CHAPTER X 



THE RESPIRATORY ORGANS 



THE principal respiratory organs consist of a series of pairs of 

 branchiaL clefts in the form of perforations in the side walls 

 of the throat, which place the pharynx in free communication 

 with the exterior. The first and most anterior of these clefts, 

 the mandibulo-hyoid cleft or "spiracle," is situated between 

 the mandibular and hyoid arches ; the second, the hyo-branchial 

 or hyoidean cleft, between the hyoid arch and the first branchial 

 arch ; and the remaining clefts between the succeeding branchial 

 arches. On the anterior and posterior walls of more or fewer of 

 the clefts highly vascular plate-like, or variously shaped fila- 

 mentous outgrowths of their lining membrane are developed, 

 which subserve the purpose of exposing the blood to the influence 

 of the oxygen - containing water, and are termed branchial 

 lamellae or " gills." In addition to their usual respiratory organs, 

 the gills, a few Fishes utilise the air-bladder either as a functional 

 lung or as an oxygen reservoir, and in others accessory breathing 

 organs of various kinds are developed. 



The arrangement of the branchial clefts and the gills may 

 be conveniently studied first in the Elasmobranchs. Excluding 

 the spiracles, there are usually in this group (Fig. 161, A), five 

 pairs of branchial clefts, but in certain primitive members of the 

 group the number may be larger. Thus, in Notidanus griseus 

 (Hexatichus) and in Chlamydoselachus there are six, and in Noti- 

 danus cinereus (Heptanchus), seven clefts. The pharyngeal aper- 

 tures of the clefts are relatively wide, but their external openings, 

 which are freely exposed on the lateral surface of the head between 

 the eye and the pectoral fin, are usually narrow and slit-like. 



The successive clefts are separated from one another by a 



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