PREFACE. 



This volume originated in a course of University lec- 

 tures prepared at my suggestion by the author. It seemed 

 important that he should bring within the reach of 

 students and of specialists among other groups, his own 

 extensive observations upon Amphioxus and other remote 

 ancestors of the Vertebrates, as well as the general litera- 

 ture upon this group. While our detailed knowledge of 

 the structure and habits of these animals has been rapidly 

 increasing in recent years, it is still in the main very 

 widely scattered in monographs and special papers. 



Probably no single group illustrates more beautifully 

 the principles of transformism ; for the Protochordates in 

 their embryonic development exhibit remarkable reminis- 

 cences of past adaptations, and, in their adult develop- 

 ment, the most varied present adaptations to pelagic, 

 deep-sea, littoral, free-swimming, and sessile life. As 

 Lankester has shown, the Ascidians alone give us a whole 

 chapter in Darwinism. But degeneration and change of 

 function constitute only one side of their history. In 



