378 FISHES 



uppermost and cleaves the water with its razor-like ventral 

 edge. 



We have next to consider respiratory adaptations. The 

 simplest and probably the most primitive condition of the 

 branchial organs, at least in the true Pisces, is that which is 

 presented by the Elasmobranchs. Here there is a series of 

 wide tranverse gill-slits at the sides of the pharynx, opening on 

 the surface of the body and separated from one another by com- 

 plete septa. The internal border of the septum is supported by 

 a cartilaginous bar or "arch" from which radial rods of carti- 

 lage extend towards the outer edge of the septum. The 

 actual respiratory organs are folds of the mucous membrane 

 forming narrow leaf-like " lamellae " on the anterior and posterior 

 surfaces of the septa ; these lamellae are situated almost parallel 

 to one another, radiating from the internal border of the septum 

 towards the outer border, and projecting into short processes at 

 their outer ends. 



In other sub-classes the branchial organs exhibit a change 

 in the relations of the branchial septa and branchial lamellae, 

 the former becoming reduced in different degrees and the latter 

 becoming more independent and elongated, until in the typical 

 bony fishes they are converted into long flat filaments attached 

 to a narrow branchial arch by their bases only. The reduction 

 of the septum is associated with the development of the gill- 

 cover, which consists of a fold of skin growing backwards from 

 the hyoid arch, i.e. the anterior boundary of the first cleft and 

 supported by flat scale-bones. This operculum occurs in the 

 Holocephali or Chimaeroids, in which the gill-septum is so far 

 reduced that it does not extend beyond the branchial lamellae : 

 but the latter are still attached along nearly their whole length. 

 In the sturgeon, the septum extends little more than half the 

 length of the folds, which project freely for the rest of their 

 length, while in Teleostei as has been said, the reduction of the 

 septum reaches its extreme and it forms merely an axis with a 

 double series of gill-filaments extending outwards from it. In 

 the lung-fishes {Ceratodus) the interbranchial septum is well 

 developed, and the gill-lamellae are attached to it along nearly 

 their whole length. 



The functions of the air-bladder are considered in another 

 place : here we have to discuss certain peculiar adaptations 



