GG DU. W. E. COLLINGE ON THE 



I. Introduction. 



There :ire few groups of animals that present greater difficulties in 

 connection with their classification than the Order Isopoda. 



Much of the work of the earlier carcinologists has, in the light of recent 

 research, proved to he very imperfect, and many of the genera and species 

 classified as related to one another have been shown to be widely separated. 

 Further, structural characters that hold good for certain divisions of the 

 Isopoda are quite unsatisfactory for others. This is particularly well illus- 

 trated in the oral appendages. I have elsewhere (7) expressed the view, and 

 shown, that in the suborder Oniscoidea, Sars, these appendages "are liable 

 to a large amount of variation in individual species, and are therefore 

 characters of only minor importance as compared with the form of the head, 

 antenna?, telson, uropoda, mesosomatic segments, and thoracic appendages," 

 and I believe that further investigations will serve to corroborate and 

 strengthen this view so far as the strictly terrestrial forms are concerned, 

 where considerable modification and degeneration have taken place. 



On the other hand, in certain suborders of marine Isopoda these same 

 appendages would seem to be fairly constant in form, and to offer excellent 

 data for the characterization of both genera and species (8-12). 



Recent work on the suborder known as Valvifera shows that the form of 

 at least two of the four oral appendages affords most valuable aid in the 

 discrimination of both genera and species, viz., the first maxilla? and the 

 maxillipedes. In this connection it is interesting to note, that in the members 

 of the family Idoteida? they differ very little in the immature stages from 

 that obtaining in the adult ; they are thus frequently of great service for 

 purposes of identification. A further interesting point relative to the two 

 pairs of maxilla?, and in a lesser degree the maxillipedes also, is the frequency 

 with which the "casts "of these appendages remain attached to the newly 

 formed ones, and they often serve a useful purpose in aiding in the elucidation 

 of minute structural details, as, being free from all muscular attachments and 

 almost void of pigment, they can easily and quickly be made transparent. 



It is therefore much to be regretted that many authors have given no 

 description or figures of the form and structure of these appendages in quite 

 a large number of species. 



In the present communication I have endeavoured to remedy this to some 

 extent by describing and figuring the first maxilla (oocasionallj r the second) 

 and the maxillipede in 21 species referable to genera of the familj' 

 Idoteida:, in none of which, so far as I am aware, have they both been 

 described, and also to correct some errors in a few of the figures and 

 descriptions where the maxillipede only has been partly described and 

 incorrectly figured. 



