278 



COMPARATIVE ANATOMY 



and thus, as in the Holocephali, the gill-slits open into a common 

 branchial chamber, which communicates with the exterior by a 

 single slit-like aperture on either side (Figs. 222 B and 223). A 



spiracle is present in Acipenser, Polyodon, 

 and Polypterus amongst Ganoids. 



As a rule Teleosts possess only four holo- 

 braiichs, 1 and this holds good for all Ganoids. 



A rudimentary gill or pseudobranch is present 

 011 the anterior wall of the spiracle of many 

 Elasmobranchs and of cartilaginous Ganoids 

 (mandibular pseudobranch) ; and the posterior 

 hyoid hemibranch, which is functional in Aci- 

 peiiser and Lepidosteus, becomes more or less 

 reduced in Ganoids and Teleosts, forming 

 the so-called opercular pseudobranch. Traces of 

 a cleft, lying behind the functional branchial 

 clefts, are found in the embryos of certain 

 Elasmobranchs. All these facts indicate that 

 Fishes formerly possessed a more extensive 

 branchial apparatus than at present. 



In the Lophobranchii the gills are replaced 

 by tufted processes, and in many Teleostei 



HOLOBRANCH OF Zygoe.ua certain accessory structures are developed in 

 (ON THE RIGHT) AND the region of the branchial chamber by a modi- 

 Gadus (ON THE LEFT), fication of the branchial arches or cavities. 

 SLIGHTLY ENLARGED. These serve to retain the water, and thus the 

 (From R. Hertwig's Fish is able to live for some time out of the 

 water (Anabas, Saccobraiichus, Heterobranchus, 

 Clarias). 



External gills are met with in young stages 

 of Elasmobranchii and Holooephali as well as 

 in Polypterus and Calamoichthys ; in Elasmo- 

 branchii and Holocephali, at any rate, they are 

 endodermal and not ectodermal in origin. 



FIG. 223. TRANSVERSE 

 SECTION THROUGH A 



Zoology). 



b, branchial arch ; z, gill- 

 rakers ; a, afferent, 

 and v, efferent branchial 

 vessels ; bl 1 , anterior, 

 and bl 2 , posterior hemi- 

 branch of the gill ; r, 

 cartilaginous gill -ray; h, 

 septum. 



Fishes breathe by taking in water 

 through the mouth, and, by the con- 

 traction 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 during 

 expiration. 



Dipnoi. These, as their name implies, possess both gills and 

 lungs, only the latter organs being functional in Protopterus during 

 its summer sleep (see p. 17). Besides the internal gills, which 

 are covered by a small operculum, Protopterus possesses three pairs 

 of external gills situated just above the operculum and supplied by 

 vessels from the arterial arches. In Ceratodus, in which, as in 

 Lepidosiren, no external gills are present, there are four complete 

 gills on the first four branchial arches, as well as a pseudobranch 



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

 less rudimentary. 



2 In the Lamprey inspiration as well as expiration takes place through the 

 gill apertures when the animal is attached by means of its suctorial mouth. 



