Vlll PREFACE. 



unnecessary to burden the text ; and, under the 

 head of Bibliography, I have given some references 

 to the literature of the subject which may be useful 

 to those who wish to follow it out more fully. 



I am indebted to Mr. T. J. Parker, demonstrator 

 of my biological class, for several anatomical draw- 

 ings; and for valuable aid in supervising the 

 execution of the woodcuts, and in seeing the work 

 through the press. 



Mr. Cooper has had charge of the illustrations, 

 and I am indebted to him and to Mr. Coombs, 

 the accurate and skilful draughtsman to whom 

 the more difficult subjects were entrusted, for 

 such excellent specimens of xylographic art as 

 the figures of the Crab, Lobster, Eock Lobster, 

 and Norway Lobster. 



T. H. H. 



London, 



November, 1879. 



