l8g4 .] THE BRITISH NATURALIST. n 



HOW DO ANIMALS BREATHE ? • 



BY A. J. JOLLEY. 



How do animals breathe ? is a question which sounds sufficiently 

 simple ; but like many other seemingly-simple questions, it is one of great 

 complexity, and the more carefully we examine it the more manifest does 

 this complexity become. Perhaps our safest answer (if asked the above 

 question) would be to ask for a definition of terms, and we should possibly 

 then be able to evade the issue. 



The first term, " animals," is not by any means easy to define, that is 

 if the definition is to be accurate; whilst the second, " breathing," is 

 still more difficult, and if our interrogator gave us a full and satisfactory 

 definition of these terms he would have answered as satisfactorily as 

 possible his own question. 



It may perhaps be well if we take the second term first. Breathing or 

 respiration are terms applied to functions of both animals and plants 

 (though some regard its application in the latter case as erroneous), and 

 these functions are the interchange of gases between the organism and 

 the surrounding medium. In animals the most important of these gases 

 are carbonic acid, which is given off, and oxygen, which is taken up ; the 

 carbonic acid originating in the union of the absorbed oxygen with the 

 carbon of the organic particles of the body ; the interchange of these 

 gases is always effected through a moist membrane. 



With regard to the first term of our question — "animals": the best 

 classification of living beings is that which divides them into three 

 groups — protists, unicellular organisms ; and plants and animals, which 

 are cell-aggregates. To the cell-aggregates the name Histiota is 

 applied, but the attempt to give a brief and concise statement of the 

 difference between the two divisions of the Histiota, plants and animals, 

 has hitherto failed. The difference in the modes of nutrition is the 

 nearest to being a sharp boundary line, most plants assimilating inorganic 

 substances, whilst animals cannot derive nourishment from inorganic 

 substances but require organic food. 



Bearing in mind that animal respiration consists essentially in the 

 inhalation of oxygen, and the exhalation of carbonic acid as a waste or 

 bye-product of the organism, we may proceed to consider the various 

 ways in which this exchange is effected in different groups of animals. 



In many of the lower animals there are no special respiratory organs, 

 the breathing being effected by the general surface of the body ; but 

 even amongst the protists we may conjecture a foreshadowing of the 

 respiratory organs of the higher animals in the contractile vacuoles of 

 many of the protozoa. 

 These vacuoles are empty spaces appearing irregularly in the proto- 



