PREFATORY NOTE. 



This memoir is the thirty-second of a series designed to illustrate investiga- 

 tions and explorations connected with the Hopkins Seaside Laboratory, an adjunct 

 of the biological laboratories of the Leland Stanford Junior University. These 

 investigations have been carried on by means of the assistance given by Timothy 

 Hopkins, Esq., of Menlo Park, California. This memoir appears in the publica- 

 tions of the California Academy of Sciences, the present edition being a reprint. 



Oliver P. Jenkins, 

 Charles H. Gilbert, 



Directors Hopkins Laboratory. 

 Date of publication, February 6th, 1904. 



