XI 



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 (fhlV 

 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 



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 



(Gfammarus, Fig. 461) and the 



Sandhoppers (Talitrus, Orchestics) 



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 ; a2, antenna; ab.lab.7, ab- 

 dominal segments ; ct/t. cephalothorax ; 

 en, endopodite ; c.c, exopodite ; /i.l,p.fj, 

 pleopods ; IV-VII, tit VIII, free thoracie 

 segments. (From Lang's Comparative 

 Anatomy, after Sars.) 



p p 2 



