GENERAL CHARACTERS. 9 



great majority being less than an inch long, and none 

 exceeding thrice that length. Like all the productions 

 of nature of diminished size, the number of indivi- 

 duals of different species far exceeds that of the larger- 

 sized Crustacea. With few exceptions, there is but little 

 external difference of form between the opposite sexes ; 

 the males, however, contrary to the ordinary rule in the 

 Annulosa, being often larger than the females. There is 

 also but little difference in form between the young and 

 adult individuals of the several species, except where 

 marked characters exist, such as the enlarged form of the 

 hands, or spines on the different segments of the body, 

 which increase in size as the animals become older. The 

 species occur in temperate or high latitudes in greater 

 proportion than in tropical climates. 



Thus it will be perceived that among the Amphipoda 

 there is a considerable variety of form, some keeping 

 closer to the typical idea of the Order, while others vary 

 more or less considerably. It is therefore desirable, both 

 for clearness of expression and in order to obtain a 

 better knowledge of the whole, that we should arrange 

 together those which more nearly assimilate to each other ; 

 whereas others, which vary in a greater or less degree, 

 should be grouped according to their respective details. 



In the works of Leach, Latreille, Milne-Edwards, 

 Kroyer, Dana, Zaddach, Liljeborg, and Bruzelius, various 

 modes of classification of these animals have been pro- 

 posed. Based upon a consideration of these various ar- 

 rangements, as well as upon the structure and respective 

 habits of the different animals (resulting from a consider- 

 able observation of their economy and modes of life), 

 and having had the advantage of studying the types of 

 Montagu, Leach, Phipps, M. -Edwards, and others, we 

 have drawn up the following tabular distribution of the 

 Order : 



