342 H. H. NEWMAN 



paired madreporic pores, or at least paired pore canals. Figure 

 46 is a simplified drawing of one of these. As is well known, the 

 advanced bipinnariae of asteroids normally develop madreporic 

 pores and pore canals only on the left side, and this asymmetrical 



Fig. 46 Outline of advanced bipennaria larva seen from the ventral side, to 

 show especially the coelomic system and the paired madreporic pores and pore 

 canals. Twenty-six other larvae of this kind were found in one culture. 



growth initiates the change from bilateral to radial symmetry. 

 The production of the paired condition in many larvae, instead 

 of the single condition typical for the group, is, in our opinion, 

 to be interpreted as only another phase of twinning. These 

 larvae are looked upon as the result of a partial physiological 

 isolation of the bilateral halves of the larva, the consequence of 



