34o 



THE POPULAR SCIENCE MONTHLY 



The chromatophores, on the other hand, are effectors which are in no 

 sense derived from muscles. These organs enable many animals to 

 make relatively sudden changes in their external coloration, and though 

 they are present in many animals, they are most perfectly developed in 

 the arthropods, mollusks and vertebrates. They are also present in 



the more complex types of eyes, where their 

 movements serve to protect the receptive ele- 

 ments from exposure to excessive light or to 

 open them to the full effects of dim light. The 

 investigation of these organs dates from com- 

 paratively recent times and van Eynberk (1906), 

 who has recently summarized our information 

 about them, has shown that the accounts already 



(given are in many respects contradictory. 

 Hence what I shall have to say I shall draw 

 ^J mostly from those fields with which I am some- 

 what acquainted at first hand. 



That some chromatophores are completely 

 independent of nervous control even though 

 they are most intimately associated with nerv- 

 ous mechanisms is well attested. The deeper 

 part of the compound eye in the shrimp, Pdlce- 

 monetes, contains a layer of cells, the retinular 

 cells (Fig. 1), which though they carry rhab- 

 domes and end proximally in nerve-fibers and 

 are therefore unquestionably sensory cells, con- 

 tain many dark pigment-granules which change 

 positions in accordance with the illumination. 

 From this standpoint these cells are true chro- 

 matophores. In an eye exposed to the light the 

 pigment-granules occupy distal positions in these 

 cells; in one in the dark they come to lie in 

 proximal positions. The place occupied by the pigment in a given eye 

 is entirely determined by the presence or absence of light in that eye, 

 for the two eyes have no sympathetic relations. Moreover if a per- 

 sistent shadow is cast on part of one eye, the condition characteristic for 

 the dark is assumed by that part even though the pigment in the rest of 

 the eye is in the position characteristic for light. These observations 

 show the physiological independence of the chromatophores in different 

 parts of the eye. These organs, though connected by nerve-fibers with 

 the central nervous organs, are also in their action independent of such 

 parts, for the movements of their pigment from the dark to the light 

 position and the reverse go on in an essentially normal way even after 

 these connections have been cut. Chromatophores then may carry out • 

 under direct stimulation somewhat complicated pigment-migrations in 



Fig. 1. Two Elements 

 from the Compound Eye of 

 a Shrimp, showing the dis- 

 tribution of pigment in the 

 light (A) and in the dark 

 (JB). b, basement membrane; 

 c, cuticula; en, cone; n, nerve- 

 fiber ; r, retinular cell. 



