vl INTRODUCTION. 
The aberrant group of Jsopoda, 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 
Amphipoda, we have classified with the normal Isopoda. 
Nor can we think that the only feature that assimilates 
Arcturus to the Amphipoda (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, Goés, De la Valette, Grube, Fritz Miiller, 
Anton Dohrn, Schobl, Schiddte, 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 offering 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 antenne, legs, and whole structure are 
distinct in type.” 
