HOW DOES THE EOLIS BREATHE? 113 



breathe at all. But lest this paradox should disturb 

 you too much, I will soften the blow by adding, that 

 when we talk of an animal breathing, we mean, or 

 ought to mean, that it employs an organ, or group of 

 organs, for the aeration of its blood ; and when the 

 animal is of so simple an organisation that it possesses 

 no such organ, although the aeration takes place quite 

 as well, it does so in a different manner. Eespiration 

 as an animal function, and Eespiration as a general 

 properti/ of tissue, are incessantly confounded in our 

 loose language ; but the distinction should always be 

 borne in mind. The ultimate fact of Eespiration is the 

 interchange of gases, and this may be effected in many 

 ways ; but although the final result is similar, there is 

 great difference between the property which all living 

 tissues, animal and vegetable, have of exchanging car- 

 bonic acid for oxygen, and the function of the special 

 apparatus by which the exchange is brought about ; just 

 as there is a wide distinction between the general pro- 

 perty of Assimilation, and the special function of 

 Digestion. The Eolis we are considering must have its 

 blood aerated ; but the means by which it is aerated 

 do not come under the term " breathing." In many 

 of the lower animals aeration is performed entirely by 

 the surface, the air or water directly bathing the deli- 

 cate tissues, and bringing to them the necessary supply 

 of oxygen, without the intervention of any special 

 apparatus ; just as food is brought to their tissues 

 without the preparatory labour of arduous digestion. 

 The Eolis has not only a delicate surface, covered with 



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