Adaptive responses to different environnients can be functional, structural, be hiavioral or a combination of these. The exposure to different environments can be (lime to changes iii environmental factors or due to the invasion of new habitats l)y a progressive migration of organisms. Invasion of the land has required a numitber of such adaptations in amphibious crabs (Bliss, 1968), foremost amiiong these beimigadaptations connected with respi ratory mechani sni s. ConiparisomiS be tweemi aquatic amid terrestrial crabs have shown that the latter have fewer gills ( Pearse, I 929, 1950) amid reduced gill surface per unit weight (Gray, 1957 ; Bliss, 1963). Also, the gills of terrestrial crabs have become highly scierotized on the edges, presumably to keep themii erect and functional in air ( Harms, 1932) . A conmiectiomi has often been mentioned betweemi the globose carapace of Gecarcinidae, Potamonidae, Pseudothelphusidae and Trichodactyhidae and their ability for aerial respiration. It has been suggested that, durimig the evolution towards a terrestrial habitat, the reduction of gills and gill area is compensated by an increasingly more important role in respiration played by the thin epithehial nienibrane that limies the branchial chamiibers (Carter, 1931; Edney, 1960). The hinimigof the gill chamber has been referred to by many authors as the “?�lung” of land crabs. Indeed, some indications of this “?�lung” exist in the vascularized epithelium of the branchial lining in Uca (Jobert, 1876) and Cardisonza and in the tufts arising from the branchial lining in species of Ocypode, Coenobita amid Birgus (Borradaile, 1903; Harnis, 1932). Of the 4,500 species of brachyuran crabs so far known, many which live in intertidal zones camiwithstand various degrees of exposure to air. However, true amphihiotms life is restricted to a few terrestrial and freshwater forms, niost of thieni belonging to the families Grapsidae, Gercarcinidae, Ocypodidae, Potamonidae, Trychodactylidae, Pseudothelphusidae, and Myctyridae (Bliss, 1968). In the present study, evidence is presemited omithe respiratory fumiction of the himiimig of the branchial chamber in several semiterrestrial brachyuran species from niarine, estuarine, and freshwater habitats. In the course of the present study, seven brachyuran species belonging to four families were used for estimates of branchial chamber volume, observations of the chamber's shape, and examination of the structure of the lining which covers the chamber. The sites of collection varied from the suprahittoral zone of sandy beaches to the mountains of tropical forest and the llanos (savanna-like forest).