SYMMETRY REVERSAL AND MIRROR-IMAGING 179 



phy in man strikes the right or inferior side in about 

 three-quarters of the cases. Amphioxus is quite asym- 

 metrical, with many structures on the left or superior 

 side that are lacking on the right. We shall now ex- 

 amine a parallel situation among the echinoderms where 

 unilateral asymmetry is very marked indeed, but still 

 capable of experimental reversal. 



REVERSED SYMMETRY IN ECHINODERM LARVAE 



In sharp contrast with the vertebrates, in which 

 unilateral asymmetry is at best only shght, stand the 

 echinoderms in which certain structures of the left side * 

 grow so much more rapidly than those of the right that 

 they almost crowd out the corresponding right-hand 

 structures altogether. The early echinoderm larva is, 

 at least morphologically, bilaterally symmetrical, and 

 it is only in relatively advanced larval stages that the 

 left side begins to show its superiority over the right. 

 During the development of the coelomic pouches a 

 left-hand hydrocoele appears, which has no counterpart 

 on the right side. This hydrocoele is the primordium 

 of the madreporic pores and pore-canals and of the radial 

 water- vascular system. In sea urchins the presence of 

 the hydrocoele also stimulates the development of the 

 so-called amniotic invagination on the left side, which 

 has no counterpart on the right. 



SPORADIC INSTANCES OF BILATERALITY IN ECHINODERM 

 LARVAE AND THEIR SIGNIFICANCE 



One of the favorite anomalies of the invertebrate em- 

 bryologist is the occasional larva in which the hydrocoele 

 and its accompaniments appear on the right side as well 



