THK KKLATION BETWEEX LIGHT AND PIGMENT-FORMATION. 553 



monochromatic light are the two chief problems dealt with 

 here. The conclusion is diawn that when monochromatic 

 light is made to fall upon all sides of the experimental animals, 

 so as to obviate a strong background effect, the result is a 

 pigmentation complementary to the colour of the incident 

 light and also to that obtained in Hippolyte by the use of 

 coloured backgrounds and white light. 



(1) Methods. 



Although none of the methods employed for rearing the 

 larva3 of Hippolyte were thoroughly successful, the record 

 of the attempts made on this very difficult problem may be 

 of assistance to other workers. A means of obtaining a 

 satisfactory solution is one of the most pressing needs of 

 experimental biology. 



The vessels used consisted of large bell-jars, supplied with 

 an air- or water-current or stirred by a glass plunger. 

 Seasoned vessels as well as sterilised ones were used; filtered, 

 '^ outside," and tank-water were respectively employed; 

 diatoms (Nitschia) and algiil cultures (Phaeocystis mixed 

 with other green flagellates) were used as food. The vessels 

 were shaded, exposed to diffuse light, and kept in darkness ; 

 tlie backgrounds were translucent, absorbent, and reflecting ; 

 the incident light used was monochromatic (red and green) 

 as well as white light. The temperature was kept down to 

 16° C. by a water-jacket, and in other cases allowed to rise to 

 18° C. or over, but in spite of all these variations of treatment 

 the lavvEe only survived about ten days. It is possible that 

 some means of removing the first sickly specimens would be 

 a great improvement, and it is, ot course, also likely that a 

 better diet could be found. The larvae, however, readily eat 

 green flagellates and seemed to digest them. 



The monochromatic screens used in the case of larvae con- 

 sisted of selected pieces of coloured glass (ruby or green) 

 combined with coloured gelatine films. These were placed 

 over the inverted bell-jars, the sides of which were converted 

 into absorbing or reflecting backgrounds. 



