G. ORGANS OF RESPIRATION. 



The typical respiratory organs of Vertebrates are closely con- 

 nected with the enteric or alimentary canal both as regards 

 position and development, and are of two kinds, gills and lungs. 

 The former, as the phylogenetically older organs, are adapted for 

 aquatic respiration, and are connected with the pharynx in the 

 region of the visceral clefts : the latter always arise as sac-like 

 outgrowths of the pharynx, which grow backwards so as to lie 

 within the body-cavity. 



Both gills and lungs may be developed in the same individual, 

 but are usually not functional at the same time. Both are sup- 

 plied with venous blood which becomes oxygenated while passing 

 through their capillaries. 



The swim-bladder or air-bladder present in many Fishes, and 

 acting as a hydrostatic organ, arises in a similar manner to the 

 lungs that is, as an outgrowth from the fore-part of the aliment- 

 ary tract : it usually receives oxygenated blood from the aorta, and 

 venous blood passes from it into the cardinal, hepatic, or hepatic 

 portal veins; but in some cases (e.g. Bony Ganoids and certain 

 Teleosts) it may act as an accessory respiratory organ. 



In some cases also the oral and pharyngeal mucous membrane 

 (e.g. certain Amphibians), or the intestine (e.g. certain Siluroid 

 Fishes), may take part secondarily in respiration, and the integu- 

 ment may be very important in this respect (e.g. in Amphibians). 



I. GILLS. 



The gills arise in connection with a series of laterally -arranged 

 outgrowths of the pharynx lying one behind the other, which be- 

 come open to the exterior. Passages or clefts separated by septa 

 are thus formed for the water entering by the mouth, and in order 

 that oxygen may become absorbed, leaf-like or thread-like vascular 

 processes, the internal gills or branchice, become developed in the 

 region of each cleft. Apart from these, external gills sometimes 

 occur, and in certain cases, both kinds are developed in the same 

 animal. 



In the development of the internal or enteric gills, the endo- 

 derm plays the chief part as the lining or covering layer, the 



