MORPHOLOGY OF ANIMALS 



The fate of the embryonic gill-pouches in different 

 classes of vertebrates is shown in the accompanying table: 



Embryonic 

 Gill Pouches 



Become in adult 



Tracheae^ Lungs. In animals which do not dwell in 

 water, and in some few which do (insect larvae, lung- 

 fishes, etc.), certain infolded portions of the body wall or 

 of the pharynx occur into which air is drawn and from 

 which it is again expelled. Among invertebrates these in- 

 folded portions are generally derived from the skin; 

 among vertebrates from a portion of the alimentary canal, 

 the pharynx. In the case of insects and allied forms 

 (Tracheata) these infolded portions have the form of 

 much branched tubes, the tracheae, which reach to all 

 parts of the body, the terminal twigs of the tracheal sys- 

 tem of tubes being found in connection with almost every 

 bit of tissue in the body (Fig. 15, B). These tracheae open 

 to the exterior though closeable pores, the spiracles, situ- 

 ated on the sides of the body (Fig. 15, A) ; air is taken in 



