REVISER'S NOTE. 



IN revising the work of Professor Orton, the writer has 

 not attempted to rewrite the book nor to introduce new 

 ideas. His plan has been to insert such changes as the 

 author would have been likely to make if he had lived to 

 revise his book. On only two points has the reviser de- 

 parted from this plan of altering only minor details. The 

 chapter on Development has been largely rewritten, and 

 the classification of the Invertebrates has been changed 

 so as to separate the worms from the Arthropoda and 

 the sponges from the Protozoa. In both these cases the 

 change seemed imperatively demanded by the progress of 

 Zoology in those directions. It is hoped that the altera- 

 tions in the book will increase its accuracy and useful- 

 ness. 



EDW. A. BIKGE. 



UNIVERSITY OF WISCONSIN. 



