EARLY DEVELOPMENT OF CRIBRELLA OCVLATA. 407 



similar around an axis, and such only holds in drifting pelagic organisms and in 

 sedentary types. It is hardly conceivable that a free-swimming bilateral animal like 

 the archi-ccelomate would degenerate to a drifting type ; at least we have no analogy for 

 such a case, and no indications of such in ontogeny ; the developing star in bipinnarise 

 is painfully out of place in its pelagic surroundings. Hence we must fall back on the 

 assumption of a fixed stage to account for the radial symmetry. Extremely sluggish 

 animals may have certain organs axially arranged in respect to some aperture, such as 

 the branchiae of Doris, but for such fundamental axo-symrnetrj' as that of echinoderms 

 fixation is indispensable. Bury promised an " attempt to show that such an assumption 

 is neither embryologically sound nor necessary as a basis for phylogenetic speculation " 

 (p. 95), but he follows this up by starting with a 'pentactula' ancestor, in which the 

 axial symmetry is already of a very marked character (the ring hydroccele), and he 

 builds the axial symmetry of the other organs around this ring by simple processes of 

 growth. Naturally, if we start with an axo-symmetric ancestor, the origin of which we 

 do not discuss, it is unnecessary to assume a fixed period for further progress. 



After fixation, the mouth and hydrocoele migrate up the left side till they reach the 

 posterior end of the larva ; a process very clearly indicated in Antedon, in many Ascidice 

 and Polyzoa. The fixed period has evidently lasted longest in the Crinoids. I may only 

 say that it appears to me easy to reconcile a b 



the position of the pre-oral lobe in asterids 

 and crinoids. MacBride (loc. cit.) argues 



a very wide divergence between the As- \\ (%M?Z> jy #ova>> 



teroids and Crinoids, because the pre-oral 

 lobe bends on to the oral side in the former, 

 but on to the aboral side in the latter. He s — :n 



states that the anterior coelom, bent orally, -ii gsv/po — 



becomes surrounded both by the left ccelo- 



1,111 1 / qqo\ Tl *• — Diagram to illustrate the evolution of the Asteroidea 



mic sac ana tne nyarocceie ^p. dyz;. ine from a fixed aucestor . (A) The fixed ailces tor, with 



pre-Oral lobe is Certainly not encircled by the adult detaching itself from the stalk, as in Antedon. 



. . (B) A type with the development abbreviated ; the 



the hydrOCCele in Cribrella, and does not w hole course of phylogeny is not repeated, the young 



appear tO be SO from MACBRIDE'S figures. leaving the stalk at an earlier stage. (C) A type, such 



* x ° as Asterina or Cribrella, in which a still greater ab- 



It appears that in asterids the axial sinUS breviation takes place, the disc never facing upwards. 



i . r ii.li ii The + indicates the cicatrix of pre-oral lobe. 



lies in a mesentery formed by the dorsal and 



ventral horns of the left posterior ccelom (left ccelomic sac), and hence is not enveloped 

 at all by the hydroccele or the posterior sac, as the rupture between axial sinus and 

 pre-oral ccelom occurs distally in the madreporic inter-radius ; if this is the case, the 

 moving of the pre-oral lobe orally or aborally along the inter-radius is a very small 

 matter, anatomically speaking. 



If we assume that this has been the true course of events, it is clear that the asterids, 

 whilst repeating the stage of fixation, do not repeat the passage of the organs to the 

 posterior end of the larva and back again. Any vestigial trace of this movement would 



