PKEFACE 



In this book I have tried to give such an account of the 

 Invertebrata as might be useful to students ia 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. 



