VI PREFACE 



appeared in other works. The sources of these are acknow- 

 ledged, but we must especially mention Claus's Lehrbuch, 

 Shipley & MacBride's Zoology, Bronn's Thierreich, Kor- 

 schelt & Heider's Textbook of Embryology, Delage & Herou- 

 ard's Zoologie Concrete, Perrier's Traite de Zoologie, the 

 Cambridge Natural History, Lang's Textbook of Comparative 

 Anatomy, Zittel's Grundzuge der Palaeontologie, Lankester's 

 Treatise on Zoology. 



I have also to state that the drawings from which figures 

 105 and 132 were engraved were made by Mr. J. C. Simpson 

 under the direction of Professor MacBride. 



Now that the work is completed something must be said 

 as to its architectural plan. This has been criticized (see, 

 for example, the review in Nature, November 23, 1905) on 

 the ground that the Arthropods are separated from the 

 Annelids, and that the Tunicates and Enteropneusta are 

 placed at the end of the Chordata. In the first volume the 

 clue given by the coelom is mainly followed and this 

 leads from the Annelids to the enterocoelic phyla the Phor- 

 onidea, the Brachiopoda and the Chaetognatha. Having 

 reached this point it was not possible to treat the Arthropoda 

 until after the Enter ocoela were finished. This accounts 

 for the position of the Arthropoda at the end of the work. 

 It may be objected that the Arthropoda should have come 

 after the Annelida in volume I. But this would have 

 involved the inconvenience of separating the Gephyrea 

 Achaeta from the Gephyrea Armata, and the Brachiopoda 

 and Polyzoa from the Chaetopoda. Moreover, the Arthro- 

 poda differ so fundamentally from the Annelida in their 

 coelomic arrangements and are such an enormous and self- 

 contained group that it did not appear that any practical 

 disadvantage would follow upon their separation from 

 the latter which in some features of their organization 

 they so closely follow. In fact we are here confronted with 

 a difficulty which the systematic Zoologist meets at every 

 turn and to which attention has often been called in the 

 course of the work. When there is more than one 

 clue, as there nearly always is, which shall we follow ? We 

 are obliged to adopt a linear arrangement, whereas Nature 



