NOTES ON ANIMAL COLOURATION. 7 



Mollusca : — Pholas, Lima, Cardium, Teredo, and others, mostly 



burrowing. 

 Crustacea : — The burrowing crabs Corystes and Thia, also the 



burrowing prawns Callianassa (white) and Gebia 



and Axius (pinkish). 

 Echinoderms : — Cucumaria (hiding). 



Ophiura albida (calcareous). 

 Tunicata : — Circinalium concrescens (calcareous spicules). 

 Fishes : — Under surfaces of Flat fishes — Plaice, Turbot, Soles ; also 

 of Ray fish and their relatives. 

 In our present obscure acquaintance with colour significance, it 

 behoves us to proceed with caution in our speculations. Still two 

 striking facts come out in perusal of the foregoing list ; first, that 

 white is associated in most cases with either the presence of a 

 calcareous skeleton (Forams, Sponges, &c), or with the habit of 

 burrowing. In the latter case it does not seem to be the absence 

 of light that prevents the development of colour — there are abundant 

 instances of deep pigmentation developed in the dark. Cases in 

 point are the burrowing purple Spatangus, the dark-green lug-worm 

 (Arenicola), pigmentation of the nerve system of Amphioxus, of the 

 mesentery of the Frog, &c. 



Animal colouration has, I believe, always direct value to the 

 possessor and is a developed attribute, being kept in esse solely 

 by the continuation of the original exciting cause or causes. The 

 principal of these are protection, warning, luring, sexuality and food 

 supply. Take the exciting factor away and gradually the colouring 

 having lost its usefulness will disappear. The production of colour 

 must mean expenditure of energy ; an allowable expenditure so long 

 as it serves a useful purpose, but which becomes waste when it 

 ceases to do so — a waste which natural selection is bound to dispense 

 with in time. What does this lead to ? Clearly that it is probable 

 that the burrowing forms that are albino (worms, crustaceans, 

 molluscs, &c.) may have dispensed with a former colouring because 

 of a present non-requirement of such. The same explanation, I 

 believe, applies to the under or white side of flat-fishes. 



IV. The Colouration of Sponges. 



In the list given above of albino animals, the long list of white 

 calcareous sponges is remarkable— no other order of animals is so 

 consistently uniform in colour. The explanation I have to offer is 

 that this is due (a) to the spongin or ground tissue of the sponge 

 being reduced to a minimum, and (b) to the shape of the spicules 

 being such that the optical effect produced by a dense closely packed 



