xi PHYLUM ARTHROPODA 56, i 



The Tanaidacea, the Isopoda, and the Amphipoda are often 

 grouped together under the heading of Arthrostraca. These orders, 

 particularly 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. 468) and the 

 Sandhoppers (Talitrus, Orchestia) so common on the sea-shore. Of 

 the Isopoda very convenient examples are Asellm (Fig. 469), 

 common in fresh- water, and the well-known Wood-lice or Slaters 

 (Oniscus, Fig. 471, 1), found under almost 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 



FIG. 468. Gammarus neglectus. abd.l abd.6, abdominal segments ; ant.l, antennule ; 

 ant. 2, antenna ; cth. cephalothorax ; E. eye ; j. f. 1, first jumping-foot ; 1. 1 I. 7, legs ; 

 mxp. maxillipede ; os. oostegite ; ov. ova ; s.f.l, first swimming-foot ; th.2 th.8, free thoracic 

 segments. (After Gerstaecker.) 



Amphipods (Fig. 468), depressed or flattened from above down- 

 wards in Isopods (Fig. 469). The normal malacostracan number 

 of segments is present, but the first thoracic segment is always 

 united with the head, so that the apparent head is really an incom- 

 plete or partial cephalothorax (c.th). In the Tanaidacea (Tanais, 

 &c.) the second segment of the thorax also unites with the head, 

 and such forms sometimes included under a distinct sub-order, 

 Anisopoda form a transition to the other Malacostraca, and 

 especially the Cumacea. In the Amphipoda and Isopoda the pos- 

 terior seven thoracic segments (th.2 th.8) are free, and those of the 

 short abdomen are usually free in Amphipods (Fig. 468, abd. 1-6), 

 often more or less fused in Isopods (Fig. 469, abd). In some 



