METAZOA ECHINODERMA. 145 



ECHINODERMA. 



Sections 5, 6. 



CYSTOIDEA. 



Most palaeontologists consider the Cystoidea as the 

 Palaeozoic ancestors of the Echinoderms, while embryolo- 

 gists hold that this view is not supported by facts. Ac- 

 cording to Bury 1 there is not the slightest embryological 

 evidence that the Echinoderms have passed through a 

 stage in which they are fixed by the aboral pole, like the 

 Cystoids. He goes on to say, " Nevertheless, almost all 

 embryologists, apparently out of deference to palaeonto- 

 logical conclusions, have thought it necessary to assume 

 that ontogeny is misleading, and that a period of fixation 

 really did take place of which all traces have since disap- 

 peared. Now this involves us in a question of fundamen- 

 tal importance. If palaeontologists have really proved 

 beyond any reasonable doubt that the Echinozoa are de- 

 rived from fixed ancestors, then ontogeny is misleading; 

 but if it is misleading to such an extent as to obliterate 

 all traces of a process of such immense importance, I for 

 my part do not see how we can trust it in other particu- 

 lars, and those who rely upon it for indications of phylo- 

 genetic history had better reconsider their position." 

 Professor Bury then takes up the question at length from 

 the embryological point of view and deduces reasons for 

 considering that the ancestors of Echinoderms were unat- 

 tached forms. 



It has been shown by Hyatt and other investigators that 

 it may be possible for all traces of an ancestral structure 

 to be lost, and still the ontogeny of the individual be abso- 



1 Metamorphosis of Echinoderms. Quart. Journ. Micr. Sci., n. s., 

 XXXVIII, 1895, p. 93. 



