CHAP, xv.] RESPIRATORY ORGANS IN ANIMAL KINGDOM. 231 



all the vascular surfaces serving aquatic respiration, all the 

 apparatus specially charged to absorb the oxygen dissolved in the 

 water, and to relinquish in exchange carbonic acid. The organs 

 of aerian respiration can be divided in a general manner into two 

 categories, the tracheae and the lungs. The tracheae are canals 

 more or less ramified, always open, plunging into the interior 

 of the body, and communicating with the exterior air only by 

 narrow openings. The lungs are internal sacs, sometimes simple, 

 sometimes subdivided into compartments more or less numerous ; 

 they communicate also with the exterior air by orifices or 

 conduits more or less narrow. 



Like all organic classifications, that of the respiratory ap- 



J9. 



FIG. 25. 



Transversal sections of annulated worms, to show the homology existing between the 

 branchis and the cirrhi. A, section of Eunice; B, section of Myrianide; p, abdominal 

 parapod ; p 1 , dorsal parapod ; br, branchiae ; br, cirrhi. 



paratus suffers a number of exceptions. There are mixed forms, 

 transitory or confused types. Thus the tracheae are aerian 

 respiratory organs according to the definition. But we can bring 

 into affinity with them the vascular ramified systems, in which 

 the water circulates in many of the invertebrates. The lungs 

 of spiders can be considered as modified tracheae ; and so on. 



In many of the inferior invertebrates (turbellariates, nsemer- 

 tians, annelidse) there are no special organs appertaining to the 

 respiration. The gaseous exchange is effected by the teguments, 

 furnished often with vibratile cilia. This last anatomical arrange- 

 ment is found, as we shall see, in the mucous membrane of the 

 respiratory passages of many superior animals. "When water 

 penetrates into the cavity of the body the surface of this cavity 



