688 Comparative Animal Physiology 



even when kept under constant conditions as regards illumination, espe- 

 cially in constant darkness. Such rhythms may persist for considerable 

 lengths of time. Idothea kept in total darkness has shown the 

 rhythm to persist at least 8 or 9 weeks.^^-"* In Uca, the diurnal rhythm 

 whereby the animals pale by night and darken by day''- •^'' is so strong that 

 only slight color change is induced by variation in background or illumina- 

 tion. ■'•* Some interesting characteristics of the rhythmical mechanism con- 

 trolling Uca chromatophores have been reported.'^'"'"' '•''"' Similar rhythms have 

 been found among vertebrates such as Lampetra,-**'^ Phoxiniis, salamander 

 larvae, frogs, Phrynosoma,^^''^ and the chameleon.-"'' 



In some instances where the diurnal changes are normally masked by re- 

 sponses to illuminated backgrounds, an underlying influence of the rhythm 

 may be evident in an increased rapidity of those responses which tend to 

 support, and the sluggishness of those which tend to antagonize, the par- 

 ticular phase of the rhythm at that moment. Thus many animals more 

 readily adapt to an illuminated white background during the night phase 

 and to an illuminated black one during the day phase, or show corresponding 

 differing susceptibilities to injections of chromatophorotropins in the night 

 and day phases. 



FUNCTIONAL ORGANIZATION OF CHROMATOPHORE SYSTEMS 



Annelids. Certain annelids become pale in darkness and dark when illum- 

 inated. This has been obser\'ed for the polvchaete, Nereis dumerWii,^'^ and 

 the leeches Piscicola geo-ntetra,^'^- '^'■^ Protoclepsis tessellata,^^'^ Heviiclepsis 

 marginata,^-- ^^ Glossiphonio complanata,^-- ^■^' ^'•"* and Placohdella parasitica 

 and P. rugosa.^''^ In no case has there been demonstrated any response to 

 color of the background. 



The rhynchobdellid leech, Placohdella parasitica, a common parasite on 

 turtles of the central United States, possesses three types of pigment cells. 

 These pigments contribute to the mottled brownish and white coloration 

 shown by these animals. The amount of white varies considerably from 

 specimen to specimen, ranging from the greater part of the dorsal surface 

 to a few minute lateral papillae and a short median line, both restricted 

 to the anterior half of the animal. One type of pigment cell containing a pale 

 yellowish, granular pigment occupies the characteristic longitudinal mid- 

 dorsal stripe, the numerous light papillae, and segmental blocks along the 

 margin. These cells show no physiological responses. Distributed over all 

 the darker areas of the body are relatively large chromatophores containing 

 dark greenish pigment. This latter pigment participates in physiological 

 color changes. Another pigment, reddish brown, is also located within func 

 tional chromatophores. These last chromatophores are much smaller than 

 the greenish ones and are located more superficially. The green pigment is 

 alcohol-soluble; the reddish brown pigment is alcohol-insoluble. The Eu- 

 ropean leech, Protoclepsis icsselata, possesses three pigments: brown, green, 

 and yellow. The Mediterranean polychaete. Nereis dmnerlii, possesses chro- 

 matophores containing yellow and violet pigments. 



1 he leech Placohdella parasitica, like other annelids showing physiological 

 color changes, blanches in darkness and darkens when illuminated. The 

 time required for the greenish pigments, which arc predominant in these 



