192 THE INFLUENCE OF INANIMATE SUKKOUNDINGS. 



lungs, reopens the siphon, and admits a stream of water through 

 it into the branchial cavity.'" Some species of Nerititia of 

 the Philippine and Pelew Islands live constantly on land, and 

 apparently go into the water only when they want to lay their 

 eggs ; other species actually living in the water often make 

 long journeys over land, as 1 myself have frequently had the 

 opportunity of observing in the Pelew Islands. In these species 

 the gills are comparatively small, and the roof of the branchial 

 cavity is furnished with a dense vascular network of which 

 the main branches unite in one large vessel ; this is inserted 

 in the heart — the auricle — and thus stands in precisely the 

 tame relation to the lungs as the pulmonary vein of the true 



Pia. iS.—Oemnlnm ntstkola, a land Orab. 



land-snails — Helicidce. Thus the branchial cavity in this case 

 seems to be capable'' of fulfilling not only its own proper func- 

 tions, but also that of a lung. We know, moreover,"' that many 

 species of crabs — Birguslatro, Gecarcinua (fig. 56), Grapsoidea, 

 Sesarma, and others — live far from all running or stagnant 

 water in damp woods, under stones or decaying trees, and are 

 even able to expose themselves to the stin for hours. In most 

 of these species trvie gills are present in the branchial cavity, 

 but they fill at most a third or fourth of the space, and the 

 cavities contain, besides water, a considerable quantity of ah-, 

 as is shown by their constantly expelling air-bubbles at the 

 sides. The supply of air thus driven out can of course be 



