COLOUR CHANGES. 443 



condition, but on the whole, as the segments of the body become 

 approximated together and lose their external distinctness 

 (and the approximation is doubtless accompanied by an 

 increase in the complexity in the co-ordinating mechanisms by 

 which the movements of their appendages are bound together) 

 so do their corresponding neuromeres merge one with another. 

 The concentration is complete in the Brachyura, where, in such 

 forms as the Spider Crabs (Maia, Fig. 241, F) the ganglia of all the 

 postoral segments are fused into a common star-shaped mass 

 in the thorax, from which the nerves radiate to the several 

 parts of the body. 



A number of species of Malacostraca with thin and trans- 

 parent cuticle are capable of changing their colour, in varying 

 degrees, in relation to their surroundings. The phenomenon 

 was investigated by Pouchet,* and more recently, in two remark- 

 able papers by Keeble and Gamble. | Only some of the results 

 can be touched on here. 



The chromatophores, in which the pigment is lodged, are in 

 the Decapods multinuclear bodies, probably clusters of cells, 

 together forming a central mass, with branching processes, 

 which anastomose with those of neighbouring chromatophores. 

 Each chromatophore may contain a number of pigments of 

 different colours. Some (reds and yellows) are light-transmitting 

 pigments, others (white, yellow and blue) yield their colour 

 when seen by reflected light. The pigment expands and con- 

 tracts within the chromatophore, without alteration in shape of 

 the latter (as has also been shown to be the case in the frog). 

 There is reason to believe that each separate pigment is contained 

 in a separate cell or group of cells. J 



The chromatophores of Decapods are classed in two systems. 

 (1) The primary system is deep-lying and mainly in relation 

 with the ganglionic centres (segmental and visceral). It is 

 developed in the larva and closely resembles that of the adult 

 Schizopod Macromysis ; (2) a secondary system, more super- 



* G. Pouchet, " Les changements de Coloration sous 1'influence des 

 nerfs," Journal de I'Anat. et de la Physiologic, T, 12 (1876). 



f Hippolyte varians,a study in Colour-change, Q.J.M.S.,vol. 43 (1900), 

 p. 589, and the Colour-Physiology of higher Crustacea, Phil. Trans. B. 

 vol. 196 (1904), p. 295. 



J In addition to pigment Hippolyte has in its chromatophores a mobile 

 colourless fat which expands and contracts with the pigments. 



