AMPHIBIOUS INVERTEBRATES 



469 



AMPHIBIOUS CRUSTACEANS (CRUSTACEA) 



Some mention has already been made of Land-Crabs (see 

 p. 443) as descended from shore -dwelling forms which have 

 lost their gills to a greater or less extent, and converted their 

 gill-chambers into organs capable of breathing ordinary air. It 

 would not therefore be surprising to find Crabs of truly amphi- 

 bious nature forming a sort of half-way house between purely 

 marine and purely terrestrial forms. Such a condition is actually 

 realized by certain shore-haunting Crabs belonging to a widely- 

 distributed family (Grapsida). The gills, though reduced in size, 

 are still useful, and the gill-chamber possesses folds and ridges 

 on its lining which enable it to be used as a lung. Such Crabs, 

 when out of water, have been observed to lift up the hinder part 

 of the body for the purpose of admitting air into the gill-chambers 

 from the back. There is also an aperture at the front end of each 

 chamber which is supposed to serve for the entry of water to be 

 used by the gills. 



Some account has been given in a preceding section (see 

 p. 220) of the habits of the Robber-Crab or Palm-Thief (Birgus 



A.L.V. 



6.L.V. 



GIL 



E.L.V. 



Fig. 574. Diagrammatic Cross Section through Breathing Organs of Robber Crab 



(Birgus latro] 



A.L.v., Vessels carrying impure blood to lung-folds; E.L.V., vessels bringing pure blood from 

 lung- folds; and E.L.V'., opening of one of these into blood-space surrounding heart (pericardial 

 space); E.G.V., vessels returning pure blood from gills to pericardial space. 



latro] of the Cocos-Keeling Islands. The breathing organs of 

 this curious creature, which is really a Hermit-Crab that has 

 taken to live on land, are in a very interesting condition (fig. 

 574). The gills are much reduced in size, and the gill-chambers 

 are divided into a small lower part for their reception, and a very 

 much larger upper part acting as a lung, and having its lining 

 raised into an elaborate system of folds. How far this animal is 



