272 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



in Stentor (stentoriii) ; in some corals and hydroids 

 there is a red pigment (polyperythrin) ; ctiloro- 



cniorin has been obtained from various Poly cb seta ; 

 pent acriai hi from Pentacrimis ; and antedoniii 



from Antedon and a Holothurian ; the terms cms- 

 taceorubrin, aplysiopurpurin, ianthiiiin, 

 and bonellein explain themselves. Zooxanthin, 

 zooerythrin, zoofulvin, and turacin have been 

 extracted from the feathers of various birds. 



The question whether the characteristic colouring 

 matter of plants (chlorophyll) is formed by animals 

 is complicated by the undoubted fact that a number 

 of lower organisms have associated with them green 

 algae, which are not so much parasitic as symbiotic, 

 inasmuch as the oxygen which they evolve in the 

 presence of sunlight is of advantage to the anima 1 

 with which they live ; such are the so-called yellow 

 cells of Anthozoa and Radiolaria. Where no cell- 

 nucleus is seen to be associated with the green cor- 

 puscles, as is the case in Spongilla and Hydra, w 

 have no reason for refusing to suppose that the 

 chlorophyll has been formed by the animal itself. 



Some animals possess the power of changing more 

 or less rapidly in colour ; as, for example, the cuttle- 

 fish or the chameleon. This property is due to the 

 presence of chromatophores, or aggregations of 

 pigment surrounded by an envelope ; the latter is 

 provided with radiating muscles, by the contraction 

 or expansion of which the chromatophore becomes 

 flattened out, and the contained pigment displayed or 

 drawn into a denser mass, so as to appear merely as a 

 dark spot. In the chameleon, where the play of 

 colour is not so rapidly effected as in the cephalopod, 

 there are no radial fibres. Similar structures are found 

 less well developed in other lizards, and in some fishes*. 



The effects of structure are best shown by what are 

 ordinarily known as metallic colours. These are 



