PIGMENTS AND COLOURS 



485 



Trigla lucerna the argentea of the eye has a golden appearance, owing to 

 the presence of guanine crystals lying subjacent to a layer of carotenoids 

 (29). 



Other common structural colours to be observed are the metallic sheen 

 on the cuticle of Nereis diversicolor and Lumbriconereis impatiens, the 

 golden crown of bristles in Pectinaria koreni and the iridescent sheen of the 

 lateral setae of Aphrodite aculeata, which Linnaeus described "as reflecting 

 the sunbeams from the depth of the sea, exhibiting as vivid colour as the 

 peacock itself spreading its jewelled train". The closely related Hermione 

 hystrix has setae showing golden iridescence, and the long-spined sea urchin 

 Diadema setosum possesses bright blue glistening eye spots. Diffraction 



wo 



80 



o 60 



40 



<* 20 



0-4 6 0% PO P2 7-4 



Wave-length (p) 



1-6 



1'8 



Fig. 



1 1.5. Reflexion of Different Wave-lengths from 

 Iridescent Pieces of Mother-of-pearl 

 One piece showed reflexion maxima at Oll/u and 1-40^, the other at 0-64^ and 1-28//. 

 These wave-length maxima are simple multiples and demonstrate the basis of inter- 

 ference effects. (From Pfund (69).) 



from fine cuticular lines may play a part in producing the colours of some 

 of these animals as distinct from purely interference effects resulting from 

 lamellar structure (17, 80). 



BIOLOGICAL SIGNIFICANCE OF PIGMENTS: 

 EXOGENOUS PIGMENTS 



Animals either obtain pigments from their food, or manufacture them from 

 suitable precursors. The most widespread of exogenous pigments are 

 carotenoids which come, ultimately, from plant sources. Animals are 

 unable to synthesize these compounds, although they can often alter them 

 to suit their particular needs. As an example may be cited the experiments 

 of M. and R. Abeloos-Parize (1) who found that specimens of Actinia 

 equina, reared on carotenoid-free food, were themselves deficient in caro- 

 tenoids on reaching maturity. Anemones deficient in carotenoids, when 

 fed on shrimp eggs rich in these substances, rapidly recovered their original 



