PEE FACE. 



THE reader who is sufficiently acquainted with the 

 progress in vertebrate physiology during the last 

 phase of physiological methods, and who knows how 

 scattered and incomplete are the investigations which 

 have been made by the same kind of physical and 

 chemical inquiries on invertebrate animals, will not 

 expect to find in the present volume any complete 

 statement of the physiology of animals, in the sense 

 in which that term is now used. Such observations 

 as have been made without especial reference to the 

 vital processes of man are, for the most part, very 

 valuable and suggestive ; but the time to write a text- 

 book of Comparative Physiology, as we now understand 

 it, has not yet arrived. 



All that I have attempted to do in this little book 

 has been to illustrate the details of structure by a 

 notice of such experimental inquiries as I have con- 

 vinced myself, or have adequate reason to believe, are, 

 in their broad outlines, correctly stated. I have much 

 more attempted to make use of what were long since 

 called the experiments that Nature makes for us, by 



