PREFACE 
In this book I have tried to give such an account of the 
Invertebrata as might be useful to students in the upper 
forms of Schools and at the Universities, who are already 
acquainted with the elementary facts of Animal Biology. The 
volume is in no sense a work for advanced students, and hence 
no references to original sources have been given, and the 
names of the various investigators who have promoted our 
knowledge have been mentioned as sparingly as possible. 
In order to keep the book within reasonable limits, I have 
not described fully certain types which are dealt with in the 
admirable elementary text-books of Huxley and Martin, and 
Marshall and Hurst; but, with this reservation, I have en- 
deavoured to describe some one example of each of the larger 
groups, and then to give a short account of the most interesting 
modifications presented by other members of the group. 
The last few years have witnessed a great extension in 
our knowledge of the structure and relationship of the Inver- 
tebrata. The earth has been ransacked for new forms, and 
improvements in microscopes and in technique have facilitated 
a more minute and thorough examination of these forms in 
the laboratory. This increase in our knowledge has neces- 
sarily been accompanied by a rearrangement of material ; 
many intermediate forms have been discovered, and unexpected 
relationships have been revealed, and these have entailed a 
revised classification. 
