8 AMPHIPODA. 



of flexible appendages (1st, 2nd, and 3rd pleopoda, p, 

 q, r), consisting of a base and two multiarticulate fila- 

 ments or rami fringed witli plumose cilia. These are used 

 in swimming, and, powerful for such a purpose, they 

 propel the animal rapidly through the water. Succeed- 

 ing to these are three pairs of appendages (4th, 5th, and 

 6th pleopoda, 5, t, v; uropoda, I. O. W.), situated upon 

 the inferior lateral margins of their respective segments. 

 They consist of a single-jointed base, supporting two 

 uniarticulate branches, inflexible and styliform, fringed 

 with spines or hairs. The posterior pair vary consider- 

 ably in form ; iy some genera they are furnished with 

 hooks, whilst in others they assume a foliaceous character, 

 circumstances which render them valuable in the recog- 

 nition of species. These appendages are also powerful 

 organs of propulsion. By folding the tail beneath the 

 body, and suddenly striking it out again, those animals 

 which exist in the water, as well as those which live on 

 the shore, are enabled to dart or leap to a considerable 

 distance. In Caprella and its near allies, the whole of 

 the appendages of the tail are absent, or present only 

 in a rudimentary and altered condition. 



The terminal segment of the animal (telson, z) is 

 represented by an imperfect or rudimentary appendage 

 or scale. From the great variety of shapes which this 

 appendage assumes, it becomes a valuable aid in the 

 determination of genera. The typical form may be con- 

 sidered to be that of an acute-angled triangular scale, 

 the apex being rounded off". Sometimes it is divided 

 into two, as in Gammarus ; again, it is deeply cleft; in 

 one genus it is represented by a hook only ; in some, it 

 is broad, flat, and foliaceous, in others it is cylindrical, 

 the intestinal canal terminating at its extremity. 



Compared with the podophthalmatous Crustaceans, the 

 animals forming the present order are of small size, the 



