Morphology of Organs. 165 



organs which fit in with a terrestrial life, just as the branchiae 

 —delicate structures which would dry up and become inefficient 

 if exposed to dry air — are suited to the aquatic life. The 

 tracheae are essentially tubes which open on to the exterior by 

 the stigmata, and at the other end branch repeatedly and 

 ramify in the body, carrying the air to the tissues of the most 

 distant organs. The tracheae, therefore, are not respiratory 

 organs in the sense that branchiae are. The latter are the 

 organs where — and where only (in the crayfish) — respiration 

 takes place ; the tracheae are merely conduits for the air, each 

 organ absorbing from them its own oxygen, and giving up its 

 carbonic acid.-' 



3. The Gills of Fishes and Tadpoles. — A third form of respi- 

 ratory organ characterizes vertebrates. It is one of the most 

 important definitions of this group that, either temporarily or 

 permanently, there are a series of slits putting the pharynx into 

 communication with the outside world. These slits, temporary 

 in Amphibia and all vertebrates lying above them in the series, 

 are permanent in fishes, and there become fringed with vascular 

 tufts, and perform the office of gills. The water taken in at 

 the mouth is passed through these slits, and as it passes the 

 delicate epidermis covering the vascular tufts gives up its 

 oxygen to the contained blood. This same process of respira- 

 tion goes on in the tadpole. 



4. Lungs of Vertebrates. — In the adult frog, and in all verte- 

 brates lying above it, respiration is effected by a pair of sacs, 

 outgrowths of the pharynx, the lungs. 



It has been attempted to be shown that these sacs are 

 really homologous with a pair of gill-slits. The latter originate 

 as outpushings of the pharynx to which inpushings of the 

 epidermis correspond. The lungs, it is suggested, are the out- 

 pushings only, which thus never acquire a communication with 

 the outside world. 



The diversity of respiratory organs contrasts, as has been 

 pointed out, with the uniformity of the excretory organs. It is 



' In some aquatic insects there is a curious modification of the tracheal 

 system. The trachese, unprovided with an external orifice, lie in out- 

 growths of the body-wall, which are perfectly comparable to branchiffi. 



