PHYLITM ARTHROPODA 



583 



is a uropod, i.e., assists the telson 

 characteristic mahicostracan tail- 

 fin : there is no tiace of the 

 entomostracan caudal styles 



The Cumacea are also a very 

 small group : Biastylis (Fig. 460) 

 is a good example. They are 

 little shrimp-like animals, differ- 

 ing from all the Malacostraca pre- 

 viously considered in having poorly 

 de\'eloped sessile eyes, sometimes 

 fused together, and in some genera 

 altogether absent. The carapace 

 [cth) is so small as to leave the 

 five posterior segments {IhlV — 

 VIII) uncovered. The first two 

 pairs of thoracic limbs are maxilli- 

 pedes, the last six, legs : of these 

 two or three pairs have exopo- 

 dites {ex). 



The Tanaidacea, the Isojpoda and 

 the Amjjhipoda are often grouped 

 together imder the heading of 

 Arthrostraru. 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 AmpUiiwda 

 are the little Fresh-water Shrimp 

 {Gammarus, Fig. 461) and the 

 Sandhoppers {Talitms, Orchcsiia) 

 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 

 (Oniscics, Fig. 464, 1), 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 

 or flattened from side to side in 

 Amphipods (Fig. 461), depressed 

 or flattened from ab<jve down- 



in the formation of the 



Fir;. 460.— Diastylis stygla. nl, au- 



tennule ; a5, jintenna ; ab.l—ab.7, ab- 

 dominal segments ; cth. cephalothorax ; 

 en, endopodite ; e.f, exopodite ; p.ljp.C, 

 pleopods; IV-VII, th VIII, free thoracic 

 scfjmeiits. (Fi'om Lang's Cohiparative 

 Anatomy, after Sars.) 



p p 2 



