INTRODUCTION. 



The terra "Sessile-eyed" has been applied to the order 

 of ci'ustaceous animals forming the subject of this work 

 in contradistiuction to that of the " Stalk-eyed " order, 

 of which Professor Bell has given an account in a pre- 

 ceding volume of this series. The name, with its Greek 

 equivalent, Edriophthalma, was first given by Dr. Leach, 

 and has been recognized by all subsequent naturalists. 

 It must not, however, be understood to characterize every 

 genus that should be classed in the order. Among the 

 Isopoda, the genera of Tanois, Paratanais, Apseudes, and 

 Munna, have their eyes fixed on pedicles. In the first of 

 these genera, the structure diff'ers so much in character 

 from that of the normal Isopcda, that it has been classed 

 with the Stalk-eyed Crustacea by Fritz Miiller and Anton 

 Dohrn. In this work Ave have placed it in an interme- 

 diate position between the AmpUipoda and the Isopoda ; its 

 most important structural characters holding a position 

 nearer, but intermediate in relation between, these two 

 orders than they bear to the Stalk-eyed Crustacea. 

 While, therefore, the eyes may be considered (as they 

 have been since the days of the Swedisli naturalist, 

 Linnaeus) as a ready and convenient means of classifi- 

 cation, separating one great division from another, this 

 character must be received as only an approximation to a 



a 2 



