6 INTRODUCTION. 



extent that they have become adapted to a fixed habit of 

 Ufe, is of course obvious ; but that they have phyloge- 

 netically undergone the immeasurable degeneration which 

 was postulated by Dohrn, is a view which is entirely 

 unjustified by facts. We shall now proceed to a presen- 

 tation of some of these facts, devoting the first two 

 chapters to the anatomy of Amphioxus, the third to the 

 development of Amphioxus, the fourth to a brief sketch of 

 the structure and development of the typical Ascidians, and 

 the fifth to a consideration of the more abstruse relation- 

 ships of the lower Vertebrates or Protochordates. 



The following classification of the forms more particu- 

 larly dealt with may be of service : — 



Group. — Protochordata. 



Division i. Hemichorda (Balanoglossus, Cephalodiscus, 



and Rhabdopleura. See Chap. V.). 

 Division 2. Urochorda (Ascidians). 

 Division 3. Cephalochorda (Amphioxus). 



