OF EESPIEATION. 221 



1 78) a series of vesicles lined with mucous membrane, and 

 richly supplied with blood vessels, are regarded as respirating 

 sacs. 



[ 383. Fishes respire by branchiae, or gills, for the sup- 

 port and protection of which a complicated framework of 

 bones, cartilages, ligaments, and muscles is provided ; the 

 form and arrangement of this apparatus varies in the different 

 families and genera. It may, however, be classified into 1st. 

 The lingual bone and branchiostegous rays ; 2nd. The bran- 

 chial arches ; 3rd. The opercula or gill covers. 



The gills are for the most part attached to the branchial 

 arches, which extend from the sides of the os hyiodes, back- 

 wards to the cranium. They are, in general, four in number 

 on each side of the head, and are composed of numerous la- 

 mellae, placed closely together, and arranged in a regular series 

 over the whole external convex margin of the branchial arches, 

 like the barbs of a feather, or the teeth of a comb. Every- 

 thing is arranged to afford the greatest possible extent of sur- 

 face for the contact of the water with the mucous membrane 

 on which a rich vascular network is spread. In the common 

 ray, the extent of surface of the mucous membrane of the 

 gills is estimated at 2250 square inches. In osseous fishes, 

 as the pike and perch, the gills adhere by their superior bor- 

 der, and are covered by moveable opercula. In the carti- 

 laginous genera, as the rays and sharks, they are attached by 

 both borders, and there are no opercula ; the water, which 

 in the former enters by the mouth and escapes by the oper- 

 cula, in the latter is expelled by a series of fissures situated 

 at the sides of the neck. In the Hippocampus and Syngnathus, 

 the gills are disposed in the form of tufts along the surface 

 of the branchial arches, resembling the tufted branchiae of 

 gastropoda and annelida. In sucking fishes, as the lamprey, 

 Petromyson, they are in the form of vesicular sacs, ar- 

 ranged on each side of the neck, into which the water is 

 introduced by a canal coming from the cavity of the mouth, 

 and discharged through the holes situated at the sides of 

 the same region. 



Most fishes, besides gills, possess a hollow organ analagous 

 to a lung, and called the air-sac, or swim-bladder ; it is situated 

 in the abdominal cavity, lying along the under side of the ver- 



