THE 



PHYSIOLOGY . OF THE INVERTEBRATA. 



CHAPTER I. 



INTRODUCTION. 



Animal pliysiology va&j be defined as that branch of biology 

 which is concerned in the elucidation of the various functions 

 which take place in the animal economy. It is a branch of 

 study quite distinct from morphology, chorology, and aetio- 

 logy ; and as a separate branch of biological science we propose 

 to treat it in the following .pages. 



Researches undertaken to investigate accurately the proper 

 physiological functions of the various organs and tissues of 

 the Invertehrata were greatly needed ; and it is only during 

 the last few years that certain biological chemists — fully 

 equipped with the necessary manipulative skill — have con- 

 siderably advanced this important but much-neglected branch 

 of biology. 



If one studies any particular organ from only one aspect, 

 incomplete or erroneous conclusions are apt to be drawn. 

 For instance, the vesicular tissue lying in the rectal loop in 

 Ascidia, and in some species extending over the intestine, is 

 well known to be renal in function. This vesicular tissue is 

 a true kidney physiologically ; .morpTiologically it is another 



