General Remarks, 



The present order being rather nearly allied to that of the Amphipoda, 

 a similar terminology may on the whole be applied to both. The body, which in 

 most cases exhibits a more or less depressed, not as in the Amphipoda com- 

 pressed, form, admits of being divided into 3 chief sections, viz., cephalon, meso- 

 some and metasome, the urosonie not being, as in the Amphipoda, defined as a 

 particular division. In the group CheUfem, as also in the family Gnathudas among 

 the FlabeUifera, the cephalon is coalesced with the 1st segment of the mesosome, for 

 which reason, in the said forms, this section may more properly be termed cephalosome. 

 As to the several appendages, those of the cephalon are the same as in the 

 Amphipoda, and are denominated in a similar manner. The 2 pairs of antennae, 

 it is true, are generally described as inner and outer, not as superior and inferior; 

 but on a closer examination it may be easily proved, that in all forms the outer 

 antennas in reality issue beneath the inner. The 2 pairs of maxilla?, in the 

 typical Isopoda, differ somewhat from those in the Amphipoda, the anterior ones 

 being generally devoid of palp, whereas the posterior ones carry outside the outer 

 lobe a lamellar appendage which ought to be regarded as a palp. The maxillipeds 

 only exhibit a single pair of masticatory lobes, answering to the basal lobes in the 

 Amphipoda. On the other hand they are provided outside with a more or less 

 distinctly developed epignath, wholly wanting in the Amphipoda. In parasitic 

 forms, as usual, the oral parts become more or less modified in their struc- 

 ture. The appendages of the mesosome, the legs, exhibit only in the terres- 

 trial Isopoda (Oniscoidea) such a uniform appearance as to justify the name 

 given to the order; but in by far the greater part of the Isopoda, the structure 

 of the legs is rather diversified, in some cases (for instance in the Munnopsidee) 

 even more so than in the Amphipoda. The 1st pair generally differ conspicuously 

 from the next succeeding pnes, being prehensile and applied to the oral region, 



1 Crustacea. 



