3 88 



ANIMAL RESPIRATION 



ORDINARY BONY FISHES (TELEOSTEI) 



In such fishes as Salmon, Perch, or Cod there is further 

 specialization in the breathing organs, resulting, it would seem, 

 in increased efficiency, which is no doubt one reason why this 

 youngest group of fishes is predominant at the present day. 



There is no spiracle, and the gill-slits are 

 usually Jive in number (though they may be 

 fewer), and close together. The most remark- 

 able peculiarity, however, concerns the gill-folds, 

 and it is one to which Chimaera and Sturgeons 

 lead up. Each such fold, instead of being 

 attached along its whole length to the side of 

 a deep gill-pouch, is here present as a thread 

 or filament, fixed by one end to a comparatively 

 narrow gill-bar (fig. 518). The gill-bars are, 

 as it were, the greatly-reduced representatives 

 of the partitions which separate the gill-pouches 

 in a Dog- Fish or Shark. An ordinary bony 

 fish possesses a large gill-cover or operculum, 

 Fig. 5 i8. -Horizontal section supported by several flat bones, and if we lift 



through the breathing-organs A x ' 



of a Teieost. The freeiy-pro- this up the gill-filaments are to be seen aggre- 



jecting gills are darkly shaded, - 1 r 1 i IM -11 i 



and the course of the breathing- gated into a number of red comb-like gills, the 



water indicated by arrows. pi i ,1 ,1 /- 1 



t. Tongue; g i. guiiet. filaments corresponding to the teeth of the 



combs. The evolution of the protective gill- 

 cover has rendered such an arrangement possible. 



Many Teleosts also exemplify a special device by which a. 

 regular flow r of water over the gills is promoted. There are 

 valvular flaps (inner lips) just within the mouth, and similar flaps 

 close to the slits behind the gill-covers. When the fish opens its 

 mouth and dilates its pharynx the latter valves close, so that water 

 enters from the front only, none passing in under the gill-covers 

 and through the gill-clefts. As the mouth shuts, its valves come 

 together, so that all the water that has been taken in is forced 

 out through the gill-clefts, and is entirely prevented from being 

 ejected again in front without having done any breathing work. 



PROTOCHORDATES 



In describing the breathing organs of fishes use has several 

 times been made of the word ''gill", and it is time to enquire 



