PHYLUM ARTHROPODA 



583 



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

 characteristic malacostracan 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 

 fused together, and in some genera 

 altogether absent. The carapace 

 (cth) is so small as to leave the 

 five posterior segments (iJilV 

 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 Isopoda and 

 the Amphipoda are often grouped 

 together under the heading of 

 Arthi\>4i'i.i<:".. 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 (Talitrus, 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, 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 above down- 



Fir.. 460. Diastylis stygia. nl, an- 

 tennule; of, antenna; ab.lab.7, ab- 

 dominal segments ; cth. cephalothorax ; 

 en, endopodite ; ex, exopodite ; p.l,p.6, 

 pleopods; IV-VII, th VIII, free thoracic 

 segments. (From Lang's Comparative 

 Anatomy, after Sars.) 



p p 2 



