NUTRITION 



vertebrates. Among the mollusks the land - dwelling 

 lung-snails have acquired a lung-sac by change in the 

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

 spiders and scorpions 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 (tracheata) we find, instead of 

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

 (trachecu), which spread through the whole body and 

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

 from without by special air-holes in the skin {stigmata 

 and spiracida). The myriapods and insects generally 

 have numbers of air-holes; the spiders only one or two, 

 more rarely four, pairs. When these air-tube animals 

 return 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 animals, or protracheata, and form the 

 link between the older annelids and the myriapods. 

 They have a number of clusters of short air-tubes dis- 

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

 have been evolved from simple skin-glands by change of 

 function. 



Gastric or internal lungs are only found in the higher 

 animals, to which we give the name of quadrupeds (or 

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

 like ancestors, the dipneusta. These internal lungs are 

 sac-shaped folds of the fore-gut, formed originally from 

 the swimming-bladder (jiectocystis) of the fishes by 

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

 appendage of the gullet, merely 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 becomes heavier; it rises to the top by 



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