2o8 THE BIOLOGY OF THE SEA-SHORE 



have proceeded in animals of truly aquatic origin it is neces- 

 sary to study the many kinds of tropical land-crabs : Grapsiis, 

 Ocypoda, Gelasimus (Fiddler-crabs), Geocarcinus and Birgus, 

 In practically all of these the largely terrestrial habits are 

 accompanied by observable modification of the respiratory 

 apparatus. In Ocypoda and Gelasimus the gill-chambers 

 are larger than usual and their lining membrane is richly 

 vascular. Air is said to enter the gill-chambers by an 

 opening, protected by a brush of setae, between the second 

 and third pairs of walking legs on each side (Caiman, 191 1). 

 The enlargement of the gill-chamber and the vascularisation 

 of its wall are closely analogous to what is seen in the tropical 

 fishes with similar amphibious habits. The modifications 

 for air-breathing in the robber-crab {Birgus latro) have been 

 studied by Semper {op. cit.), who found that the gill-cavity 

 is divided into an upper and a lower half by a transverse fold. 

 The lower half contains the gills which, though numerous, 

 are reduced in size ; the upper portion functions as a true 

 lung and contains nothing but air ; it also has its lining 

 membrane produced into numerous arborescent tufts 

 which are extremely vascular. A further striking feature is 

 to be found in the arrangement of the blood-vessels, which 

 is precisely that of a true lung : blood poor in oxygen entering 

 the chamber from the body and the efferent vessels opening 

 directly into the heart. 



Members of the genus of hermit crabs Coenobita, which 

 is related to Birgus, in addition to the modification of the 

 giU-chamber, have the soft skin of their abdomen richly 

 vascularised, so as to be capable of eflFecting an exchange of 

 gases, a specially active circulation being maintained in this 

 region by a pair of contractile vesicles or accessory " hearts," 

 situated at the base of the abdomen (Caiman, op. cit.). 



Cowles (1908) has made an exceedingly interesting study 

 of the habits of Ocypoda arenaria, a land-crab which frequents 

 the Atlantic shores of the more southern states of America. 

 Although Ocypoda spends most of its time on land (in damp 

 burrows), it is obliged to go down to the ocean from time to 

 time in order to moisten its gills. On such occasions the 



