482 



THE BIOLOGY OF MARINE ANIMALS 



skeleton of crustaceans. Although usually located in the skin, they also 

 occur in deeper tissues, as in the parietal epithelium of the plaice Pleuro- 

 nectes platessa. In echinoderms (Holothwia, Diademd) the melanin pig- 

 ment is elaborated by amoebocytes which deposit it in the body wall. 

 The inky material produced within the ink sac of cephalopods is a melanin 

 and tyrosinase is found in the epithelial tissue of that organ. When the 



Br— 



V-^N 



O o = 



N^\^ 



—Br 



6 : 6' — DIBROMINDIGO 



-CH 2 

 H °— %^ CHCOOH 

 NH 2 



TYROSINE 



-CJL 



HO— 



H0— S^ CHCOOH 

 NH 2 



DOPA 

 DIHYDROXYPHENYLALANINE 



.-COOH 



HALLACHROME 



animal is disturbed it discharges an inky secretion from its mantle cavity 

 and takes to flight behind the black screen (7, 17, 19, 21, 22, 41, 50, 54, 59, 

 67). 



Purines. Purine substances, important in the surface coloration of certain 

 groups of animals, are derived from the metabolism of nucleoprotein 

 (p. 294). Purines are themselves white or yellow in colour, but because of 

 their crystalline structure they are often responsible for the structural 

 colours encountered in so many animals (p. 484). Among coelenterates 

 uric acid deposits occur in the anemone Metridium senile, where they form 

 white bands in the endoderm of the tentacles (Fig. 11.3). In the nudi- 

 branch Janolus cristatus the conspicuous chalky white tips on the dorsal 

 papillae and the white patches on the skin are due to the deposits of 

 guanine (Fig. 1 1.4). Structural colours produced by guanine are character- 

 istic especially of Crustacea, cephalopods and fishes. The iridescent layers 

 of the skin of teleosts and cephalopods are due to crystals of guanine, which 

 are located in iridophores underneath the epidermis. A particularly abun- 

 dant deposit of guanine crystals in the dermis is responsible for the stratum 

 argenteum of fishes. When sufficiently thick, guanine particles fail to show 

 iridescence and reflect all the incident light; under these conditions they 

 form opaque white surfaces, seen for example in the bellies of selachians 

 and bony fishes (17, 22, 80). 



