Page:Popular Science Monthly Volume 75.djvu/344

340 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 elements from exposure to excessive light or to open them to the full effects of dim light. The investigation of these organs dates from comparatively recent times and van Rynberk (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 mostly from those fields with which I am somewhat acquainted at first hand.

That some chromatophores are completely independent of nervous control even though they are most intimately associated with nervous mechanisms is well attested. The deeper part of the compound eye in the shrimp, Palæmonetes, contains a layer of cells, the retinular cells (Fig. 1), which though they carry rhabdomes and end proximally in nerve-fibers and are therefore unquestionably sensory cells, contain many dark pigment-granules which change positions in accordance with the illumination. From this standpoint these cells are true chromatophores. 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 persistent 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