. The biology of marine animals. Marine animals; Physiology, Comparative. 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 -CH2 H°—%^ C
. The biology of marine animals. Marine animals; Physiology, Comparative. 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 -CH2 H°—%^ CHCOOH NH2 TYROSINE. -CJL HO— H0—S^ CHCOOH NH2 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. ). 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. ). 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 al
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