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TEXT-BOOK OF ZOOLOGY 



supplies them with arterial blood. This blood, after giving up its oxygen 

 (see Part I, p. 7, Section 6) and receiving carbonic acid gas in exchange, 

 once more returns (V.) to the heart, thus completing the circulation. 



(c) As the water which bathes the gills 

 is deprived of its oxygen, it requires to be 

 constantly renewed. This is effected in the 

 following manner : After being taken in by 

 the mouth, the water is pressed through the 

 branchial clefts, and after having bathed 

 the branchial leaflets is expelled through 

 the gill-slits. (For the course of the re- 

 spiratory current in rays also many sharks 

 the lamprey and the lancelet, see under 

 the respective species.) 



(d) Fish soon die out of 

 the water, although they 

 have at their command 

 a much larger quantity 

 of oxygen than in the 

 water. The branchial 

 leaflets dry up, stick to- 

 gether, and get entangled, 

 respiration is arrested 

 (for air only penetrates 

 animal membranes with 

 ease when these are 

 moistened compare 

 with the cutaneous re- 

 spiration in amphibians) , 

 until finally the animals 

 die of suffocation. 



(<?) It results from TRANSVERSE SECTION 

 DIAGRAMMATIC OUTLINE SKETCH OF the simple structure of 



A FISH (CARP), SHOWING THE ClRCU- fofa heart fl^ gl()w cir . 

 LATION OF THE BLOOD. . ' 



Bf., barbels ; Br., pectoral fins ; A., CUlatlOn and branchial 



anal fin ; s., caudal fin ; R., dorsal respiration, that the body 



fin : Sch,, swimbladder ; Lg., pneu- , , 



maticductofswim-bladdfr. The temperature of fishes 



other letters are explained in the varies icitll that of the 

 text. ,. ~. 



surrounding m ediu in ; 



whereas in all other vertebrates the blood courses twice through the 

 heart, the circulation thereby receiving a double stimulus, the simple 

 heart of the fish imparts to the blood only a single impulse to circulate. 



THROUGH A BRAN- 

 CHIAL ARCH, WITH 



Two OF THE BRAN- 

 CHIAL LEAFLETS, 

 SHOWING THE CIR- 

 CULATION OF THE 

 BLOOD (DIAGRAM- 

 MATIC). 



B, branchial arch. 

 The other references are 

 explained in the text. 



