HOW DOES THE EOLIS BREATHE? 107 



the organ is distinctly recognisable, and to compare it with 

 the dorsal papilloe of the Eolis is to demonstrate tliat two 

 such dissimilar organs cannot play the same part. Indeed, 

 the Doris seems to me higher in the scale of organisation 

 than the Eolis, although less active in its movements. It 

 has a specialised liver, a more perfect vascular system, the 

 commencement of a respiratory system ; and it has not the 

 arborescent intestines which the Eolis has in common with 

 the PlanaricB and Pycnogonidos. I should propose, there- 

 fore, to remove the Eolis from the Nudibranchiates, and caU 

 it Abranchiate. 



How, then, does the Eolis breathe ? He does not 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 

 (juite as well, it does so in a different manner. Respiration 

 as an animal function, and Respiration as a gewQToS. property 

 of tissue, are incessantly confounded in our loose language ; 

 but the distinction should always be borne in mind. The 

 ultimate fact of Respiration 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 ex- 

 changing carbonic acid for oxygen, and the function of the 

 special apparatus by which the exchange is brought aljout ; 

 just as there is a wide distinction between the general 



