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Differences in coloin- between the sexes have also been noticed, though they 

 do not appear to be of common occurrence. In the Oelasimi one cheliped of the 

 adult male is not only enormously enlarged, but its chelaj are also brilhantly 

 coloured. In one species of this genus the brightly coloured chelaj are obtru- 

 sively displayed by rival males, apparently to attract the female. And in more 

 than one species of the same genus the carapace of the adult male is known to 

 be more brilliantly coloured than that of the female, at any rate under the 

 influence of sexual rivalry and excitement. 



Several Brachyura {e.g., species of Matnfri, Oci/jwda and Metaplax) possess 

 organs for the production of sound. These organs consist of two opposable file- 

 like ridges, the ^^brations of which, when they are rhythmically rubbed against 

 one another, give rise to grating and whirring sounds of considerable power. 

 The more perfect of the two ridges is placed on some part of the flexor aspect of 

 the cheliped ; the opposed ridge may either be placed on another part of the 

 cheliped or below the orbit. 



In Mai Ufa and Ocijpoda both sexes possess these organs and have been 

 observed to use them when moved by fear or anger. In MefapJa.c the male alone 

 has them, so that in this case they are probably used to express sexual emotions. 



As regards the part they play in Nature, the crabs, in common with other 

 Crustacea, are generally regariled as scavengers. They are more particularly 

 the scavengers of the sea-shore, and in tropical islands land- and strand-crabs 

 are of much importance as land-scavengers also. 



On the other hand the marine species are one of the principal sources of 

 food of numerous fishes, especially of Rays, and remains of small crabs have 

 been found in the stomach of starfish. Among the chief depredators of the 

 shore and freshwater and amphibious species in this country may be mentioned 

 water-lizards {Varanus) and ci'ocodiles and occasionally frogs, swimming- and 

 Avadiug-birds of all kinds, kites and crows, jackals and other small carnivorous 

 mammals and sometimes pigs and monkeys. 



[Ou the general bionoiiiy of N[alapo.straca see Ortmanii, in Bioim's Tliier-Rcicli, Malacos- 

 traca,pp. llSl-1262]. 



II, Gexeeal Classification' of thk Bi;A('uvri;A. 



Class CRUSTACEA. 



Order MALACOSTRACA. 



Suborder Beachyuka, Leach, Boas, Stebbing. 



Caneri Brachyuri, Liuneus, SyBtema Naturu-, ed. xii. torn. I, para ii. p. 1038 (1767). 

 Kleistagnatha {paxt), Fabrioius, Entomologioe Systcmaticiu Supplementum, p. 307 (ITli'^l. 

 Caneri Brnchyuri (part), Lamarck, Syat. des Animaux sans vertebres, p. 148 (1801). 



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