VI INTRODUCTION. 



The aberrant group of Isopoda, although containing, 

 and perhaps based upon, the most characteristic genus of 

 Dana^s supposed order of Anisopoda. yet must not be con- 

 sidered synonymous with it, since all the parasitic forms 

 that possess such extremely aberrant characters in the 

 adult females, possess the true character of the normal 

 Isopod, both in the young and adult male. Thus the 

 genera Arcturus, Bopyrus, and the rest of the parasitic 

 Ampldjjoda, we have classified with the normal Isopoda. 

 Nor can we think that the only feature that assimilates 

 Arcturus to the Aniphlpoda (the forward direction of 

 the second pair of pereiopoda), can be considered of suf- 

 ficient importance to narrow the distinction between it 

 and the Amphipoda, whereas other characters of greater 

 importance induce a natural separation that is strongly 

 marked. 



The consideration of the structure of the Sessile-eyed 

 Crustacea has, until recently, but little attracted the 

 attention of zoologists. The observations of Loven, 

 Lilljeborg, Goes, De la Valette, Grube, Fritz Miiller, 

 Anton Dolirn, Schobl, Schiodte, and others, have done 

 much to show the large amount of novel and interesting- 

 subjects of biological knowledge that have been, and still 

 are to be, developed by the study of this hitherto much 

 neglected class of animals. 



The structure of these animals, though oflFering a very 

 palpable distinction from the higher forms, is indubitably 

 formed upon the same common type. So clearly can 

 this be demonstrated, that we are somewhat surprised to 

 find that Mr. Dana (" United States Exploring Expe- 

 dition," vol. i. p. 1404) should say that " they have not 

 a macrural characteristic, but have a body divided into 

 as many segments as they have legs (hence our name 

 Choristopoda) ; the antennse, legs, and whole structure are 

 distinct in type." 



