THE CHROMATOPHORES OF ANIMALS. 105 



of highly coloured animals in which the value of colour is 

 still hypothetical, and in which there is no peculiarity of 

 form to distinguish any purely chromatic cells from other 

 pigmented cells which may be inferred to discharge re- 

 spiratory, nutritive or other functions ? The Turbellaria 

 and Echinoidea afford cases in point. 



Two attitudes are possible. The term chromatophore 

 may be applied to all pigmented cells which have not been 

 shown to discharge a respiratory or other non-chromatic 

 function ; or it may be restricted to pigmented cells which 

 are without doubt chromatic in function. The latter attitude 

 seems to me preferable, since it begs no question and yet 

 conduces to precision of language. 



Chromatophores are thus pigmented cells specialised 

 for the discharge of the chromatic function. 



This definition evades the difficulty presented by pig- 

 mented cells which, although respiratory or excretory, etc., 

 in function, do, nevertheless, contribute to the normal colora- 

 tion of the animal, as, for example, the elements of the vaso- 

 fibrous tissue of leeches. There still remains the question 

 whether the term chromatophore is applicable to the cells 

 of an epithelium in which pigment is deposited for in- 

 dubitably chromatic purposes, but which are not distin- 

 guished by any other features from the unpigmented ele- 

 ments of the same tissue. This case is presented by the 

 Insecta and Brachyura, in which groups the colours of the 

 pigments deposited in ordinary cells of the epidermis (hypo- 

 dermis of authors) frequently assume a high protective 

 significance. It is far from improbable that the distinctly 

 specialised chromatophores of other Arthropods have been 

 developed from such unspecialised pigmented cells as those 

 of the Insectan epidermis, so that the latter may certainly 

 be regarded as incipient chromatophores ; but the fact that 

 they are in no way set apart for chromatic purposes, that 

 this function is merely superadded to their ordinary task of 

 secreting the protective cuticle of the body, seems to me to 

 be decisive against their claim to the title chromatophore in 

 any other than a purely etymological sense. 



With the ground thus cleared we may proceed to the 



