66 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Introduction. 



The Isopod Ligia 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 Grnathiidae) 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 limbs 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, exclu- 

 sively natatory - - - 1. Cheliferae. 

 II. 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. 



