ON THE ORAL APPENDAGES IN SOME TERRESTRIAL ISOPODS. 287 



On the Range o£ Variation o£ the Oral Appendages in some Terrestrial 

 Isopods. By Walter E. Collingb, M.Sc, F.L.S., F.E.S. 



(Plates 20 & 21.) 



[Read 5th February, 1914.] 



Contents, 



Page 



I. Introduction 287 



II. Species Examined 288 



1. Ligia oceanica, Linn 289 



2. Trichofiiscus roseus (Koch) 289 



3. Oniscus asellus, Linn. . . . : 289 



4. Porcellio scaber, Latr 290 



5. Porcellio Icevis, Latr 291 



6. Porcellionides jwuinosus (Brandt) 291 



7. Armadilliclium vulgar e (Latr.) . 292 



III. Summary and Conclusion 292 



IV. BiWiography 292 



I. Introduction. 



In classifying the different Orders o£ animals, zoologists have endeavoured to 

 separate species and genera from one another by certain characters which 

 are constant for a particular genus or species ; thus in Insects the form of 

 the antennse and the venation of the wings has been largely used, in 

 Molluscs the generative organs and nervous system. 



Many of the systems propounded by earlier writers have been discarded 

 because the characters relied upon for this separation, have on further study, 

 been found to vary greatly in individual species and therefore to be un- 

 reliable. 



In the various attempts that have been made to classify and diagnose the 

 different genera and species of Terrestrial Isopoda, students have laid 

 particular stress at different times upon the shape and form of almost every 

 part of the external structure. 



Considerable reliance has of recent years been placed upon minute 

 differences in the oral appendages, and the object of the present paper is to 

 show that these particular 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, antennae, telson, uropoda, 

 mesosomatic segments, and thoracic appendages. 



Lereboullet (Compt. Rendus, 1849, xx, p. 346) stated that in his opinion 

 the oral appendages are in no case available for characterizing the genera and 



