INTRODUCTION. 



THE structure of the Crustacea is so little known to 

 the students of Natural History in this country, and there 

 are so few works which give even the most superficial 

 information on the subject, that it appears very desirable 

 and even necessary to introduce the study of the British 

 species, by a brief account of the general organization and 

 physiology of this class of animals. Not only indeed is 

 the subject itself one of great interest, but without some 

 such introductory information it would not be possible to 

 comprehend the descriptions of the different genera and 

 species ; for it will be found that in scarcely any other 

 class of animals, is there a greater variety of form and 

 structure, or more striking apparent anomalies in the 

 modifications of the typical plan of organization, or in some 

 cases greater difficulties in ascertaining the true homologies 

 of the different elements, than in the present. 



It is not, indeed, a very easy matter even to express, 

 in a clear and definite phrase, the characters which, whilst 

 belonging strictly to all the forms of Crustacea, shall dis- 

 tinctly exclude those of the approximate ones ; for the 

 variations which occur in every organ and function, in 

 the different groups belonging to the crustacean type, are 

 so considerable, as to render it almost impossible to include 

 them all within one common and well-defined expression. 

 The typical characters are so astonishingly modified, in 

 some cases being totally changed, and in others absolutely 

 lost, that the inexperienced student examining some aber- 



