ENDOCRINOLOGY OF CRUSTACEANS 185 



their study to demonstrate the presence of a dark-adapting hormone was 

 under the environmental condition of complete darkness. 



ClIROMATOPIIORES 



The first systematic study of a physiological process in crustaceans 

 which gave evidence of endocrine regulation was that of color change, the 

 effectors in this case being the pigment cells located within the hypodermis 

 and on the deeper-lying organs of the body. The readiness with which these 

 chromatophores may be discerned depends, among various crustacean 

 species, on the degree of transparency of the overlying tissues and exo- 

 skeleton. The chromatophoral systems of different species may range 

 through a complexity of colors, morphology, and distribution over the 

 body surface ; the physiological responses are effected by centrifugal or 

 centripetal streaming of the chromatophoral cytoplasm in which the pig- 

 ment granules are carried, resulting in the dark or colored phase of the 

 animal when the pigment is dispersed through the interlacing processes of 

 the chromatophores, and in the light phase of the animal's color change 

 when the pigment granules are withdrawn from the chromatophoral pro- 

 cesses and are concentrated near the center of the cell body ; intermediate 

 conditions between the extremes of response are also possible. The colored 

 appearance of an animal may be determined by the abundance and distri- 

 bution of a particular type of chromatophore ; where there are physio- 

 logically responsive chromatophores containing an assortment of pigments 

 in the system of an animal, the animal may be able to assume a variety of 

 colors, depending on which chromatophore components have their pigment 

 granules dispersed and which concentrated. In some species color changes 

 may occur in adaptation to the color of the background, in others to change 

 in light intensity. In investigations of color change, the most obvious com- 

 ponent of the variegated chromatophore system has been the one usually 

 studied, while those chromatophores which are less abundant have been 

 relatively slighted. 



The studies establishing an endocrine basis for color change in crus- 

 taceans, the subsequent observations on the diversity of response of differ- 

 ent chromatophore systems in a variety of crustacean species, and some of 

 the attendant problems that arose from these studies have been amply 

 leviewed by the authors mentioned in the introductory section of this 

 paper. In recent years a large proportion of the studies of color change 

 has been concerned with the demonstration by extraction and injection 

 methods of the presence of different active principles from the central 

 nervous system of crustaceans. Many of these studies were prompted by 

 an effort to resolve one of the early problems in this field ; i.e., whether 

 the regulation of the various types of chromatophores in a color-changing 



