2 Stewart, Branchial Lamellae of Ligia oceanica. 



Bate and Westwood (i) quote it as occurring in the 

 crevices of rocks, just above high-water mark, and observe 

 that " it is seldom found under water." 



Delage (2) is of the opinion that "although they Hve 

 an exclusively terrestial life, and cannot stay long in 

 water without being asphyxiated, Ligias are marine 

 animals, in the sense that the immediate neighbourhood 

 of sea-water is as indispensable to them as the direct 

 contact of it is harmful." 



Webb and Sillum (5) remark that Ligia occurs on 

 the sea-coast " at low tide, beneath stones." It would 

 appear from this that the usual habitat of Ligia is the 

 littoral zone of the shore, somewhere in the region of 

 high-water mark, but it is worthy of note that the Isopod 

 is occasionally found at a considerable distance from the 

 sea-shore. (Hewitt (3) in St. Kilda found a number of 

 specimens at a considerable height above the sea-level 

 and quite out of reach of the spray.) 



The fact that one often finds Ligia in the rock-pools, 

 or crawling amongst the Ficcus which covers the shore 

 between tide-marks, leads one to speculate whether, if 

 ■overtaken by the tide, the sea-louse could withstand 

 several hours of total immersion in water. 



The gills of all Isopods have approximately the same 

 structure ; it follows, therefore, that adaptation to a dif- 

 ferent habit is essentially an adaptation of the gills to 

 different osmotic pressures, and it was with a view to 

 investigating the effects of altered surroundings upon the 

 gill structure that the following experiments were under- 

 taken. 



It may be remarked here that the results obtained 

 are inadequate as yet to form a basis for any definite 

 statement ; numerous points remain to be cleared up, 

 and a further study of the structure of the gills in relation 



