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 

 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, any- 

 thing 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 descriptions of 

 extinct groups ; (3) by dealing only very briefly with 

 embryology. Opinions must differ to a certain ex- 

 tent as to the best selection of groups to be treated of 

 in an elementary manual of this kind. But it ap- 

 peared advisable to us that the groups of rare occur- 

 rence and uncertain relationships should be omitted 

 or only briefly dealt with, the greater part of the 

 space being devoted to the more familiar represent- 

 atives of the large phyla. 



It is universally agreed that a course of laboratory 

 and museum instruction, supplemented by work in 



