66 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
INTRODUCTION. 
The Isopod Liagia oceanica (Linn.) has been selected as 
the type for this Memoir on account of its comparatively 
large size, being the largest British Isopod, and also 
because it is one of the most interesting of the group, 
being mid-way between the aquatic and terrestrial forms. 
The Isopoda, with the Amphipoda, form the sub-order 
Arthrostraca, and are characterised by being Malacostraca 
with seven distinct thoracic segments, each bearing a 
pair of limbs (except the Gnathiidae) and _ possessing 
sessile eyes; on account of the last character, they are 
usually classed together in the sub-order Edriophthalmia. 
The Isopoda possess a dorsoventrally depressed body. 
The thoracic hmbs do not bear branchial appendages, as 
in the Amphipoda, but respiration is carried on by means 
of the abdominal appendages, which are modified for that 
purpose, the modification varying in the different tribes. 
The terrestrial Isopods, the Oniscoidea, are the only 
members of the group which exhibit such a uniformity 
in the character of the thoracic appendages as to justify 
the name. 
The following classification of the Isopoda is that 
given by Sars (1896), each tribe being defined by three 
characters—those of the first pair of legs, the uropoda, 
and the pleopoda or abdominal appendages : — 
I. First pair of legs cheliform; Uropoda terminal ; 
Pleopoda, when distinctly developed, exelu- 
sively natatory - - - 1. Cheliferae. 
Il. First pair of legs not cheliform. (1) Uropoda 
lateral. (i) Pleopoda for the most part nata- 
tory, forming a caudal fan with the terminal 
segment of the metasome - 2. Flabelliferae. 
