Marsh. 221 



siderable amount of force in the apparently remarkable 

 statement that if you keep the mouth of the frog or toad 

 propped open, you may suffocate it. 



How different, then, is this method of breathing from that 

 of a mammal, and yet, again, how different from the method 

 it adopted when it was a tadpole ! Let us consider for a 

 moment the question, " What breathing does ! " From the 

 air breathed in, oxygen is absorbed by the blood, and into the 

 same air, before it is expired, pass used-up gases from the 

 blood, of which carbonic acid gas is the chief. The tissues 

 of the body are constantly requiring oxygen, with which 

 they combine, and one of the waste products of bodily action 

 is carbonic acid gas, which must be constantly carried off, 

 or it will have a poisonous effect. 



The methods adopted by animals for this purpose are 

 various. In fishes the venous blood, poorly supplied with 

 oxygen but rich in carbonic acid gas, passes through 

 the membranous plates called gills. Water is taken into 

 their mouths, passed over these plates, and out of the gill 

 cavities behind. As this water, well-supplied with oxygen, 

 passes over these thin gill membranes, the oxygen, which is 

 dissolved in the water, passes through the thin membrane 

 into the blood, whilst the carbonic acid gas passes in the 

 opposite direction. This is what happens in the tadpole and 

 other creatures that live in the water; but many of the 

 lower forms of animal life have no proper breathing organs. 

 In these, respiration is carried on through the surface of the 

 body, and to a certain extent the skin of the adult toad and 

 frog is used for this purpose. In others of the lower animals 

 molluscs, insects, etc. the air enters through external holes, 

 called spiracles, which lead into tubes and chambers, through 



