FISUES. 207 



Capt. Hall has seen them pass over a space of 200 yards ; 

 but they cannot alter the direction of their course, and the 

 expanded fins, when in the air, serve only to make the descent 

 more gradual.* 



Eespikation. — The heart of fishes is composed of two 

 cavities only. It receives the blood which has circulated 

 through the system, and propels it to the gills. These are 

 the great organs for respiration, and in the greater number of 

 fishes ai"e arranged in the form of arches on each side of the 

 hinder part of the head. The water is taken in at the mouth, 

 and passes out between these arches, whore the venous blood, 

 in the gills is purified by the air diffused through the water. 

 The delicate membrane by which the minute ramifications of 

 the blood-vessels are supported, forms no obstacle to the free 

 action of the water on the impure or carbonated blood. The 

 details connected with the circulation will be more easily- 

 understood by an examination of the annexed figure (184) 

 than by any formal description. The true cause of death in 

 a fish kept out of water is an interesting question, which 

 appears to have been satisfactorily answered by ]\I. Fleurens, 

 a French physiologist. Though the gill-cover be raised and 

 shut alternately, the gills themselves are not separated. 

 Their fine filaments rapidly dry and cohere together. The 

 blood can no longer circulate through them, and hence it is 

 not affected by the vivifying infiuencc of the oxygen of the 

 air. " The situation of the fish is similar to that of an air- 

 breathing animal enclosed in a vacuum, and death by suffoca- 

 tion is the consequence. "t The gills vary considerably in 

 form and arrangement. Some are convohited, some are in 

 little tufts, some are enclosed in cavities, with circular orifices, 

 and others furnished with gill-covers composed of distinct 

 bones, to which certain fixed names are appropriated. 



Food. — Some fishes live upon marine vegetables. The 

 species of one genus (Scarus) are known to browse upon the 

 living polypes which built up the coral reefs ; and as the 

 polypes retreat, when touched, into the star-shaped cavities of 

 theii* support, these fishes are furnished with a dental apparatus 



* Fragments of Voyages and Travels. Second series, vol. 1, p. 220. 

 A more recent writer asserts that the fins are used as wings ; vide Note 

 in Edinburgh Phil. New Journal, April, 1817, p. oSl, from Gardner's 

 Travels in ISrazil. 



t YarreU, vuL i. p. C7. Owen, p. 60. 



