NUTRITION 



vertebrates > Among the moUufeks the land - dwelling 

 lung-snails haVe acquired a lUng-sac by dhange in the 

 work of the gill cavity-: among the articulata the lung- 

 spiders and soorpions have two or more trachea-lungs; 

 that is to say, cutaneous sacs^ in which are enclosed fan- 

 wise a number of trachea-leaves. In the other air- 

 breathing articulates (ttacheata) we find, instead of 

 these Simple or branched, and often bushlike, air-tubes 

 (trachecB), which Spread through the whole body and 

 conduct the air ditect to the tissues. They take the air 

 frotn without by special air-holes in the skin (stigmata 

 and spiracula). Th€! myriapods and insects generally 

 haVe numbers of ait-holes; the spiders only one or two, 

 morfe rarely four, pairs. Wh6n these air-tube animals 

 rettirn to an aquatic life (as happens With the larvae of 

 various groups of insects), the outer air-holes close up, 

 and new thread^shaped Or leaf-shaped trachea-gills are 

 formed, which take the air from the surrounding water 

 by osmosis. The oldest and lowest tracheata are the 

 primitive air-tube aniinals, or protracheata, and form the 

 link between the older annelids afld the myriapods. 

 They have a numbef of clusters of Short air-tubes dis-- 

 tributed over the whole skin, and it is clear that these 

 have been evolved frbm siihple skin-glands by change of 

 function, 



Gastric or iiiternal lungs are only found in the higher 

 aiilmals, to which w6 give the name of quadrupeds (or 

 tetrapoda), the amphibia and arflniotes, and their fish- 

 like ancestors, the dipneusta. These internal lungs are 

 sac-'Shaped folds of the fore^gut, formed originally from 

 the Bwiflmling'bladder {nectocfstis) of the fishes by 

 change of function. This air^filled bladder, a sac-shaped 

 appendage of the gullet, mefely serves the purpose of a 

 hydrostatic organ, by varying the specific weight, in the 

 fishes. When the fish wishes to descend it contracts 

 the bladder and beconies heavier; it rises to the top by 



231 



