48 



RESPIRATORY MECHANISMS 



Fig. 23. Head of Anabas scandens 

 showing respiratory "labyrinth" in spe- 

 cial cavity. (Henninger.) 



fish is breathing air the operculum is kept closed and the whole 

 cavity- alternately expanded and contracted. 



In the South-American "electric eel," Gymnotus ( = Electro- 

 phones) the surfaces in the mouth and branchial cavities are 



enlarged by rigid excres- 

 cences supplied with a cap- 

 illary net-work just below 

 the surface (Figs. 21, 22), 

 while the gills are reduced 

 and unable to provide suffi- 

 cient oxygen even in well- 

 aerated water (Boker, 1933; 

 Carter, 1935). 



In several distinct groups 

 of fresh- water fishes, of which 

 Anabas (Figs. 23, 24) and Clarias (Figs. 25, 26) are good ex- 

 amples, special rigid outgrowths from one of the branchial 

 arches, situated in a separate cavity with a valved opening, 

 function as air-breathing organs, and these fish leave the water 

 at night and often travel from 

 one pond to another (Das, 

 1928). 



In others {Saccobranchus) 

 the branchial cavity is ex- 

 tended backwards as a long 

 sac with richly vascularized 

 walls serving the same pur- 

 pose. In all these cases the 

 organs are regularly venti- 

 lated by respiratory move- 

 ments (Das, 1928). 



A number of fishes again 

 respire air by means of a por- 

 tion of the intestinal canal. Some (0 pyocephalus and Amphip- 

 nous) develop pouches from the pharynx for the purpose, and 

 in some species these are divided up by epithelial ridges into 

 "alveoli" so as to present the same appearance as the lungs of 



Fig. 24. Enlarged view of laby- 

 rinth showing larger blood vessels. 3/1, 

 (Henninger.) 



