Tribe 5. 



ONISCOIDA. 



Remarks. This tribe comprises the air-breathing or terrestrial Isopocla, 

 to which the name of the order may be found to be more properly applicable. 

 Indeed, in all the known forms of this tribe the legs exhibit a very uniform ap- 

 pearance, being all ambulatory in character; and in the general appearance a 

 much greater uniformity also prevails in this tribe than in other tribes of the Iso- 

 poda. The body is more or less depressed, oval or oblong in form, and may 

 in some cases admit of being rolled into a ball. The cephalon is generally small, 

 and more or less sunk into the 1st segment of the mesosome, exhibiting no true 

 rostral projection, whereas the lateral parts may be more or loss expanded. The 

 mesosome is composed of 7 well-defined and rather uniform segments, the lateral 

 parts of which are generally expanded to thin fornicate plates. The metasome, 

 in by far the greater number of the forms, is divided into 6 well-defined segments, the 

 lateral plates (epimerse) of which may also be expanded in a similar manner to 

 those of the mesosome. More generally, however, this is not the case with the 

 last, and the 2 anterior segments, which, as a rule, are smaller than the 3 middle 

 ones. The 1st pair of antenna? are always very small, and are placed inside the 

 2nd pair, for which reason they cannot properly be termed "superior antenna?", 

 but may more conveniently be named "antennula?", on account of their small 

 size. They are never composed of more than 3 joints, the last of these being often 

 rudimentary. The 2nd pair of antennae, or true antenna?, are of normal struc- 

 ture, being composed of a 5-articulated peduncle and a flagellum generally di- 

 vided into a restricted number of articulations. They are of moderate length, 

 seldom exceeding half the length of the body. The buccal mass is more or less 

 prominent, and the oral parts are adapted to biting and triturating the food. 



20 Crustacea. 



