BRITISH 

 SESSILE-EYED CRUSTACEA. 



Order AMPHIPODA * 



This name was given by Latreille to the present 

 order -f of Crustacea on account of the animals contained 

 in it having both swimming and walking legs, and to 

 distinguish it from the order Isopoda, in which the legs 

 are adapted for walking only. 



The Amphipods exhibit the characters of the great 

 class of which they are a part, more typically perhaps 

 than any other Crustacea. In the higher orders, the 

 head, from its great development, encroaches upon the 

 body, and in the lower orders, the body encroaches upon 

 the head. The type J of a class, order, or indeed any 

 other group, is to be found in its centre, and not at 

 either extremity of the series. 



The Ampliipoda are formed upon the Macroural type, 

 from the normal condition of which they differ in the 

 three following important particulars : first, there is no 



* Derived from tlie Greek u.fiipa>, both ; a-aSsf, feet. 



■!• It must be borue in mind that, for the considerations set forth in the 

 Introduction, the order Lsemodipoda, proposed by Latreille for Caprella and 

 its allies, has been rejected — that group being regarded as an aberrant 

 division of the Ampliipoda. 



X The following definition of a type is given by Professor Whewell, as the 

 92nd aphorism concerning ideas, in his " Philosophy of the Inductive 

 Sciences:" — "Natural groups are best described, not l>y any definition 

 which marks their boundaries, but by a type which marks their centre. The 

 type of any natural group is an example which possesses, in a marked degree, 

 all the leading characters of the class." 



B 



