214 



Arthropoda. Class 1. Crustacea. 



Order 4. Isopoda. 



The body is dorso-ventrally compressed, enclosed in a hard, 

 often brittle, dermal skeleton ; the abdomen is short, at most with six 

 segments, for the last (seventh) is absent. Of the remaining segments 

 the terminal one is usually large : owing to fusion there often appear 

 to be fewer than six. The carapace is absent; the eyes 

 (lateral) are sessile, the exopod of the second antenna is 

 usually wanting. The first thoracic segment is fused with the head, 

 but the remaining seven are free, movable, and well 

 developed. The first thoracic appendage is modified as a maxilli- 

 ped; its inner edge is usually provided with hooks, by means of 

 which it may be coupled with its fellow. The other seven pairs of 



Vig. 176. 1 Aega; 1—3 Cymothoa, dorsal and ventral. /land VII second and eighth 

 thoracic segments ; 1,2,6 first, second, and sixth abdominal segments; iTj sixth abdominal 

 leg; E^, K4 etc. second, fourth, etc. thoracic limbs ; R plate of brood-ponoh.— After 

 H. Milne Edwards. 



thoracic feet are powerful ambulatory appendages, without 

 exopod and without epipod. The abdominal appendages are pecu- 

 liar in having the inner ramus of some of their number modified as 

 a gill; this ramus is membranous, and provided with a delicate, close 

 capillary net- work ; as a rule, there are no other respiratory organs. 

 The Isopoda possess a marsupium under the thorax, formed of 

 the lamellate appendages of the basal joints of the thoracic limbs, 

 as in the Mysidse ; the young ones leave the egg as non-motile 

 nauplii, with three pairs of stumpy appendages; or they may be 



