AUTHOR S ABSTRACT OF THIS PAPER ISSUED 

 BY THE BIBLIOGRAPHIC SERVICE, FEBRUARY 7 



ON THE OCCURRENCE OF PAIRED MADREPORIC 



PORES AND PORE-CANALS IN THE ADVANCED 



BIPENNARIA LARVAE OF ASTERINA (PATIRIA) 



MINIATA TOGETHER WITH A DISCUS- 



SION OF THE SIGNIFICANCE OF SIMI- 



LAR STRUCTURES IN OTHER 



ECHINODERM 



H. H. NEWMAX.i 



INTRODUCTION. 



Ever since the theory became current that the bilaterally sym- 

 metrical larvse of echinoderms afford phylogenetic evidence that 

 this group of radially symmetrical animals was derived from 

 bilaterally symmetrical ancestors, larvse that showed more than 

 the normal tendencies toward persistent bilaterality have had a 

 special significance. 



Normally, the first evidence of the encroachment of the adult 

 radial symmetry upon the the larval bilaterality is seen in the 

 development of a distinct hydroccel with a madreporic pore and 

 pore-canal on the left side, and none on the right. This failure 

 of the right side to keep pace with the left has been considered 

 as the mechanical cause for the twisting around of the serially 

 repeated primordia of the radial water canals and the assumption 

 of the adult radial symmetry. 



The occurrence, therefore, in the larvse of at least two classes 

 of echinoderms, of paired right and left madreporic pores, pore- 

 canals, and other derivatives of the hydrocoels, looks like the per- 

 sistence of the ancestral bilateriality and tends to strengthen the 

 current theory as to echinoderm phylogeny. 



As early as 1892 Field, in his account of the larval development 

 of Asterias vulgaris, described the transitory appearance in all 



i From the Hopkins Marine Station of Leland Stanford University and 

 the Hull Zoological Laboratory of the University of Chicago. 



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