EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 405 



MacBride and Bather chiefly in the presence of the right lateral coelom (later the 

 epigastric ccelom), and its comparison with the right collar-cavity of Balanoglossus. 

 MacBride compares the latter with his right hydrocoele, the vestigial remnant of which 

 he finds in the central ccelom. I agree with Bury in regarding this as a median organ. 



Transition to Echinoderms. 



The subject of the phyletic history of Echinoderms is at present in the state in 

 which the speculations have so out-distanced the progress of research — and the same 

 remark appears to apply to most branches of biology — that I shall merely indicate in the 

 briefest way what further light the facts here narrated help to throw upon the ancestry 

 of the echinoderms. Bather # has recently reviewed the leading theories of echinoderm 

 descent in an able and suggestive paper, and has put forward his own views in a clear 

 and concise manner. The earlier history up to the conception of a bilateral ancestor so 

 resembles my own views of the descent of Archi-ccelomata,t in which I have suggested 

 the inclusion of Echinodermata, that we are necessarily in agreement, but naturally I 

 now desire to make the reservations here expressed in regard to the homology of the 

 epigastric and hypogastric coelom. Whether Bather will be prepared to accept this 

 view on the evidence submitted remains to be seen. 



We may now trace the steps by which this archi-ccelomate ancestor was transformed 

 into an axially symmetric echinoderm. We are at present concerned with asterid 

 ontogeny, and here there are two evident changes of symmetry, which succeed but over- 

 lap each other ; it is important to keep these, together with their respective causes, 

 carefully apart. The first of these changes, already apparent in stage D of Cribrella, is 

 the transition to left-handed asymmetry. The organs of the left side become predominant, 

 the left lateral ccelom becomes radiate, the left posterior ccelom elongates dorsally and 

 ventrally, and finally, at a very late stage, the oesophagus and mouth appear on the left 

 side. The second change is the assumption of axial symmetry. This appears later than 

 the asymmetry, and commences in stage E by the radiate growth of the hydrocoele, 

 followed by that of the hypogastric coelom. It terminates in the formation of the star- 

 fish. Hence the starfish owes its underlying bilateral symmetry to its free-swimming 

 period, its oral-aboral arrangement to its period of asymmetry, and its axial symmetry 

 to its period of axial fixation. 



1. Asymmetry. — This asymmetry has been recognised by many. Quite early 

 Butschli \ attempted to account for its appearance by assuming a fixation by the right 

 side. MacBride supposes that, upon fixation, the factors inducing bilateral symmetry 

 are in abeyance and permit the organism to run riot ; by accident, the riot develops on 

 the left side. But we must recollect that bilateral symmetry is included in axial. If 



* Bather, F. A., .Town. London Coll. Soc, viii. (pp. 21-33), May 1901. 



t Masterjian, A. T., Proc. Roy. Soc. Edin., Dec. 1898, and Quart. Journ. Micr. Sci., Aug. 1897. 



X Butschli, 0., " Versuch der Ableitung des Echinoderms aus einer bilateralen Urform," Zeitsch.f. w. Zool., vol. liii. 



TRANS. ROY. SOC. EDIN., VOL. XL. PART II. (NO. 19). 3 p 



