142 SECTION A. GENERAL ZOOLOGY. 



SECTION A. GENERAL ZOOLOGY. 



Friday, 26 Atigtist, at the Guildhall, at 2 p.m. 



Chairman, The President. 



Prof. E. W. MacBride read the following paper: 

 On the Origin of Echinoderms. 



Although all the great divisions of the animal kingdom are 

 isolated from each other, in the sense that no forms exhibiting 

 intermediate types of structure are known, yet this is true in a 

 very special sense of Echinoderms. In the case of the other phyla 

 there is sufficient fundamental agreement in structure to enable us 

 to at least picture to ourselves an evolutionary series by which they 

 could have been derived from the same ancestors ; but this, if we 

 have regard only to the structure of the adult, is impossible in the 

 case of Echinoderms. 



These are radial in structure, yet the older attempts of Cuvier 

 and the elder Agassiz to ally them with the Coelenterata fail on 

 account of the omission to explain the important fact that the 

 radial symmetry is broken by the asymmetry of the madreporite 

 and the stone canal so intimately connected therewith. The at- 

 tempt repeated in modern times to get suggestions as to the origin 

 of the group by seizing on the indications of bilateral symmetry 

 found in some modern Holothuridea and in some ancient forms 

 such as the Cystidea, is equally unsatisfactory, since the bilateral 

 symmetry of Holothurids has every mark of secondary origin : it 

 is totally unconnected with the bilateral symmetry of Spatangids, 

 as Cuenot has pointed out, and of the internal structure of Cystids 

 we know next to nothing. 



Since the examination of the adult structure yields so little result, 

 there has been a general tendency to look for light on the question 

 in the structure of the larva. Leaving the Crinoids apart for the 

 moment, Joh. Miiller has demonstrated the fundamental similarit}' 

 in structure of the larvae of Ophiurids, Echinids, Asterids and 

 Holothurids. In every case we have a strongly marked bilateral 

 form, with a complete alimentary canal and a ventral ciliated band 

 passing in front of the mouth and then on each side fringing a 

 well-defined ventral skin area, behind which is found the anus. 

 All the bizarre arms and processes which give these larvae such 



