38 ANIMAL LIFE 



well as enabling it to punt over the sea-bottom or to 

 climb up burrows of earth (fig. 35, p. 200). 



Among Crustacea we encounter precisely the same 

 arrangement. Paired appendages are found on each 

 segment, but now, being placed nearer together on 

 the under-side of the body, their action is stronger and 

 more precise. Moreover, the appendage is divided 

 into inner and outer divisions, the outer one bearing 

 the leaf-like structures and the inner branch becoming 

 jointed and lobecl. Such animals swim either on 

 their backs by strong sculling movements inwards and 

 backwards, or {Daphnia, fig. 24), standing vertically, 

 they tread the water, rising at each stroke and falling 

 slightly between the strokes. In higher Crustacea 

 the outer division disappears from the limbs of most 

 segments, and the inner one is converted, in the 

 middle of the body, into a substantial jointed leg, 

 capable both of clinging to holdfasts against currents 

 and of supporting the weight of the body in walking ; 

 but the original character of the limb is seen in the 

 swimmerets found under the tail of prawns and 

 lobsters. These still beat from before backwards 

 and ensure the forward swimming of the animal. 

 Thus the body becomes divided into three portions — 

 a head provided with jaws and antenna;, a firmer middle 

 part for walking, and a ilexible tail for swimming. 

 The highest Crustacea, crabs, depend altogether on 

 the mobility and strength of their legs for walking 

 or swimming. In these animals the swimmerets 

 are turned to other accounts, and are chiefly used for 



