PREFACE TO THE SIXTH EDITION. 



THE present edition has been submitted to thorough revision, 

 and it is hoped that the most important facts which have 

 been brought to light since the appearance of the last edition 

 will be found to be incorporated in it. No very important 

 changes in classification have been introduced, save that the 

 Echinodermata, in accordance with the views of many dis- 

 tinguished naturalists, have been raised to the rank of a sub- 

 kingdom. This alteration necessitates the abandonment of 

 the Annulotda, as a sub-kingdom, and the reference of the 

 Scolecida to the Annul osa ; and though this arrangement is far 

 from being wholly satisfactory, it would seem to be the best 

 that can, under the circumstances and with our present know- 

 ledge, be adopted. 



Lastly, a considerable number of additional illustrations 

 have been introduced, by which, it is hoped, the work of the 

 student will be proportionately lightened ; and it is to these, 

 chiefly, that any increase of bulk in the present edition is to 

 be ascribed. 



UNITED COLLEGE, ST ANDREWS, 

 May 24, 1880. 



