i6o THE LIFE OF CRUSTACEA 



and journey for short distances overland. On the 

 other hand, since free-swimming larvae would be 

 liable to be swept out to sea, most of them have 

 a direct development, the young only leaving the 

 protection of the mother when they have attained 

 the form and habits of the adult. When all these 

 factors have been taken into account, however, there 

 still remain many cases where the distribution of 

 individual species or of groups is hard to explain, 

 and shows indications of dating from a time when 

 the outlines of continents and the connections of 

 river systems were different from what they are now. 



Before proceeding to mention some of the more 

 characteristic forms of fresh-water Crustacea, it 

 should be mentioned that in large lakes, as in the 

 sea, we can distinguish a littoral fauna in the shallow 

 waters close to the shore, a plankton fauna of the 

 surface waters, and a deep-water fauna. The littoral 

 fauna does not differ in general characters from that 

 found in smaller ponds and gently-flowing rivers; 

 the plankton comprises many peculiar species show- 

 ing adaptations for flotation, as in the case of the 

 marine plankton ; and the deep-water fauna is very 

 poor in species and in individuals, and shows some 

 relations with the subterranean fauna to be men- 

 tioned later. 



Of all the subclasses of Crustacea, the Branchio- 

 poda are the most characteristically fresh-water 

 animals, only a few Cladocera being found in the 



