This deviation is the adoption of an aquatic mode of life by the European fresh-water spider (Argyroneta) and by the marine spider Desis, which is found on the shores of the Indian and Pacific Oceans from Cape Colony to eastern Australia.
The Hexapoda, being aerial, terrestrial and fresh-water animals, are but occasionally preserved in stratified rocks, and our knowledge of extinct members of the class is therefore fragmentary, while the description, as insects, of various obscure fossils, which are perhaps not even Arthropods, has not tended to the advancement of this branch of zoology.
They occur up mountain slopes as far as vegetation extends, in tropical valleys and forests, in open grassy plains, in sandy deserts, and even in fresh-water ponds and between tide-marks on the seashore.
In the later stages arms of the sea were cut off and were converted at first into lagoons and then into brackish or fresh-water lakes which continued to occupy much of S.
Of doubtful position, but commonly referred to the Trachylinae, are the two genera of fresh-water medusae, Limnocodium and Limnocnida.
The Yellow Sandstone contains Archanodon, the oldest known fresh-water mollusc, and plant-remains; the Coomhola Grits are marine, and are sometimes regarded as Carboniferous, sometimes as uppermost Devonian.
The genuine Isopoda are divided among the Flabellifera, in which the terminal segment and uropods form a flabellum or swimming fan; the Epicaridea, parasitic on Crustaceans; the Valvifera, in which the uropods fold valve-like over the branchial pleopods; the Asellota, in which the first pair of pleopods of the female are usually transformed into a single opercular plate; the Phreatoicidea, a fresh-water tribe, known as yet only from subterranean waters in New Zealand and an Australian swamp nearly 6000 ft.
In fresh-water Hydromedusae the life-cycle is usually secondarily simplified, but in marine forms the life-cycle may be extremely complicated, and a given species often passes in the course of its history through widely different forms adapted to different habitats and modes of life.
It is also the largest market for fresh-water fish in America, and handles large quantities of lumber and grain.
Salmon, lampreys and eels are caught in some of the larger rivers; trout abound in the streams of the northern provinces; but many fresh-water fish common elsewhere in Europe, including pike, perch, tench and chub, are not found.