RESPIRATORY SYSTEM 767 



The openings of the clefts to the exterior differ in different groups 

 of fishes. Among the elasmobranchs each cleft usually opens separately 

 and is not covered by any flap or operculum, although in Chlameidose- 

 lachus, the primitive frilled shark, each cleft has a backwardly directed 

 flap or gill cover. In the Holocephali the first three clefts are covered 

 "by an operculum, and only the fourth or the last functional cleft opens 

 freely to the outside. In the great majority of teleostomi and in the 

 Dipneusti the five clefts are covered with a flap-like operculum, capable 

 of opening and closing, thus effectively protecting the branchial filaments 

 irom injury. In some of the eels and in other specialized teleosts, the 

 gills are completely covered with a fold of skin, the only exit being 

 through one or two small water pores. There are two quite different 

 and distinct kinds of gills found among fishes, namely : external and in- 

 ternal gills. 



External gills are purely larval or embryonic organs. They are not 

 functional in any adult fish; although their homologues are found in the 

 perennibranchiate amphibia, believed to be paedogenetic or permanent 

 larval types. External gills are finely branched processes of the ectoder- 

 mal epithelium of the branchial tract. They are found in the embryos 

 of many elasmobranchs and in some teleosts. A notable case of larval 

 trills is seen in the advanced larva of Polypterus (Fig. 368). 



The true functional gills of adult fishes are internal. They are finely 

 divided diverticula of the endodermal epithelium of the branchial clefts. 

 The nasal cavities are blind sacs which do not communicate with the 

 mouth. Such communication begins with amphibia. 



THE AIR-BLADDER AND ACCESSORY ORGANS OF 

 RESPIRATION 



In all of the groups of fishes above the elasmobranchs, there is a 

 single or paired air-bladder (probably homologous with the lungs of 

 "higher forms), a sac-like diverticulum of the pharynx, derived from 

 either dorsal or ventral sides of the alimentary tract. It is in all cases 

 supplied with blood from the pulmonary artery (which, in turn, arises 

 from the last efferent artery of either side), and, primitively at least, 

 subserves two functions: (1) that of a hydrostatic or buoyancy organ, 

 and (2) that of an accessory respiratory organ or primitive lung. In 

 the most primitive teleostome fishes, the Crossopterygii, it is used as a 

 lung when the water is foul ; in Amia, it is constantly functional as an 

 air-breathing apparatus ; while in the Dipneusti (lung-fishes) it is an 

 elaborately pouched lung, used to tide the fish over a period of drought. 



In certain other fishes that have acquired terrestrial habits, such 

 as the climbing perch, Anabas (Fig. 371), which will drown if immersed 

 in water, and the air-breathing eel, Clarias, there is an extensive post- 

 "branchial chamber, provided with labyrinthine or arborescent elabora- 



