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 



spiraile is present in Acipeuser, Polyodon, 

 and Polypterus amongst Ganoids. 



As a rule Teleosts possess only four holo- 

 branchs,^ and this holds good for all Ganoids. 



A rudimentary gill or pseudobranch is present 

 on the anterior wall of the spiracle of many 

 Elasmobranchs and of cartilaginous Ganoids 

 (mandibular jjseudobranch) ; and the posterior 

 hyoid hemibranch, which is functional in Aci- 

 penser and Lepidosteus, becomes more or less 

 reduced in Ganoids and Teleosts, formuig 

 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 

 certain accessory structures are developed in 

 the region of the branchial chamber by a modi- 

 fication of the branchial arches or cavities. 

 These serve to retain the water, and thus the 

 Fish is able to live for some time out of the 

 water (Anabas, Sacoobranchus, Heterobranchus, 

 Clarias). 



External gills are met with in young stages 

 of Elasmobranchii and Holocephali 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 

 Sectiojt through a 

 HoLOBBANCH OF Zygcena 

 (on the right) and 

 Gadus (ON the left). 

 Slightly Enlarged. 

 (From R. Hertwig's 

 Zoology). 



h 



branchial arch ; z, gill- 

 rakers ; «., afferent, 

 and V, efferent branchial 

 vessels ; bl^, anterior, 

 and bP, posterior hemi- 

 branch of the gill ; r, 

 cartilaginous gill-ray; h, 

 septum. 



traction of the latter, 

 slits.^ In this process 



Fishes breathe by taking in water 

 through the mouth, and, by the con- 

 forcing it out again through the gill- 

 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 



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

 less rudimentary. 



- 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. 



