vir. 4 



RESPIRATION 



197 



Respiration is produced by a current passing in a single direction, 

 as in elasmobranchs, namely, in at the mouth and out over the gill 

 lamellae, but the mechanism by which the current is produced is some- 

 what different. The pumping action is produced by a buccal pressure 

 pump and opercular suction pumps, resulting from sideways move- 



A B 



Fig. 119. Arrangement of organs of head and gills in A, a shark, and, 



B, a teleostean fish. 



gb. gill bar; GC. outer opening of gill cleft; GF. gill filament; gr. gill-rakers; cv. gill vessels; 



J, ]'. upper and lower jaw; M. mouth; N, n'. openings of nasal chamber; op. operculum; 



sp. spiracle; ST. septum between gill filaments. (From Dean, Fishes, Living & Fossil, 



The Macmillan Co.) 



ments of the operculum, enlarging the branchial cavity. The branchio- 

 stegal folds, below the operculum, prevent inflow of water from 

 behind. When the operculum moves inward dorsal and ventral flaps 

 in the throat prevent the exit of water forwards. 



The gill lamellae differ from those of elasmobranchs in the great 

 reduction of the septum between the respiratory surfaces (Fig. 119). 

 This has the effect of leaving the lamellae as free flaps, increasing the 

 surface available for respiration. 



The area of the gills varies greatly, being relatively larger in more 

 active species. The rate of respiration is controlled by a medullary 

 centre but the most active rate of respiratory exchange is only some 

 four times the standard rate (as against twenty times in man). The 



