The species of _Coenobita_ possess a very curious adaptation for aerial respiration. The soft skin of the abdomen is traversed by a network of bloodvessels and acts as a kind of lung, and a pair of contractile vesicles at the base of the abdomen serve as accessory hearts in promoting a specially active circulation in that part of the body. The lining membrane of the gill chambers also appears to aid in respiration as in other terrestrial Decapods. [Illustration: _PLATE XXVII_ THE COCONUT CRAB, _Birgus latro_. (MUCH REDUCED)] The "Robber Crab" or "Coconut Crab" (_Birgus latro_--Plate XXVII.) also belongs to the family Coenobitidæ, and has attracted much notice from its relatively gigantic size and its singular habits. Although resembling _Coenobita_ closely in essential structure, _Birgus_ differs from it and from most other Hermit Crabs in not making use of a portable shelter, perhaps owing to the difficulty of obtaining one of suitable size. The necessary protection for the abdomen is obtained by a redevelopment of the shelly plates (terga) on the upper surface of the abdominal somites. The abdomen is carried doubled underneath the body to protect the soft under-surface, and the animal, when threatened, seeks a shelter for its vulnerable hinder part in the nearest hole or cranny. The swimmerets are absent in the male sex, and are present only on one side of the abdomen in the female. This unsymmetrical development of the appendages is interesting as indicating the derivation of the Robber Crab from ancestors adapted to living in the unsymmetrical shells of Gasteropod Molluscs. The last pair of abdominal appendages, which in other Hermit Crabs serve to hold the body in the shell, are here much reduced in size, and quite useless for that purpose. The carapace is very broad posteriorly, owing to the great development of the branchial cavities, which are much too capacious for the very small gills. As in the true Land Crabs, the lining membrane of the gill cavity is thick and spongy, and traversed by numerous bloodvessels; but in this case its efficiency as a lung is added to by numerous tufted papillæ, which increase the surface exposed to the air. As in other Hermit Crabs, the last two pairs of legs are shorter than the others, and they end in small chelæ. The last pair are very slender, and are usually carried folded up within the gill chambers, which they possibly serve to keep clear from foreign bodies. The penultimate pair of legs are stouter, and the two pairs in front of these are long walking legs. The chelipeds are very strong and are of unequal size. When attacked, the animal defends itself, not, as might have been expected, with its chelipeds, but with the first pair of walking legs, the sharp points of which form very efficient weapons.