46 THE LIFE OF CRUSTACEA 



thoracic somites, or all but the first, are distinct. 

 The antennules are two-branched, the antennae may 

 have a scale-like exopodite, and the last pair of ab- 

 dominal appendages form, with the telson, a tail-fan. 

 The eyes are sometimes stalked, but in one species 

 they are sessile. The thoracic limbs, which are not 

 clearly divided into maxillipeds and legs, carry a 

 double series of plate-like gills or epipodites. As 

 will be shown later, the living Syncarida are espe- 

 cially interesting on account of their resemblance to 

 certain very ancient fossil Crustacea. 



The second division of the Eumalacostraca, the 

 PERACARIDA, includes five orders, the members of 

 which differ very greatly in appearance. They all 

 agree, however, in certain important points of struc- 

 ture, of which the most conspicuous is the posses- 

 sion, in the female sex, of a brood-pouch for carrying 

 the eggs and young. This brood-pouch is formed 

 by a series of overlapping plates attached to the 

 bases of the thoracic limbs. 



The first order of the Peracarida, the MYSIDACEA, 

 consists of small, free-swimming, shrimp-like animals 

 (Fig. 16). Many species are common in the sea 

 round the British coasts, and from their possession 

 of a brood-pouch, in which the young are carried, 

 they are sometimes known as " Opossum Shrimps." 

 The eyes are stalked, and the carapace is well 

 developed, although it does not unite with all the 

 thoracic somites. The antennae have a flattened, 



