PREFACE 



In planning the present work the aim of the authors has 

 been to provide a manual embodying a course of study 

 adapted to the requirements of the student chiefly in higher 

 classes of schools, and to some extent in junior classes of 

 universities. To make this, within the necessarily narrow 

 limits of space imposed, anything more than a bare synopsis, 

 it has been necessary to restrict the extent of the ground 

 covered. This has been done (i) by leaving out altogether 

 certain classes of existing animals; (2) by omitting all de- 

 scriptions of extinct groups ; (3) by dealing only very briefly 

 with embryology. Opinions must differ as to the best selec- 

 tion of groups for an elementary manual of this kind. But 

 broadly, there can, it has appeared to us, be little doubt that 

 what should be omitted, or only briefly dealt with, are the 

 groups of rare occurrence and uncertain relationships, the 

 greater part of the space being devoted to the more familiar 

 representatives of the large phyla. 



A course of laboratory and museum instruction, supple- 

 mented by work in the field and on the seashore, is abso- 

 lutely necessary in order that any sound knowledge of 



