retained. This type further presents the following remarkable 

 feature which may or may not have been primitive ; the mouth shifts 

 from the left side to the hind end, where it lies alongside the anus. 



Class ASTEROIDEA { 



Star -shaped or pentagonal forms with the body flattened in the 

 or-anal axis. The arms are not sharply marked off from the disc 

 and have an ambulacral groove from which the tube feet project. 

 The madreporite is on the abactinal surface. 



* Brooks and Field have asserted that in bipinnaria a second madre- 

 poric pore normally occurs, but this statement has not been confirmed. 



t See Lang's Comparative Anatomy, Pt. 2, p. 548. 



j E. W. MacBride, " The development of Asterina gibbosa," Q.J.M.S., 

 38, 1896, p. 339. H. Ludwig, " Asteroidea " in Bronn's Thierreich, 

 Leipzig, 1894-8. Id., Die Seesterne des Mittelmeeres , Neapel, 1897. 

 S. Goto, Metamorphosis of Asterias pallida, etc., Journ. Coll. Sc. 

 Japan, 10, 1898, p. 239. Id., Metamorphosis of Asterina gibbosa, 

 ibid., 12, 1898, p. 227. W. P. Sladen, " Report on the Asteroidea," 

 Challenger Reports, vol. 30, 1889. E. Perrier, " Echinodermes," in Exped. 

 Sci. du Travailleur et du Talisman, Paris, 1894, and in Mission Scient. du 

 Cap Horn, vol. vi., Paris, -1891. See also works of Ludwig, Cuenot, 

 Hamann, Delage et Herouard, loc. cit. 



