PHYLUM ARTHROPODA 



The Cumacea are also a very small group : Diastylis (Fig. 430) is a good 

 example. They are little shrimp-like animals, differing from all the Malacostraca 

 previously 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 

 (th IVVIII) uncovered. The first two 

 pairs of thoracic limbs are maxillipedes, 

 the last six legs : of these two or three 

 pairs have exopodites (ex). 



In the Arthrostraca we come 

 once more to a very large and 

 important order, containing 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 

 (Grammarus, Fig. 431) and the 

 Sandhoppers (Talitrus, Orchcstia) 

 so common on the sea-shore. Of 

 the Isopoda very convenient ex- 

 amples are Asellus (Fig. 432), 

 common in fresh-water, and the 

 well-known Wood-lice or Slaters 

 (Oniscus, Fig. 434, 1). found under 

 any piece of wood, stone, &c., 

 which has lain undisturbed on the 

 ground for a few weeks. 



The body is usually compressed 

 or flattened from side to side in 

 Amphipods (Fig. 431), depressed 

 or flattened from above down- 

 wards in Isopods (Fig. 432). The 

 normal malacostracan number of 

 segments is present, but the first 

 thoracic segment is always united 

 with the head, so that the ap- 

 parent head is really an incom- 

 plete or partial cephalothorax 

 (c.th). In some genera (Tanais, 

 &c.) the second segment of the 

 thorax also unites with the head, 

 and such forms sometimes in- 

 cluded under a distinct sub-order, 

 Anisopoda form a transition to 



FIG. 430. Diastylis stygia. ai, an- 



tennule ; a;?, antenna; ab.l ub.7, ab- 

 dominal segments ; cth. cephalothorax ; 

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

 pleopods. (From Lang's Coiiipa,-nti.i:>: 

 Anatomy, after Sars.) 



