HIGH SCHOOL ZOOLOGY. 



43 



55. As we shall .see hereafter, the air-bladder in some fishes 

 is so well supplied with blood, and communicates so freely with 

 the (jesophagus that it can act as a breathing organ. Such is 

 obviously not its function in the catfish. Again there are fishes 

 which live in deep watei", and can by altering the amount of 

 air in the air-bladder, accommodate their specific gravity to that 

 of the water at any particular level. But such an hydrostatic 

 arrangement must be of less service to a catfish than to many 

 other groups. The connection with the ear renders it likely 

 that the functions discharged by the air-bladder are of a complex 

 character, but they are not yet well understood. 



5G. Respiratory System. — We have 



already studied the skeleton of the gill- 

 arches ; there remain to be examined the 

 soft parts wliich clothe these. Within the 

 cavity of the mouth there may be observed 

 certain tubercles which fit into each other 

 when the gill clefts are closed, these are 

 the gill-rakers ; they are sometimes of con- 

 siderable size in other fishes, and may act 

 as strainers of the water which flows out 

 through the clefts, over the gills. On the 

 convex side are the gill-filaments, disposed 

 in two I'ows. (Fig. 17). 



51. The vessels which supply the gill- 

 filaments ascend the arches in a groove, 

 which is easily seen on their convex side in 

 the dry condition. Of the four arches, the 



O, bony arch; ea, efferent i . • i • i n , i i . . i ,i 



artery; "aa, afiferent artery ; i^st IS decidedly the shortest, and the same 

 "' "'''^^- is true of the slit behind it. All the slits 



open freely into the branchial cavity, and this by a very wide 

 aperture to the outside, the apertures of the right and left sides 

 being only separated from each other below by a narrow isth- 



Fig. 17 — Diag-rammatic Sec- 

 tion of Gill-arch. 



