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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 (Y.) to the heart, thus completing the circulation. 



(e) As the water which bathes the gills 

 is deprived of its oxygen, it requires to be 

 constantly truncal. 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.) 



(</) Fish soon die oat 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 

 moi s t e n e d — c ompare 

 with the cutaneous re- 

 spiration in amphibians) , 

 until finally the animals 

 die of suffocation. 



(c) It results from 

 the simple structure of 

 their heart, the slow cir- 

 culation and branchial 

 respiration, that the body 

 temperature of fishes 

 varies with that of the 

 surrounditlff medium ,• explained in the text. 



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. 



Diagrammatic Outline Sketch of 

 a Fish (Carp), showing the Circu- 

 lation of the Blood. 

 Bf. , barbels ; Br., pectoral fins ; A., 

 anal tin ; S., caudal lin ; R., dorsal 

 fin ; Hch., swimbladder ; Lg., pneu- 

 matic duet of swim-bladder. Tlie 

 other letters are explained in the 

 text. 



Transverse Section 

 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 



