370 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



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forced through the gill slits. The current, quickened 

 by bringing down the opercles, runs between all the teeth 

 of the fringes and aerates the blood 

 >jS^re J-ll This series of movements is 



y have n'.y repeated. 

 Panics of =^ion of the blood in gill 

 fferent frc,y means of the free oxy- 

 voned. It i in the water. If water 

 be boostal. , as to drive out all the 

 dissolved gases, it will no lbnger sup- 

 port life. If the gills of fishes be 

 kept moist and the fringes separated, 

 life out of water may be maintained 

 for some time. Some fishes have the 

 means of keeping their gills moist. 

 Such fishes often leave the water and 

 crawl about on land for hours. 



Variation of Gills among 

 Fishes. — We have taken teleosts as 

 a type. Gills of other fishes may be 

 regarded as modifications of this 

 type. In sharks (Fig. 252) there are 

 five gill slits in the throat and five 

 corresponding separate gill openings 

 on the sides of the head, but not covered by an opercle. 

 There are cartilaginous plates between the gill openings 

 and the throat slits, and on these are fixed the gill 

 fringes. The breathing is similar to that already de- 

 scribed in teleosts, except in regard to the movements 

 of the opercle. 



In the lamprey (petromyzont, Fig. 253) we have 

 seven holes in the throat and corresponding holes on 

 the side of the neck, connected each by a pouch. In this 

 pouch are arranged the fringes. The breathing is the 

 same (Fig. 254). 



Kig. 251. — Transverse 

 section of a gill arch 

 GA, showing a pair 

 of fringe plates ; A t 

 artery ; V, vein ; a, 

 arteriole ; v, veinlet. 



