106 THE LIFE OF CRUSTACEA 



Of somewhat similar habits are the numerous 

 species of the genus Gelasimus (" Fiddler Crabs " 

 Plate XV.), which abound on sand and mud flats of 

 tropical shores. These little Crabs are remarkable 

 for the great dissimilarity between the sexes in the 

 form of the chelipeds. In the female both chelipeds 

 are small and feeble, but in the males one of them, 

 either the right or the left, is enormously enlarged, 

 sometimes exceeding in length and breadth the body 

 of the Crab which carries it. What the precise use 

 of this enormous claw may be does not seem to be 

 quite certainly known. It is said to be used as a 

 weapon by the males in fighting with one another, 

 but it seems too clumsy to be very efficient for this 

 purpose. It is often brilliantly coloured, and has 

 been supposed to be a sexual adornment. 



In Ocypode and Gelasimus the respiratory apparatus 

 is modified for the purpose of breathing air. The 

 gills are similar to those of purely aquatic Crabs, and 

 no doubt serve for respiration when the animal is in 

 the water; but the gill chambers are much more 

 spacious than usual, and the lining membrane is 

 richly supplied with bloodvessels. Air is admitted 

 to the gill chambers by an opening, protected by a 

 brush of hairs, between the second and third pairs of 

 walking legs on each side. It is believed that in 

 this way the gill chamber is fitted to be used as a 

 lung when the animals are out of the water. Similar 

 arrangements in some of the more exclusively 



