COLOUR CHANGES 



513 



tint of the background. Octopuses also display complex colour trans- 

 formations, whereas squid (Loligo) and the paper nautilus (Argonauta) 

 have relatively simple chromatic responses (7, 35, 54). 



In the pelagic pteropods such as Cymbulia and Tiedemannia, there are 

 chromatophores which are very similar to those of cephalopods. As in the 

 latter animals they consist of a central pigmented sac surrounded by a 

 series of radial muscle fibres (54). 



Colour Responses of Crustacea. Among the Crustacea, chromatophores 

 occur in copepods, isopods, many amphipods, mysids, euphausiids, and 

 are particularly characteristic of decapods (Figs. 12.3 and 12.5). An amphi- 

 pod Hyperia galba, which is commensal on medusae, changes colour 

 according to the incident illumination, and is deep reddish-brown in light 



Fig. 12.5. Expanded White Chromatophores in the Prawn Palaemon 

 (a) From Palaemon adspersus, and (b) from P. serratus. (Drawn from photographs 

 of Knowles (39), and Stephenson, 1946.) 



and colourless in darkness. Dark animals also become colourless when 

 attached to medusae or an inanimate object, regardless of the colour of 

 the background. Schlieper concluded that colour changes are regulated in 

 Hyperia both by light acting through the eyes, and by tactile stimuli acting 

 on receptors in the legs. The pale phase is of value to the animal when it is 

 attached to its host, a transparent medusa (54). 



Littoral isopods are provided with chromatophores and demonstrate some 

 ability to alter their shade and colour. Species of Ligia and Idotea, which 

 have been used for experimental studies, have melanophores, xantho- 

 phores, guanophores and a non-cellular white pigment as well. Ligia 

 oceanica, Idotea tricuspidata and /. baltica show well-marked background 

 responses, darkening on black backgrounds and blanching on white back- 

 grounds. Apart from changes in shade from light to dark, these animals 

 show no true colour responses to match the tint of the background, and 

 this is referable to the limited variety of chromatophore pigments present. 

 Besides these pigments the colour of cuticle and of underlying tissues 



M.A. — 17 



