SYMMETRY REVERSAL AND MIRROR-IMAGING 183 



miliaris by subjecting the eggs and early larvae to hyper- 

 tonic sea water. In spite of the fact that the bilateral 

 condition is experimentally produced under abnormal 

 conditions, he adheres to his view that the development 

 of a right-hand hydrocoele in addition to the normal 

 left-hand one, is '^an indication that the common bilateral 

 ancestor of the Echinodermata had, corresponding to 

 the hydrocoele, a paired organ equally developed on 

 both sides of the body, and that, whilst the organ on the 

 left side became further developed until it grew to be 

 the water-vascular system and its appendages, the organ 

 on the right side dwindled and disappeared." 



It seems more logical to me to conceive of the ances- 

 tral echinoderm as a simple bilateral organism which 

 through some mutational change in the germinal proto- 

 plasm acquired an asymmetry which enabled only 

 the left-hand hydrocoele to develop. This originated 

 the first step in radial symmetry which culminated 

 in the condition now present. When both sides develop 

 hydrocoeles we have a condition equivalent to twinning, 

 which can hardly be thought of as an ancestral remi- 

 niscence. Moreover, we now know that instances of 

 reversed symmetry are ahnost as common as cases of 

 bilateral symmetry in echinoderm larvae, and it would 

 be impossible to consider a right-handed individual as 

 ancestral to a left-handed one. 



CASES OF SYAIMETRY Rj:VERSAL IN 

 ECHINODERMS 



A significant paper by Oshima (192 1) has recently 

 appeared in which that writer gives an account of the 

 discovery in certain laboratory cultures of Echinus mili- 



