vi PREFACE 
of the creatures there is much to afford entertainment, 
and almost every new observer finds something singular 
to relate.. 
In examining the structure both external and internal, 
whether in new species or in those that have been long 
established, the acutest powers of observation may be 
trained and profitably employed. Moreover, the highest 
ingenuity is excited and finds scope in the effort to explain 
the meaning of the facts observed. For, judging by dis- 
coveries already made, we are warranted in supposing 
that, down to the finest hair, every detail of every organism 
has its motive and meaning. Nor need man despair of 
finding out something for his private and personal benefit 
while investigating the physiology of a shrimp. 
It is needless to insist that a hundred volumes such as 
the present would not suffice to discuss the subject in all 
its bearings, since a hundred volumes would be but a 
small fraction of what has been already written upon it, 
and the incessant stream of publications widens and 
deepens as it flows. 
By the references made to some of the most recent 
and to some of the most important authorities, the student 
will be guided in general to adequate lists of literature. 
In consulting these bibliographical notices he will be 
perhaps as much amazed by the multitude of writers and 
writings as at first by the multitude of the genera and 
species of the Crustacea themselves. He will be led to 
consider it not unreasonable that the present volume 
should have been content to deal with one half of the 
entire class, leaving the other half for a future occasion. 
