THE CLASSIFICATION OF CRUSTACEA 55 



appendages are divided into two sets : the first three 

 pairs have each two slender, many-jointed branches, 

 and are used in swimming; the last three pairs 

 are short, stiff, and 

 directed backwards, 

 and are used in 

 pushing the animal 

 through mud or 

 among water-weeds. 

 In many Amphipods, 

 such as the Sand- 

 hoppers, the last 

 three pairs of abdo- 

 minal limbs are used 

 in jumping by sudden 

 backward strokes of 

 the abdomen. 



Two families of the 

 Amphipoda differ so 

 much in general ap- 

 pearance from the 

 others that they de- 

 serve mention. The 

 Caprellidae (Fig. 22) 

 have the body drawn 

 out to a thread-like 



slenderness, and the abdomen reduced to a mere 

 vestige. The fourth and fifth pairs of thoracic limbs 

 are generally absent, though the corresponding gills 



FIG. 23 Paracyamus boopis, THE 

 WHALE-LOUSE OF THE H0MP- 

 BACK WHALE. (From Lankester's 

 "Treatise on Zoology," after 

 Sars.) 



A, Male, dorsal view, enlarged ; B, 

 the maxillipeds detached and fur- 

 ther enlarged, a', Antennule ; a", 

 antenna; abd, vestigial abdomen; 

 br, gills ; gn, gnathopods ; IV, V, 

 fourth and fifth thoracic somites 



