ON COLOURING MATTERS OF VARIOUS ANIMALS. 21 



instability of highly complex chemical substances would 

 render the existence of any but a very imperfect phylum of 

 pigments impossible. 



A considerable number of animal colouring matters with 

 banded spectra may be made to yield two diiferent spectra, 

 according as they are rendered acid or alkaline ; and they 

 exist in the animals in which they occur either in the acid 

 or alkaline condition, as shown by the spectra. In other 

 animals, as in the case of Antedonin in the Holothurian 

 and the Antedon, the colouring matter has three phases, 

 acid, alkaline, and neutral, and exists in the animals in the 

 intermediate neutral condition. 



Colouring Matters of Deep-Sea Animals. — Very little, if 

 any, light can penetrate from the surface of the sea to depths 

 such as 1000 or 2000 fathoms, and I believe that experiment 

 has shown that little or no effect is produced on sensitized 

 paper at the moderate depth of sixty fathoms. It is probably, 

 as far as solar light is concerned, absolutely dark at depths 

 of 1000 fathoms and upwards, and the fact that two blind 

 decapod Crustacea were dredged by us in 450 to 490 fathoms 

 (off Sombrero, D. W. I., March 15th, 1873) seems to point to 

 a condition of extreme darkness at much less depths. Never- 

 theless, several facts show that at these depths light of some 

 kind must exist. Some deep-sea animals are entirely desti- 

 tute of the eyes possessed by their shallow-water congeners, 

 and appear, like the blind cave animals, to rely on touch 

 alone, being provided with specially long antenna hairs, or 

 fine rays, for the purpose of feeling. Other animals, how- 

 ever, living in very deep water, have enormously enlarged 

 eyes, and hence some light must exist ; and a further evi- 

 dence that such must exist is the fact that several small deep- 

 sea Lophioid fishes have the dangling lures on their heads 

 specially developed, and apparently rendered attractive 

 with a view to enticing their prey, as in the case of 

 the Angler. Professor Sir C. Wyville Thomson and Dr. 

 Carpenter have suggested that phosphorescent animals 

 form the source of light in the deep sea. All the Alcyona- 

 rians dredged by the Challenger in deep water were found 

 to be brilliantly phosphorescent when brought to the surface, 

 and their phosphorescence was found to agree in its manner 

 of exhibition with the same conditions as are observed in 

 the case of similar shallow-water forms. There seems no 

 reason why the animals should not emit light when living 

 in deep water just as do their shallow-water relatives. The 

 light emitted by phosphorescent animals is quite possibly 

 in some instances to be regarded only as an accidental 



