PREFACE Vll 



is content with nothing less than arrangement in three 

 dimensions. 



Following then the clue of the coelom and having reached 

 so near the Chordata, it seemed natural and proper to take 

 them after the Chaetognatha. I might of course have 

 placed the Echinoderms here and treated the Enteropneusta 

 after them. That was in fact the original intention, but 

 after careful consideration there seemed no reason based on 

 zoological affinity why this should be done and it would have 

 been almost an outrage to have placed the Echinoderms 

 next to Sagitta. It was accordingly decided to take the 

 Chordata at this point, beginning with Amphioxus and re- 

 serving the Enteropneusta to the end to lead on to the 

 Echinoderms. 



Though I offer this explanation I quite recognize the 

 validity of the criticism. All zoological arrangements are 

 compromises, and none of them can be, now or ever, entirely 

 natural. 



A. SEDGWTCK. 



CAMBRIDGE, 



November 25, 1908. 



