from their size and powers of swimming or creeping, can make their way upstream and spread throughout a river system, and in some cases they can leave the water and journey for short distances overland. On the other hand, since free-swimming larvæ 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 showing 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 mentioned later. Of all the subclasses of Crustacea, the Branchiopoda are the most characteristically fresh-water animals, only a few Cladocera being found in the sea, and some Anostraca in salt lakes and brine pools.