IX. Genetic Relations to other Echinoderms. 1915 394 



which proceed the perradial canals; the hydropore, the genital 

 apertures, and the anus are at the apical pole ; the coil of the gut is 

 the same. The orientation of an Asteroid is essentially the same, 

 except that the genital apertures are marginal, and that the gut 

 is not obviously coiled ; the relative positions of the anus, when 

 present, and of the madreporite are, however, in accordance with 

 a solar coil (see Treatise, 1900, pp. 21 and 34), and such a coil does 

 appear in early stages of development (Gemmill, 1914). An Ophiuroid 

 differs from an Asteroid in the entire absence of anus, and in the 

 position of the hydropore on the oral surface. 



Therefore, the passage from a pelmatozoan to an echinoid or 

 asteroid type involves: (1) the translation of the mouth, the water- 

 ring, and the proximal ends of the perradial water-vessels, from an 

 upper to an under position; (2) the translation of the hydropore, the 

 anus, and, to a less extent, the genital apertures, from the oral 

 to the apical surface; (3) the retention of the solar coil of the gut 

 when viewed from the oral surface. Two modes of effecting the 

 passage are conceivable. The first demands (a) the closure of the 

 original mouth and the breaking out of a new one at the opposite 

 pole ; (b) the corresponding sinking of the circumcesophageal water- 

 ring and the evolution of an entirely fresh set of perradial water- 

 vessels starting from this new oral pole; (<?) the reversal of the coil 

 of the gut ; it does not demand any essential alteration in the position 

 of anus, madreporite, or genital apertures, or in the normal position of 

 the theca as a whole. The second mode demands (a) the turning 

 upside-down of the whole theca; (b) the migration of hydropore and 

 anus along the posterior interradius towards the aboral pole, and, as 

 a consequence, the elongation of the stone-canal ; (<?) a change in 

 position of the genital apertures ; it does not demand any alteration 

 in the original mouth, in the coil of the gut, or in the other relations 

 of the water-vascular system. The position of the genital apertures 

 may be neglected, for in either case there must have been an entirely 

 independent evolution of pentamerism in the gonads, and probably 

 the bursting through of a fresh set of apertures. However this may 

 be, there is little doubt but that the second mode of passage is 

 the more in accordance with general echinoderm evolution. The 

 migration of the hydropore may be paralleled in holothurians, and 

 the migration of the anus in crinoids and echinoids. There remains 

 only the initial step — the overturning of the whole theca. 



It is clear that a change of this magnitude was no sudden one. 

 As Professor MacBride has happily remarked, "No animal ever went 

 to bed with one set of habits and woke up in the morning with 

 another." In the absence of direct evidence we can only speculate, 

 but our speculations must be controlled by two distinct sets of facts 

 and principles. The changes imagined must be consistent, first, with 

 the ordinary processes of life and the general character of such 

 changes in the Echinoderma, secondly, with the facts of development 

 in the recent Echinoderma concerned. 



A possible mode of origin of Asterozoa from some early Edrioasteroid 

 has been suggested briefly in the Encyclopedia Britannica, Supplement, 

 vol. 27, p. 623 (July, 1902) and Eleventh Edition, vol. 8, p. 877 



H 



