apparent. In the latter case, the 3 chief divisions, cephalon, mesosome and meta- 

 some, may, us a rule, be tolerably well distinguished, though not very sharply 

 defined from each other. Of appendages, the cephalon carries generally in 

 front two pairs of rudimentary antennae, and is produced below to a conical pro- 

 minence exhibiting on the tip the oral aperture. The oral parts are much 

 reduced, only the mandibles and maxillipeds being distinctly developed, the for- 

 mer being styliform and converging to the oral orifice, the latter lamelliform. 

 The legs, when present, are all alike, prehensile, terminating in a minute hand. 

 The pleopoda are in some instances rather large, and all of them branchial in 

 character, whereas in the larvae these organs are natatory. The uropoda, when 

 present, are always very small and simple, and do not in any instance form 

 with the last segment a caudal fan. 



As to the systematical relation of this tribe to other Isopodous tribes, 

 Dr. H. J. Hansen has suggested, that it is closely related to the Cymothoidce and 

 cannot therefore be set apart from the tribe FlabeUifera. In this opinion I am, 

 however, by no means prepared to agree with the distinguished Danish author. 

 The apparent resemblance to the CymotlioidoR is only due to the parasitic habits 

 of both ; but in the more essential points of organisation, as also in their develop- 

 ment, both these groups are in reality widely different, and no transition 

 forms between them are as yet known. 



Of recent authors, who have made this tribe their special study, may 

 in the first place be named the two distinguished French naturalists M.M. 

 Giard and Bonnier, who have published several admirable treatises on these 

 interesting Isopoda, accompanied by excellent illustrations. It is however to be 

 regretted, that these authors have introduced considerable confusion as to the 

 definition of species and genera, owing to an assumption, according to which 

 it is postulated, that one and the same species of parasite cannot be found 

 on different species of Crustacea, and that, as a rule, parasites infesting 

 different genera af Crustacea must also be generically different. This assump- 

 tion is most certainly unmaintainable, and a great number of species, and - per- 

 haps also genera founded only upon this principle, must of course be cancelled. 

 The grouping of the several genera, into families has also to a great measure 

 been made according to a similar principle. No less than 7 families have thus 

 been established by the above-named authors, viz. 



1. Microniscidos parasitic on Copepoda. 



2. Cyproniscidce Ostracoda. 



3. DajidcB Schizopoda. 



4. Cabiropsidce Isopoda and Amphipoda, 



