I3O GENERAL PRINCIPLES OF ZOOLOGY. 



Development of the Respiratory Apparatus. In the 



air-breathing animals the breathing apparatus must be 

 derived either from the digestive canal, or from the skin. 

 With the vertebrates the former is the case, since the lungs 

 containing air here stand, directly or by the mediation of 

 the trachea and bronchi, in connection with the lumen of 



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FIG. 59. Anterior end of Terebella nebulosa. (After Milne Edwards.) //i, pharynx ; z'</, 

 dorsal, vv, ventral, blood-vessel ; 6r, gills ; t, tentacles. 



the digestive tract. On the contrary, in the case of inver- 

 tebrated animals (snails and spiders) when the term " lung ' 

 is used, it refers always to an invagination or sac of the 

 skin ; of such a nature are the tracheae of insects, tubes 

 containing air, beginning at the surface cf the body with 

 a hole or stigma, and branching internally (Fig. 56, st). 



Distinctions between the Respiratory Systems of 

 the Vertebrates and Invertebrates. In general, then, a 

 distinction can be drawn between the respiratory systems 

 of vertebrated and invertebrated animals: in the former, the 

 digestive tract, or derivatives from it, function for air- and 



