1915 395 F. A. Bather — Studies in Edrioasteroidea. 



(Feb., 1911), and more fully expressed in " What is an Echinoderm ?" 

 (Journ. London Coll. Sci. Soc, vol. 8, pp. 21-33, May, 1901 ; Italian 

 edition, Oct., 1901). This hypothesis has been criticized on the 

 ground of the structure of certain fossils by Mr. W. K. Spencer 

 (1904), who, however, now recognizes that he " overstated " his 

 "case . . . It is not difficult to imagine a more primitive Edrioasteroid 

 which would show, at any rate, near relationships with the ancestral 

 Eleutherozoa " (1914, p. 6). More weighty objections have been 

 raised by Professor MacBride and Dr. Gemmill on the ground of 

 their admirable observations on Asteroid embryology. But while the 

 hypothesis which they themselves adopt, as expressed in Professor 

 MacBride's Textbook of Embryology : Invertebrata (.1914, pp. 560-5), 

 is difficult of acceptance, on the other hand some of their own recent 

 observations seem actually to support the hypothesis which they 

 reject. Let us consider these conflicting views more closely. 



We now agree on many points of fundamental importance. We 

 agree, namely, that the ancestral Asterozoa were attached by a stem 

 homologous with that of the Crinoidea in so far as it was derived 

 from the same portion of the larva. We agree that the ancestral 

 Asterozoa fed by a subvective system of ciliated grooves. We agree 

 that radiate symmetry was — in Eleutherozoa as in Pelraatozoa — 

 a consequence of this mode of life. It is admitted that processes 

 subsequent to the fixation of the bilaterally symmetrical ancestor by 

 its anterior end eventually brought the stem : (a) in Crinoidea, to 

 an aboral position outside the water-ring; (b) in Asterozoa, to an 

 excentric oral position within the water-ring. We even appear to be 

 agreed as to the changes that produced the plan (#). 



The difference arises with regard to the number and order of the 

 steps that produced the plan (b). 



Whereas some of us have expressed the opinion that the ancestors 

 of the Eleutherozoa passed through a true pelmatozoan stage, with 

 mouth uppermost, and that to this stage the Echinoderma owe their 

 coiled gut and radiate symmetry, MacBride and Gemmill hold that 

 "the Echinoderm stock became split into two stems" shortly after 

 the ancestral Dipleurula had become fixed by its prae-oral lobe, and 

 after the organs of its left side had begun to grow more rapidly than 

 those on the right, but "before the hydrocoel was a closed ring, and 

 before radial symmetry was completely attained ". From such 

 a stage, they maintain, the primitive Asterozoon evolved immediately 

 and directly, with its mouth turned towards the sea-floor, so that the 

 left hydrocoel, as it developed into a hydrocircus, grew round 

 the stem, which was therefore on the oral face ; and it was while 

 the creature was so supported that radial symmetry was acquired. 

 Figures illustrating this view are given in MacBride's classical paper 

 on "The Development of Asterina gibbosa" (1896, Quart. Journ. 

 Micro. Sci., vol. 38, pi. 29, figs. 157-9) and are reproduced in his 

 Textbook of Embryology (1914, p. 563). It should be noted that in 

 the Cambridge Natural History (1906, Echinodermata, p. 621) 

 a mistake crept into the diagram of the primitive Pelmatozoon, in 

 that the gut was given a contrasolar coil. 



It is, presumably, the position of the stalk within the hydrocircus, 



