INTRODUCTION. Xlll 



admit water are lined by the delicate membrane through 

 which the blood is aerated. In the Lampreys, the external 

 apertures of the branchial cells are placed on the side of the 

 neck ; but in the Myxine, which feeds upon the internal 

 parts of its prey, and buries its head and a part of its body in 

 the flesh, the openings of the respiratory organs are removed 

 sufficiently far from the head to admit of respiration going 

 on while the animal's head is so inserted. 



The branchiae or gills in fishes possess complex powers, 

 and are capable of receiving the influence of oxygen not 

 only from that portion of atmospheric air which is mixed 

 with the water, but also directly from the atmosphere itself. 

 When fishes confined in a limited quantity of water are 

 prevented by any mechanical contrivance from taking in at- 

 mospheric air at the surface, they die much sooner than 

 others that are permitted to do so. The consumption of 

 oxygen, however, is small ; and the temperature of the body 

 of fishes that swim near the bottom, and are known to pos- 

 sess but a low degree of respiration, is seldom more than two 

 or three degrees higher than the temperature of the water at 

 its surface. Dr. John Davy, however, in a paper read before 

 the Royal Society of London in 1835, on the temperature of 

 some fishes allied to the Mackerel, all of which are surface- 

 swimmers with a high degree of respiration, observed that 

 the Bonito had a temperature of 90 degrees of Fahr. when 

 the surrounding medium was 80 5' ; and that it therefore 

 constituted an exception to the generally received rule, that 

 fishes are universally cold-blooded. Physiologists have 

 shown that the quantity of respiration is inversely as the de- 

 gree of muscular irritability. It may be considered as a law, 

 that those fish which swim near the surface of the water have 

 a high standard of respiration, a low degree of muscular irri- 

 tability, great necessity for oxygen, die soon almost imme- 



