SIGNIFICANCE OF CERTAIN LARVAE OF ECHINODERMS. \JJ 



Bather, and serve to carry it one step further. Each of these 

 students has reconstructed the hypothetical ancestor both in its 

 bilateral free swimming stage and the stage during which it be- 

 came radially symmetrical. The same plan is followed in this 

 paper. 



The papers of Bury, 1 McBride 2 and Bather 3 in which the 

 hypothetical bilateral ancestor of the echinoderms is recon- 



FIG. 9. Pluteus of JMellita testudinata. Original. A, anus; Ec/i., developing 

 sand dollar ; HI, mouth. Ciliated bands stippled. 



structed and figured, are so well known and the reasons for every 

 detail of the anatomy of the creature are therein so well set forth 

 that it would be a waste of the reader's time to again do more 

 than give an outline of the supposed structure of the hypothetical 

 organism, discussing such points only in which a change is made. 



1 Henry Bur}', " The Metamorphosis of Echinoderms," Q. J. Mic. Sc. , No. 149, 

 1895. 



2E. W. McBride, "The Development of Asterina Gibbosa," Q. J. Mic. Sc.,No. 

 151, 1896. 



3F. A. Bather, "A Treatise on Zoology." Part III., " The Echinoderma." 

 Edited by E. Ray Lankester, 1900. 



