COLORATION AND MIMICRY 283 



Sexual selection of course does not apply. 



Attraction to prey does not come in, for they feed on 

 infusorians, etc., brought within range by the currents 

 caused by the action of their cilia. 



There then remains warning colour sponges well stuffed 

 with flinty needles are apparently not good food for any- 

 thing, yet one animal eats them. The Nudibranch mollusc 

 Doris browses on them quite regardless of colour. Then, 

 again, how about the equally nasty ones that have no 

 warning colour, nor mimetic colour either. 



Evidently this does not apply. Beddard, in the intro- 

 duction to his "Animal Coloration," mentions this red 

 of sponges. He attributes it to a pigment named 

 Zoonerythrin, which, he says, has the property of absorb- 

 ing oxygen and converting it into ozone, " hence it is 

 clearly of great importance as a respiratory pigment, being 

 in a way analagous to the chlorophyll of plants." 



This does not seem to explain it, at least not the colours 

 of sponges generally, only a few species have (on our shores) 

 this red colour : Hymeniacidon Ingallii, Halichondria 

 sanguinea, and the ihin encrusting Ophlitaspongia. 



The vast majority are yellow, brown, grey, and white, 

 so do not possess this red Zoonerythrin, and obviously have 

 as much need of respiration as their neighbours. Yet 

 there must be a reason for these bright tints, and no solution 

 has yet been given. 



Passing to the Echinoderms, we have brilliant colours 

 in many rich purple and bright green in the same species 

 (Echinus lividus) and these strikingly opposite colours in 

 individuals living side by side under the same boulder. 



Sexual selection does not apply to animals the fertilisation 

 of whose ova takes place in open sea. 



Warning colour is not needed : the bristling, sharp, and 

 rigid spines would be sufficient warning to any investigating 

 nose. 



