PREFACE. 



When I first accepted the invitation of the editors 

 of the International Scientific Series to supply a 

 book upon Primitive Nervous Systems, I intended 

 to have supplemented the description of my own 

 work on the physiology of the Medusce and 

 Echinodermata with a tolerably full exposition of 

 the results which have been obtained by other 

 inquirers concerning the morphology and develop- 

 ment of these animals. But it soon became 

 apparent that it would be impossible, within the 

 limits assigned to me, to do justice to the more 

 important investigations upon these matters ; and 

 therefore I eventually decided upon restricting this 

 essay to an account of my own researches. 



With the exception of a few Avoodcuts in the 

 last chapter (for the loan of which I am indebted 

 to the kindness of Messrs. Cassell), all the illustra- 

 tions are either original or copies of those in my 

 Royal Society papers. In the letter-press also I 

 have not scrupled to draw upon these papers. 



