viii PREFACE. 



they are destined to play an important part in the elucidation 



of many problems still requiring solution. 



The following work gives an account of some of the most 

 important researches on the subject, which have been published 

 during the past fifteen or twenty years ; and I have also 

 included an account of my own researches in the present 

 volume, more especially as these have appeared in the 

 Proceedirigs of the Eoyal Societies of London and Edinburgh, 

 and have also attracted the attention of the Acad^mie des 

 Sciences (I'lnstitut de France), to the extent that its Council 

 thought proper to award me an " honourable mention " in 

 connection with the Prix Montyon, which is given annually 

 for researches in experimental physiology and physiological 

 chemistry. Besides, several well-known biologists have 

 informed me that a work on the physiology of the Invert e- 

 hrata would be a welcome addition to biological literature. 

 Consequently, I hope that this work (although I am fully 

 cognisant of its many imperfections and shortcomings) may 

 prove of some utility to those scientists and students who are 

 desirous of investigating biological problems involving the 

 applications of chemistry and physics. 



I take this opportunity of expressing my gratitude and 

 best thanks to Sir Eichard Owen, K.C.B., P.E.S., for the 

 great interest he has always taken in my investigations, and 

 for the many letters of friendly criticism which I have received 

 from him, 



I am also grateful to Mr. F. E. Beddard, P.E.S.E. ; the 

 Eev. W, H. Dallinger, LL.D., F.E.S. ; Mr. H. H. Dixon (of 

 the University of Dublin) ; Prof. J. C. Ewart (of the Univer- 

 sity of Edinburgh); Prof. L4on Fredericq (of the University 

 of LiSge) ; Dr. A. Giard (of Paris) ; Mr, S. T. Griffiths ; 



