SYMMETRY REVERSAL AND MIRROR-IMAGING i8i 



look upon this situation in a somewhat different light. 

 While still holding that the appearance of bilaterally 

 symmetrical larvae is a sort of twinning phenomenon, 

 I believe that it is really a phase of symmetry reversal. 

 In the typical larva, with but one hydrocoele complex 



Fig. 64. — Ventral view of an advanced bipennaria larva, one of many- 

 observed by me, of the starfish Patiria miniata. The water pores, 

 pore-canals, and hydrocoele vesicles are symmetrically developed on 

 both sides. (Original.) 



