250 COMPARATIVE ANATOMY. 



reduced, so that the apices of the latter extend freely outwards ; the 

 whole branchial region is moreover covered over by the operculum 

 and branchiostegal membrane (comp. pp. 66 and 69), and thus 

 the gill-slits open into a common branchial chamber, which com- 

 municates with the exterior by a single slit-like aperture on either 

 side (Fig. 203). 



As a rule, the Teleostei possess only four gill-bearing visceral arches, 1 

 and this holds good for all Ganoids. A rudimentary gill or pseudobranch 

 is present on both mandibular and hyoid arches of most Elasmobranchs, on the 

 mandibular arch of all Cartilaginous Ganoids (spiracular gills), and on the 

 hyoid of Teleostei, and a, more complete hyoidean gill is seen in Acipenser and 

 Lepidosteus: this indicates that all these Fishes formerly possessed a more 

 extensive branchial apparatus than at present. 



In many Teleostei certain accessory structures are developed in the region 

 of the branchial chamber by a modification of the branchial arches or cavity. 

 These serve to retain the water, and thus the Fish is able to live for some time 

 out of the water (Anabas, Saccobranchus, Heterobranchus, Clarias, &c.). 



External gills are met with in young stages of Elasmobranchii, Holo- 

 cephali, Polypterus, and Cobitis ; these possibly have a nutritive function 

 in connection with the absorption of the yolk. 



Fishes breathe by taking in water through the mouth, and, by 

 the contraction of the latter, forcing it out again through the gill- 

 slits. 2 In this process the gill-arches rise and fall, separating 

 from one another during inspiration, and approximating cftiring 

 expiration. 



Dipnoi. These, as their name implies, breathe both by gills 

 and lungs. 



The branchial apparatus deserves particular attention, inas- 

 much as, in Protopterus (Fig. 54, K\ besides the internal 

 gills situated on the branchial arches, there are also three ex- 

 ternal gills. These are situated on the posterior and upper 

 border of the pectoral arch, to which they are fixed by connective- 

 tissue and vessels, which latter pass to them from the third and 

 fourth arterial arches. 



As in Ganoids and Teleosteaus there is only a single external 

 branchial opening on either side, covered by a somewhat rudimentary 

 operculum. 



Amphibia. In all Urodele larvae as well as in the Perenni- 

 branchiata there are three gill-tufts, lying one over the other, and 

 decreasing in size from the dorsal to the ventral side ; these extend 

 backwards, projecting freely to the exterior, and are composed of 

 connective-tissue, unsupported by cartilage. They either have 

 the form of tufts, or they may be delicately branched, showing the 

 most varied arrangements for increasing the respiratory surface. 



1 They may be reduced to three, or to two, and even these may become more or 

 less rudimentary. 



2 In Petromyzon, when holding on by its suctorial mouth, inspiration as well 

 as expiration takes place through the gill-apertures. 



