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 with 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, s } 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 ; in 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 
