122 CRUSTACEA PERACARIDA chap. 



characters, they are plainly allied to the other Peracarida, and 

 an easy transition is effected from the Mysidacea to the Isopoda 

 through the Chelifera or Anisopoda. Only one thoracic segment 

 is usually fused with the head, the appendage of this segment 

 being the maxilhpede ; in the Chelifera among Isopoda, and the 

 Caprellidae among Amphipoda, two thoracic segments are fused 

 with the head. 



The Isopoda are distinguished from the Amphipoda by the 

 dorso-ventral flattening of the body, as opposed to the lateral 

 flattening in the Amphipoda, by the posterior position of the 

 hearc, and by the In'anchial organs being situated on the 

 abdominal instead of on the thoracic limbs. 



The Isopoda, following Sars' ^ classification, fall into six sub- 

 orders — 'the Chelifera, Flabellifera, Valvifera, Asellota, Oniscoida, 

 and Epicarida, — to which must be added the Phreatoicidea. 



Sub-Order 1. Chelifera. 



The Chelifera, including the families (1) Apseudidae and (2) 

 Tanaidae, are interesting in that they afford a transition between 

 the ordinary Isopods and the Mysidacea. The important features 

 in which they resemble the Mysidacea are, first, tlie fusion of the 

 first two thoracic segments with the head, with the coincident 

 formation of a kind of carapace in which the respiratory functions 

 are discharged by a pair of branchial lamellae attached to the 

 maxillipedes ; and, second, the presence of very small exopodites 

 on the first two thoracic appendages of the Apseudidae. 



The second pair of thoracic limbs, i.e. the pair behind the 

 maxillipedes, are developed both in the Apseudidae and Tanaidae 

 into a pair of powerful chelae, and these frequently show marked 

 sexual differences, being much more highly developed in the 

 males than in the females. The biramous and flattened pleopods 

 are purely natatory in function, and the uropods or pleopods of 

 the sixth pair are terminal in position and slender. 



Both families, of which the Apseudidae contain the larger 

 forms, sometimes attaining to an inch in length, are littoral in 

 habit, or occur in sand and ooze at considerable depths, many of 

 the genera being Ijlind. .Many Tanaids (e.g. Leptorhelia, Tanais, 



' "Crustacea of Norway," vol. ii., Isopoda, 1899, in which iiiany references to 

 literature will be found. 



