colours 55 



produced in a gland which discharges them into the water. When 

 the two suhstances, called luciferin and luciferase, are mixed in 

 the presence of oxygen the luciferin hecomes oxidised and this 

 process excites the luciferase to produce light. The interesting 

 feature of this reaction is that the oxidation of luciferin is catalysed 

 by luciferase. so that the latter plays the dual role of an activator, 

 and of a substance which becomes activated. Luciferin and luci- 

 ferase are present in the discharges of other luminescent crusta- 

 ceans, but they show a certain degree of specificity. The luciferase of 

 a decapod will not produce light with luciferin from Cypridina, but 

 the luciferase from Cypridina will produce light with luciferin from 

 another ostracod, Pyrocypris. 



The chemistry of the non-discharging light organs in the bodies 

 of Crustacea has not yet been worked out, but it may well prove to 

 be more complicated than the system found in Cypridina. Other 

 animals with non-discharging luminescent organs, such as fireflies, 

 need other substances as well as luciferin, luciferase and oxygen 

 before they can produce light. 



The dark colours of Crustacea are due to two different types of 

 pigment which have been confused in the past; these are the 

 melanins and the ommochromes. The latter pigments are found 

 particularly in the eyes of Crustacea, but they also colour the body; 

 for instance, the brown colour of the shrimp Crangon is due to 

 ommochromes, so is the dark pigmentary pattern of the water slater 

 Asellns. A curious feature concerning the distribution of melanin 

 in the decapods is that it is only found in the Brachyura. and when 

 found it is associated with considerable concentrations of riboflavin, 

 which is an important component of the vitamin B complex, but 

 we do not know why this association occurs. 



The eyes of many Crustacea contain white reflecting pigments, 

 whose chemical nature is uncertain, but pterins have been identified 

 in the eyes of a number of decapods. The pterins were at one time 

 thought to be end products of the metabolism of nitrogen-contain- 

 ing substances in the body, but they now appear to play an active 

 part in the chemistry of an animal; the various functions which 

 have been discovered, or suggested, include a part in the manufac- 

 ture of melanin and in the chemistry of vision. The pterins are 

 usually coloured yellow, and one such pigment, xanthopterin, or at 

 least something very similar to it, has been isolated from the 

 carapace of the edible crab Cancer pagnrus. Several pterins are 

 colourless, but in ultra-violet light they fluoresce brilliantly. 



