RESPIRATION OF MARINE ANIMALS. 503 



breathing creatures ; and the loss to the latter, 

 from the vast amount of oxygen thus removed 

 from the atmosphere, it might have been beyond 

 the power of the most profuse vegetation to 

 repair. Although sea-water contains nitrogen 

 in solution as well as oxygen, yet its propor- 

 tion is only small, and we may regard the water 

 as the diluent for oxygen in the ocean, as nitro- 

 gen is for it in the atmosphere. 



The manner in which the dissolved oxygen 

 is received by fish from the medium in which 

 they live is very similar to that in which air- 

 breathing creatures receive it from the air. In 

 the gills, and other modifications of the respi- 

 ratory organs in marine beings, there exists a 

 similar provision for the exposure of the blood 

 to the influence of oxygen as in those of air- 

 breathers. The gills are composed of numerous 

 lamince, or plates of tissue, covered with innu- 

 merable minute blood-vessels, and exposing 

 a very large surface to the influence of the 

 oxygen dissolved in the water. The water 

 becoming partially deprived of its dissolved 

 oxygen, is discharged from under the gill 

 covers, a fresh portion being taken in at the 

 mouth. Thus a constant current of fresh water 

 is caused to flow over the Iamina3, from which 

 a constant supply of oxygen is obtained by the 

 fish. The blood, after becoming thus oxygen- 



