I 



PREFACE. 



T HAVE been anxious to render the articles on various groups of Animals written by 

 me for the Encyclopaedia Britannica more readily accessible to the University 

 student than they are when bound up in the large volumes of that great work. The 

 Publishers have very kindly met my wishes in this respect by consenting to issue the 

 present reprint. With my articles on Protozoa, Hydrozoa, Mollusca, Polyzoa, and 

 Vertebrata, are here included, by the kind consent of the authors, the article on Sponges 

 by Professor Sollas, that on Planarians by Professor von Graff, that on Nemertines by 

 Professor Hubrecht, that on Rotifera by Professor Bourne, and that on Tunicata by 

 Professor Herdman. The volume thus forms a treatise on a considerable section of the 

 animal kingdom. Obviously it does not profess to be a complete handbook. Since the 

 articles are reprinted from the original plates, and issued at a low price, it has not been 

 possible to introduce any large additions into the text. Here and there an error, due to 

 oversight, has been corrected, and one or two new figures have been added, rendering the 

 work more complete. The chief additions are the woodcut illustrating recent discoveries 

 concerning the Dinoflagellata (p. 37) ; the note by Professor Sollas on the classification of 

 Monaxonida (p. 39); the woodcut of Scyphomedusre from the Deep-Sea (p. 57); the 

 woodcut fig. 19 on p. 107, which replaces a similar but incorrect figure in the original 

 article, and the woodcut, fig. IA on p. 159, showing forms connecting the Eupolyzoa and 

 other Gephyrsea. 



There are one or two matters, by way of addition to or correction of my own articles, 

 which this preface gives me the opportunity of mentioning. 



In regard to the Protozoa, the reader should note that Professor Butschli's treatise in 

 Bronn's Thicrreich is now completed. He has rejected the classification of the Ciliata. 

 which we owe to Stein, and adopts the following Branch A. Gymnostoma ( = Holotricha 

 with chitiuised pharynx, Prorodon, Trachelius, &c.) ; Branch B. Trichostoma ( = the 

 remaining Ciliata, all of which have the pharynx ciliated, if present). The Trichostoma 

 are divided into two classes the Aspirotricha, and the Spirotricha. The Aspirotricha 

 are the rest of the Holotricha of Stein, not comprised in the Gymnostoma of this classifi- 



