Aquatic Respiration of Fishes 



The respiratory organs of fishes form the so-called 'gills'. These 

 are diagnostic of all chordates and form a series of slits between 

 the pharynx and the outside on both sides of the animal. In the 

 earliest forms, a large number were present and they functioned 

 not only for respiration but especially during feeding. This 

 condition persists in primitive chordates such as Amphioxus and 

 the sea squirts. During evolution the number of gill slits became 

 reduced and most modern fishes have five pairs and never more 

 than eight pairs. Even among terrestrial vertebrates a gill-slit 

 stage is passed through in the embryo but the gills are not usually 

 functional. The formation of the gills mainly involves activity 

 of the endoderm which forms a series of pouches on both sides 

 of the pharynx. These meet slight intuckings of the ectoderm and 

 perforate to form gill slits, separated by the gill bars. It is the 

 epithelium of these bars which becomes folded to form a series 

 of filaments attached to the anterior and posterior face of each 

 bar. Thus each gill or holobranch consists of two hemibranchs or 

 sets of filaments attached to a gill bar and belonging to adjacent 

 sides of two consecutive gill pouches. The gill bars are supported 

 by the skeleton of the branchial arches. 



The formation of gill slits is usually in step with the muscular 

 segmentation but not invariably, for example in the Ammo- 

 coete larva of the lamprey. There seems little doubt that the two 

 are segmented independently and that during phylogeny they 

 have become intimately associated with one another. 



In addition to these true gills, more superficial external gills 

 are present during the development of fishes and Amphibia. 

 Some of these, as in the early stages of all Elasmobranchs and 



