Amphibious Animals. ] 



and darker coloured, as frogs and lizards, they require less oxygen 

 than the warmer animals with a greater quantity and more scarlet 

 blood; and thence, though they have only lungs, they can stay long 

 under water \vithout great inconvenience; hut are all of them, like 

 frogs, and crocodiles, and whales, necessitated frequently to rise above 

 the surface for air. 



Jn this circumstance of their possessing a one-celled heart, and 

 colder and darker blood, they approach to the state of fish; which 

 thus appear not to acquire so much oxygen by their gills from the 

 water as terrestrial animals do by their lungs from the atmosphere; 

 whence it may be concluded that the gills of fish do not decompose 

 the water which passes through them, and which contains so much 

 more oxygen than the air, but that they only procure a small quantity 

 of oxygen from the air which is diffused in the water; which also is 

 further confirmed by an experiment with the air-pump, as fish soon 

 die when put in a glass of water into the exhausted receiver, which 

 they would not do if their gills had power to decompose the water and 

 obtain the oxygen from it. 



The lamprey, petromyzon, is put by Linneus amongst the nantes, 

 which are defined to possess both gills and lungs. It has seven spira- 

 cula, or breathing holes, on each side of the neck, and by its more 

 perfect lungs approaches to the serpent kind; Syst. Nat. The means 

 by which it adheres to stones, even in rapid streams, is probably owing 

 to a partial vacuum made by its respiring organs like sucking, and 

 may be compared to the ingenious method by which boys are seen to 

 lift large stones in the street, by applying to them a piece of strong- 

 moist leather with a string through the centre of it; which, when it 

 is forcibly drawn upwards, produces a partial vacuum under it, and 

 thus the stone is supported by the pressure of the atmosphere. 



The leech, hirudo, and the remora, echeneis, adhere strongly to 

 objects pi-obably by a similar method. I once saw ten or twelve 

 leeches adhere to each foot of an old horse a little above his hoofs, 

 who was grazing in a morass, and which did not lose their hold when 

 he moved about. The bare-legged travellers in Ceylon are said to be 

 much infested by leeches; and the sea-leech, hirudo muricata, is said 



