420 A HISTORY OF RECENT CRUSTACEA 



CHAPTER XXVII 



TRIBE VII. ONISCOIDEA 



THIS tribe consists of the land Isopocls, adapted for breath- 

 ing air, but probably in all cases requiring that the air 

 should be humid. In general uniformity and regularity 

 of shape they are strikingly contrasted with the preceding 

 parasitic tribe. The head is small, the seven segments of 

 the perason are distinct, the pleon is narrower, and usually 

 has six distinct segments, of which the sixth is the smallest. 

 The first antennae never have more than three joints, and 

 may have fewer, or may be wholly wanting. The man- 

 dibles are without ' palp.' The inner plate of the first 

 maxillae bears either two or three plumose setae. The 

 second maxillae have two plates or only one. The maxilli- 

 peds are occasionally seven-jointed, but seldom more than 

 four-jointed. The seven pairs of trunk-feet are similar to 

 one another ; the hinder more or less exceeding in length 

 those that precede ; all are gressorial. The pleopods are 

 branchial. Of the five pairs rarely the first is obsolete ; 

 the second and sometimes also the first have sexual organs 

 in the male. The uropods are of variable character, never 

 powerful organs ; their branches are single-jointed. Five 

 families are included in the tribe, the Ligiidae, Tylidae, 

 Helleriidae, Oniscidae, and Armadillididae. 



Family 1. Ligiidce. 



The pleon consists of six segments, of which the first 

 two are narrower than the third. The first antennas are 

 small, three-jointed. The first maxillae have three seta3 

 on the inner plate. The second maxillae have two plates. 



