194 THK INFLUENCE OF INANIMATE SURROUNDINGS. 



Now, since this lung perfectly corresponds, morphologically, to 

 the upper half of the branchial cavity of other crabs, proof is 

 furnished that here a portion of the gill-cavity has been trans- 

 formed into an organ for breathing air, and has at the same 

 time undergone very characteristic modifications of structure. 



The objection which certain morphologists feel to admit- 

 ting that land crabs, and above all Birgus latro, are animals 

 actually breathing air and provided with, lungs, appears to be 

 caused by their incapacity for understanding that the same 

 organ which to-day acts as a lung may be used to-morrow as 

 an organ for breathing water by the employment of the gi)ls 

 placed in it, or close to it. This objection is all the more to 

 be wondered at, since the same zoologists readily admit that 

 this same process — the transformation of a gill-cavity into a 

 lung — has actually taken place in snails. The lung of Ampul- 

 laria spoken of above might be here adduced ; still it might 

 perhaps be said that it is not proved that it belongs morpho- 

 logically to the branchial cavity, and that it may be a new organ 

 occurring only in this genus. But the same objection could 

 not be made with regard to the lungs of the HelicinidiB, 

 Cyclostomaceae, Siphonariadse, and hermaphrodite Pulmonata, 

 for aU these Molluscs have only one respiratory cavity, which 

 breathes air, but which, by reason of its position and its 

 relation to the other organs, may be regarded as a gill-cavity 

 transformed into a lung, and which is even regarded as such by 

 all zoologists. But there are among them certain forms — 

 Siphonaria, the aquatic Pulmonata, Auricula — which bear a 

 small gill or gill-hke organ in this lung, notwithstanding that it 

 is filled with air ; consequently even those zoologists who dis- 

 pute the air-breathing powers of Birgus latro, merely because 

 it possesses gills, must regard the above-mentioned molluscs 

 as water-breathers also. This, however, they do not do, for 

 they cannot deny the fact that these creatures breathe air; 

 hence they will be obliged by degrees to accustom themselves 

 to the idea that land-crabs breathe air, and to regard the lungs, 

 in them as well as in the snails, as gill-cavities which have 

 exchanged their normal or primary function for another. 



At the first glance it certainly appears singular that an 



