P22 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 maxillipede; 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 
heart, and by the branchial 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, the 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 thoracie limbs, é.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 blind. Many Tanaids (eg. Leptochelia, Tanais, 
1 «Crustacea of Norway,” vol. ii., Isopoda, 1899, in which many references to 
literature will be found. 
