518 THE BIOLOGY OF MARINE ANIMALS 



all three physiological groups distinguished above (shrimps Palaemonetes 

 and Crangon; hermit crab Pagurus; blue crab Callinectes; shore crab 

 Carcinus). 



The sinus glands of Crangon also yield yet another factor, relatively 

 insoluble in alcohol. When injected into eye-stalkless animals, it results in 

 concentration of red and black pigments in the telson and uropods and, 

 consequently, lightening of the tail. This principle is known as the Crangon 

 tail-lightening hormone. It occurs in the sinus glands of the first two groups, 

 but is absent from the third group (brachyurans) (10, 12, 23 58a). 



Subsequent evidence has shown that chromatophorotropins are also 

 secreted in other parts of the nervous system. When the central stubs of 

 eye-stalkless animals are stimulated electrically, changes in coloration 

 result, indicating the release of chromatophorotropins from sources other 

 than the sinus glands. Extracts of the tritocerebral commissure of Crangon 

 contain two principles, a body-lightening hormone and a body-and-tail- 

 darkening hormone. These same factors have been identified in the central 

 nervous system of many other malacostraca. The f/ca-darkening hormone, 

 previously recorded in the sinus gland, occurs in optic and supra- 

 oesophageal ganglia, whereas a white-pigment-concentrating principle is 

 most concentrated in the circumoesophageal connectives (6, 13, 61, 68). 



The complex picture of chromatophore control which is now emerging 

 may be underlined by reference to the prawn Palaemon serratus which con- 

 tains many types of chromatophores, differing in position, shape and colour. 

 These are differently affected by extracts of sinus glands and post-com- 

 missures, and at least five, and possibly more, factors appear to be involved 

 in their concentration and dispersion (41, 42, 62). 



Partial separation of the chromatophorotropins of P. serratus by electro- 

 phoresis has yielded two substances, one of which concentrates all the red 

 pigments of the body, and another which concentrates the large red 

 chromatophores and disperses the small red chromatophores of the body 

 and tail. They are possibly hormonal precursors giving rise to more mobile 

 agents that produce specific responses of each kind of chromatophore (45). 



The control of crustacean chromatophores, although solely hormonal, 

 is nevertheless extremely complicated. Five or more chromatophorotropins 

 are involved, some of which have antagonistic effects. They occur both in 

 the sinus glands and in other regions of the central nervous system. Some 

 are found in all Crustacea examined; others are more restricted in distribu- 

 tion. Although the total effect of these various chromatophorotropins has 

 not been determined for all the pigment cells of each group, it would seem 

 that the multiplicity of factors involved should eventually provide an 

 explanation for the intricate combination of chromatophore movements 

 that go to make up the complex colour responses of these animals (23a, 

 44a). 



Colour Responses of Fishes. The lower marine vertebrates charac- 

 teristically possess chromatophores, and respond by colour changes to 

 alterations in their environment (Fig. 12.2). The hagfish Myxine glutinosa 



