INTRODUCTION 



Most people have seen a crustacean at one time or another; crabs, 

 lobsters, shrimps and woodlice are common enough, but there are 

 also myriads of smaller less conspicuous Crustacea unknown to the 

 layman. The aquarist probably knows Daphnia as ' the water flea ' 

 and does not consider it as a relative of the lobster. 



Why are such different creatures included in the same group? 

 One answer would be that they both have external skeletons and 

 jointed limbs — features which they share with other arthropods 

 such as insects, spiders and centipedes; they also have other 

 similarities such as a pair of mandibles which act as jaws, and they 

 have two pairs of appendages in front of the mouth — characters 

 which distinguish them from other arthropods. But such an answer, 

 although it is technically correct, is not very satisfying. In order 

 really to understand why water fleas and lobsters belong to the same 

 group it is necessary to know about other creatures belonging to 

 the group and to know about their various ways of life. 



In this book an attempt is made to treat the biology of Crustacea 

 as a whole. The difficulties of this approach will become evident in 

 the first chapter on origin and radiation; the origin may be obscure, 

 but the results of their radiation abound throughout the world. No 

 other class of animals exhibits such diversity of form. Nevertheless 

 the essential unity of the Crustacea as a group becomes more 

 apparent the more one learns of them. 



xv 



