PREFACE. Vll 



clear and well-illustrated discussion of the structure of the Pteropoda, and of the inferences 

 which may be drawn therefrom as to their affinities. 



Iii regard to the article Polyzoa, I may mention that I think it preferable to make use 

 of the established term "Gephyrsea" in place of that introduced in this article, viz., 

 " Podaxonia," The Gephyreea, then, include the Sternaspidomorpha, Echiuromorpha, 

 Sipunculomorpha, Phoronidomorpha, Polyzoa (Eupolyzoa of the article), Brachiopoda, and 

 Pterobranchia, Concerning the affinities of the first four of these classes with one another, 

 there is little doubt : as to the affinities of the last three with one another, and with the 

 first four we are still in a very uncertain state, and are likely to remain so for some time, 

 owing to the absence of satisfactory embryological data and the difficulty of obtaining such. 



The subject matter of the article Vertebra ta is much more extensive than that of the 

 other chapters, and, owing to limited space, is treated in a much more general way than is 

 the case with the latter. In regard to the Craniata, the intention was to give only a sketch 

 of leading features which should be supplemented by the study of such works as Gegenbaur's 

 Comparative Anatomy, Wiedersheim's Anatomy of Vertebrates, and the special articles on 

 Fishes, Eeptiles, Birds, and Mammals, written for the Encyclopaedia by eminent autho- 

 rities on those groups. The treatment of the Cephalochorda (Arnphioxus) and its relations 

 to the Urochorda is a little more complete, and I therefore take occasion to refer the reader 

 to recent publications, in which our knowledge of this most interesting member of the 

 Vertebrate group has been largely extended. They are Contributions to the Know- 

 ledge of Rhabdopleura and Amphioxus (ubique citata), by E. Ray Lankester (London: 

 J. & A. Churchill, 1889); "The Development of the Atrial Chamber of Amphioxus," by 

 E. Ray Lankester and Arthur Willey, in the Quart. Jour, of Mic. Sci., vol. xxxi., 1890 ; 

 "The Later Larval Development of Amphioxus," by Arthur Willey, B.Sc., in the same 

 Journal, vol. xxxii. ; and "The Excretory Organs of Amphioxus," by F. E. Weiss, B.Sc., 

 also in the Quart. Jour, of Mic. Sci., vol. xxxi. 



The article " Sponges," by Professor Sollas, contains the only summary account of the 

 Porifera written since the recent extraordinary advances in our knowledge of this group. 

 Its incorporation in the present volume cannot fail to be welcome to students. In 

 Professor Bourne's article on Rotifera are given the only extant woodcuts of the important 

 genus Pedalion. This most important form is not figured or discussed in any other general 

 treatise accessible to students. The articles on Planarians and Nemertiues, by Professor 

 von Graff and Professor Hubrecht respectively, are brief summaries of what is known, 

 written by the chief living authority on each group. 



E. RAY LANKESTER. 



OXFORD, December 1890. 



