XI 



PHYLUM ARTHROPODA 



583 



18 a uropod, i.e., assists the telson in the formation of the 

 characteristic malaeostracan tail- 

 fin : there is no trace of the 

 entomostracan caudal styles 



The Cumacea are also a very 

 small group : Diastylis (Fig. 460) 

 is a good example. They are 

 little shrimp-like animals, differ- 

 ing from all the Malacostraca pre- 

 viously considered in having poorly 

 developed sessile eyes, sometimes 

 {used together, and in some genera 

 altogether absent. The carapace 

 (cth) is so small as to leave the 

 five posterior segments (thlV — 

 VIII) uncovered. The first two 

 pairs of thoracic limbs are maxiili- 

 pedes, the last six, legs : of these 

 two or three pairs have exopo- 

 dites (ex). 



The Tanaidacea, the Isopoda and 

 the Amphipoda are often grouped 

 together under the heading of 

 Arthrostraca. These orders, par- 

 ticularly the two last, comprise a 

 great number of genera and 

 species, many of them strangely 

 modified in correspondence with 

 special habits of life. The best 

 known examples of the Amphipoda 

 are the little Fresh-water Shrimp 

 (Gammarus, Fig. 461) and the 

 Sandhoppers (Talilnis, Orchcstia) 

 so common on the sea-shore. Of 

 the Isopoda very convenient ex- 

 amples are Asellus (Fig. 462), 

 common in fresh-water, and the 

 well-known Wood-lice or Slaters 

 (Oniscus, Fig. 464, /), found 

 under almost any piece of wood, 

 stone, &c, which has lain undis- 

 turbed on the ground for a few 

 weeks. 



The body is usually compressed ^^T^SS^SSUeS X 



Or flattened from Side tO Side in dominal segments J cth. cephalothorax ; 



. , . , /T -,. tn-,\ i i «' l > cndopodito ; ex, cxopuditc ; /).l,p.O, 



AmphipodS (rig. 461), depressed pleopods; IV-VII, tii VUI, free thoracic 



or flattened from above down- 'fi^JStotf*** o-*~*» 



p p 2 



