Crustacea 205 



which he presented to the Cambridge University Museum in 

 1868. 



It has been thought that the addition of certain facts con- 

 cerning the structure and habits of the Crustacea occurring 

 in the district, would render this account of more interest 

 to the general reader, and it has accordingly been done. The 

 extremely local and variable distribution of many of the 

 forms to be mentioned must also be noticed. A species, 

 which is not to be found at one time, may appear in enormous 

 quantities shortly after. The reverse is of course equally 

 true, so that it by no means follows that every species will be 

 found in the particular spot mentioned. 



It will perhaps be most satisfactory to take a survey of 

 the various Orders which constitute the class Crustacea, and 

 so to consider in turn the principal characteristics of the 

 more important forms which come under consideration. 



In the first instance it will be sufficient for our purpose 

 to divide the Crustacea into the two great groups of Ento- 

 mostraca and Malacostraca. The former includes the smaller 

 and more simply organized forms and, in the present instance, 

 the great majority of those we have to consider. The group 

 Malacostraca contains generally speaking the larger and higher 

 forms, most of which are inhabitants of the sea. 



ENTOMOSTRACA. 



PHYLLOPODA. The Order Phyllopoda, which contains the 

 lowest of all Crustacea, contains also a large number of most 

 varied forms which are characteristic of inland waters, and 

 well represented in the Cambridge district. 



It is not indeed the most primitive types, the Branchio- 

 poda which occur here, but rather members of the more 

 specialised sub-order Cladocera, the so-called "water fleas." 

 With very few exceptions, the whole of the Cladocera live in 

 fresh water, and so may well be abundant in a country of 

 pond, ditch, and fen. The common Dap/tnia, which is a 

 most familiar object to all who have studied pond life, may 



