LUKGS IN A LAND-CBAB. 193 



replaced only by air, since the animals live in the air, and it 

 obviously follows that they generally breathe air with their 

 branchial cavities, and only exceptionally water.^^ In one 

 single case this change of function has induced a modifica- 

 tion of structure ; this case is that of Birgus latro, according to 

 my own observations. The lower portion (see fig. 2, p. 5) of 

 the gill-cavity, which contains numerous but small gills, is 

 divided from the upper half by a transverse fold which tiirns 

 inwards at the edge of the thorax-plate. The cavity thus 

 enclosed is a true lung (see fig. 2), since it never contains any- 

 thing but air, and the arrangement of the vessels traversing its 

 walls proves that blood poor in oxygen ent«rs it from the 

 body, and the vessels leading from it open directly into the 

 auricle. The skin on the outer and upper lung-covering bears 

 a great number of ramified tufts, which add to the extent of 

 the respiratory surface, and contain in their interior an extra- 

 ordinarily developed network of vascular spaces, intervening 

 between the afierent and efierent pulmonary vessels. These 

 spring from two large vessels, proceeding one on each side from 

 the anterior half of the body cavity ; each divides close to the 

 foremost angle of the lung into four pulmonary vessels, of which 

 three ramify over the upper and one over the lower lung-cover- 

 ing. These are merged in the before-mentioned network which 

 traverses the villi of the lungs. Erom this again proceed 

 several vessels which unite at the angle of the lung to form a 

 large trunk, the afierent pulmonary vessel ; this passes at first 

 from the front backwards, then bends round, corresponding to 

 the curve of the thorax-shield, and passes from behind forwards, 

 uniting with the branchial vein coming from the interior, shortly 

 before it enters the auricle of the heart. This arrangement of 

 the vessels is such as we should expect to find in a true lung ; 

 the expansion of the respiratory surface which is here afforded 

 by the villous structure of the lungs equally corresponds to the 

 typical structure of all organs for breathing air ; finally, it is 

 positively established that the animals pass the greater portion 

 of their life on land, and that their lung-cavities contain 

 actually air, and never water in any greater quantity than is 

 requisite for maintaining the moisture of the respiratory surface. 



