INTRODUCTION 1 1 



the shrimp. When a number of the eye-stalks of pale 

 Palaemonetes were crushed and extracted with sea- 

 water a solution was obtained which on being injected 

 with proper precautions into a dark shrimp of the same 

 species caused the pigment in its chromatophores to 

 concentrate and the shrimp to blanch. Perkins showed 

 further that the cutting of nerves in Palaemonetes had 

 no effect whatever upon its color changes, but that a 

 temporary obstruction to the flow of blood in certain 

 of its blood-vessels was followed by a very profound 

 color change. These various observations have been 

 repeated and confirmed by subsequent workers and with 

 such a wealth of detail that we are now fully justified 

 in concluding that among crustaceans, as among am- 

 phibians, nerves play no direct part in the control of 

 chromatophores which at least in these groups of ani- 

 mals are under the exclusive influence of special hor- 

 mones, the neurohumors. Such secretions are produced 

 in some distant part of the body under excitation re- 

 ceived from the eye and transported from their region 

 of origin by the blood to the responding color-cells. 

 Thus it appears that color changes may be controlled 

 in animals through one or other of two radically different 

 physiological systems: a direct nervous control as seen 

 particularly in fishes and in reptiles, and a secretory or 

 neurohumoral one as exemplified in crustaceans and am- 

 phibians. It is now proposed to examine these two 

 types of control and to ask the question, are they as 

 different as at first sight they appear to be, or have they 

 elements enough in common to allow them to be brought 

 under one general plan of action (Parker, 1932)? In 

 attempting this analysis I shall discuss the color changes 

 of two fishes, the common smooth dogfish, Mustelus 

 canis, and the killifish, Fundulus heteroclitus, a small 

 top-minnow of the Atlantic coast. 



