PREFACE. 
—a 
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 Meduse 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 woodcuts 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, 
