402 Dr. F. A. Bather — Studies in Edrioasteroidea. 



from that of the arms, and that only later on do the two sets become 

 apposed or harmonized. The accounts given by the embryologists 

 seem to prove that in certain Asteroids there actually is such a shifting 

 of the whole hydrocoel that each lobe of it becomes applied, not to the 

 ray to which it would (especially on any homology with Pelmatozoa) 

 naturally belong, but to the neighbouring ray, just as though one 

 were to twist a clock-face backwards, so that when the clock struck 

 twelve the hands should point to twelve minutes past two (i.e. one- 

 fifth of the clock-face). 



This interpretation is confirmed by the exception ; for in Bury's 

 Dipinnaria, where the hydrocoel closes in the M plane, the fusion of 

 the main hydrocoel lobes with the rays " is effected ", says Bury, 

 " without that rotation of the two series of organs noticed by Ludwig 

 in Asterina" (1895, p. 68). 



It seems legitimate to infer that in some remote ancestor of the 

 Asteroids the closure of the hydrocoel took place in the M plane, and 

 that the lobes 1 to 5 corresponded with the rays numbered I to V 

 in the Pelmatozoa, but numbered respectively II, III, IV, Y, I in 

 the adult Asteroid by MacBride and Gemmill. 



Why this torsion took place in some Asteroids we do not know. 

 It may have been connected with the migration of the anus and the 

 dragging of the mesenteries. That is a point which the embryologists 

 do not as yet seem to have discussed. 



It was, however, pointed out in " What is an Echinoderm ? " that 

 if any Asteroids were evolved from a pelmatozoon in which the 

 food-grooves had a strong contrasolar curve (as in Edrioaster big&byi), 

 then some such contrasolar torsion of the oral region through 72° 

 would naturally accompany the straightening of the rays. Examination 

 of Text-figures 3 and 4 will at once make this clear. This hypothesis 

 works quite well so far as the hydrocoel is concerned, but seems to 

 create a difficulty with regard to the hydropore, which, one might 

 suppose, would also have been involved. We have to remember, 

 however, that the hydropore and stone-canal did not necessarily 

 accompany the hydrocoel, and when the madreporite had attained the 

 aboral surface and escaped the influence of the rays, it might have 

 been pulled back into its original interradius by the stone-canal. 



Any exceptions, such as Bury's Bipinnaria, would, on this same 

 hypothesis, be readily accounted for by supposing those starfish to 

 derive from an Edrioasteroid like the Cambrian Stromatocystis, in 

 which the rays have not acquired a curvature. 



To consider the possible relations of the Pelmatozoa (Edrioasteroid 

 or other) to the Echinoidea would unduly prolong the argument. 

 Those to whom the present paper is intended to appeal agree with me 

 that the Echinoidea no less than the Asteroidea must have had 

 a fixed ancestor, and Professor MacBride, for one, would derive all 

 Eleutherozoa from the' Asteroid stem. If we can but agree as to the 

 Asteroid ancestor, the rest follows. 



The essential difference between us is that Messrs. MacBride and 

 Gemmill believe the Asteroid to have evolved directly, mouth down- 

 wards, from the Dipleurula, whereas I would insert a true Pelmatozoic 



