Chromatophores and Color Change 689 



changes, to complete their concentration in darkness or dispersion in light 

 is approximately one hour. 



There are several types of evidence all pointing to nervous control of the 

 green chromatophores. Decapitation or any other transection or injury of 

 the nervous system results in a darkening of pale animals kept in darkness.'^" 

 This darkening may persist for many hours. If a uniformly pale leech in dark- 

 ness is stimulated faradicallv at either the anterior or posterior end the 

 whole animal darkens. If, hovvex'er, the experiment is repeated with a speci- 

 men whose nerve cord has been transected in the middle, only that half of 

 the animal receiving the stimulus darkens, the body beyond the point of 

 transection remaining light. The experiments furthermore give little evidence 

 lor direct action of light on the chromatophores. 



The two eyespots at the anterior end appear to play an active but by no 

 means an exclusive role in the responses. Decapitated specimens show the 

 characteristic changes even though responding more sluggishly. When speci- 

 mens are brought from darkness into light and one half of the body is im- 

 mediately covered with opaque paper, the covered portion remains largely 

 in the dark-adapted phase while the uncovered portion becomes com- 

 pletely light-adapted. The results are more striking when the posterior end 

 is covered than when the anterior end is. These experiments suggest a role of 

 generally distributed photoreceptors operating through segmental reflexes. 



The reddish brown pigment of Placohdella parasitica appears to respond 

 independently of the green and shows no predictable responses to background 

 or to light intensity. Its condition is more or Jess variable in normal speci- 

 mens. When dispersed it can usually be made to concentrate by intense 

 stimulation of the animal. In animals with transected cords, the response to 

 electrical stimulation, when it occurs, passes as in the case of the green 

 cells, only to the point of transection. Thus it would appear that the net- 

 vous system controls the reddish-brown chromatophores directly, but that 

 excitation induces concentration rather than dispersion, as in the case of 

 the green. 



Echinoderms. It was reported many years ago-'*^ that the Mediterranean 

 sea-urchins, Arhacia pustidosa and Centrostephanus longispinus, became 

 lighter in color on transfer from light to darkness. These observations have 

 recently been confirmed."^ Illuminated Arhacia pustulosa are blackish in 

 color whether on a white or on a black background, but in darkness become 

 brown in color. Centrostephanus longispinus, which are dark purple in light, 

 change in darkness to gray. The color changes require about one or two 

 hours for their completion. Microscopic examination of tube feet removed 

 from light-adapted and dark-adapted individuals show numerous reddish 

 brown chromatophores with their pigment dispersed in the light-adapted and 

 concentrated in the dark-adapted individuals. The color changes in these 

 urchins therefore appear to be due to the movements of pigments within 

 definite chromatophores. The chromatophores of isolated tube feet which 

 have been mounted on a microscope slide respond to illumination and dark- 

 ness in the same manner as when they are present in the intact animal, in- 

 dicating that the chromatophores are responding to the light either directly 

 or by way of local reflex pathways comparable to those known to function 

 in the locomotor movements of the tube feet. 



