506 THE BIOLOGY OF MARINE ANIMALS 



pigment movement is an inherent activity of a branched colour cell (Figs. 

 12.2, 12.3). There has long been some doubt about how the alteration in 

 the distribution of pigment is brought about in branched chromatophores. 

 Some workers believed that the chromatophore was an amoeboid cell, and 

 that it altered its pigmented surface by putting forth or retracting pro- 

 cesses in an amoeboid manner (expansion or contraction of the cell). This 

 can hardly be the case, however, for it has been observed in Crustacea that 

 a given chromatophore, on contraction and re-expansion, assumes the 

 same shape that it had before. Either the pigment material flows to and 

 fro in an irregular cell of fixed outline, or the cell expands and contracts in 

 preformed spaces of fixed pattern. In Palaemonetes and Hippolyte the 

 movement of pigment is due to the ebb and flow of cytoplasm through 

 fixed tubular spaces, which collapse when the cell is contracted and fill 

 out when the cell is expanded. Similar movements are also seen in the 

 chromatophores of teleosts (Gadus, MuIIus, Fundulus). During expansion 

 and concentration of the melanophores in these fish the pigment granules 

 flow along stable cellular processes that maintain a fixed position. Pigment 

 movement itself results from alterations in the sol-gel condition of the 

 endoplasm of the cell, and flowing of the cytoplasm (54). 



Direct and Indirect Responses 



From the physiological point of view chromatophores may be said to 

 respond to two modes of excitation; direct and indirect. In direct excitation 

 the chromatophore itself is responding to external stimuli impinging 

 directly upon it from the environment, and is acting as an independent 

 effector. In indirect excitation, on the other hand, the stimuli activate 

 receptor organs, usually but not always the eyes, and set in train a series 

 of events which finally influence the chromatophores. The terms primary 

 and secondary responses also have been employed frequently in referring 

 to stimulation through extra-ocular regions and to stimulation through the 

 eyes, respectively. 



It is usual for changes in the environment to be registered in the 

 animal's nervous system, which controls chromatophore activity through 

 efferent nervous impulses or endocrine secretions. It is known, however, 

 that blinded animals often continue to show chromatophore movements 

 in response to illumination, although true background responses are 

 usually lost. Studies on blinded Ligia, Palaemon and Hippolyte have shown 

 that these animals blanch (due to chromatophore contraction) in darkness, 

 and darken (due to chromatophore expansion) in light. The results of 

 some experiments on normal and blinded sea-slaters Ligia oceanica are 

 shown in Table 12.1. Blinded animals failed to show any difference in their 

 chromatic behaviour on white and black backgrounds, but displayed 

 graded chromatophore responses according to the intensity of overhead 

 illumination. In experiments of this kind it is undecided whether the light 

 is acting upon the chromatophores directly, or exerts its effect through 

 photoreceptors other than the eyes. Tait (66) observed no direct effect of 



