NATURE AND OKIGIN OF FRESH-WATER ORGANISMS 355 
marine. On the other hand, the small but interesting group of lung- 
fishes (Dipnoi) is wholly fluviatile, while many Teleostean fishes are 
common inhabitants of our rivers and lakes. 
Descending in the scale, we find both the Tunicates and AmpMoxus 
unknown save in the sea. 
Among the Arthropoda we find examples of both typical fresh- 
water and typical marine forms. The insects, myriapods and 
arachnids, are mainly terrestrial animals, but nevertheless a number 
of adult insects, and a still larger number of insect larvae, are 
inhabitants of ponds and streams, while the family of the Hydrach- 
nidas is almost entirely confined to fresh water. The Crustacea, 
being principally aquatic, aftbrd examples of both groups. Of the 
lower forms, the Cirripedia are entirely marine ; the Copepoda and 
Ostracoda are abundant both in the sea and in fresh water, though 
present in greater variety in the sea ; and the Branchiopoda are most 
common in fresh water. The great majority of the higher Crustacea 
are marine, the Cumacea and Stomatopoda exclusively so, and the 
other groups to a very large extent. The Isopoda, however, together 
with a number of terrestrial forms, includes the characteristic fresh- 
water genus Asellus ; in like manner the genus Gaimuarus^ species of 
which are common in fresh w^ater, occurs amongst the Amphipoda. 
The Decapoda too, in addition to a great many marine types, contains 
the crayfishes, Astaciis and its allies, certain prawns {Palcemon^ 
Caridina, etc.) and crabs (principally Potamonidae), which are 
characteristic of fresh water. 
All the Brachiopoda are marine, and so are most of the Polyzoa, 
although the sub-group of the Phylactolaemata is confined to fresh 
water. Turning to the Mollusca, \ve find a number of types belong- 
ing both to the Gasteropoda and to the Lamellibranchiata which are 
well known in, and characteristic of, various fresh waters, though these 
divisions have a much larger number of species in the sea. Amongst 
others, we may indicate Planorbis, Limncjea^ Paludina^ and Unio from 
fresh water, and Buccmiim^ Trochus^ Patella^ and Cardiinn from the 
ocean, as being typical genera belonging to the two groups. The 
Cephalopoda are found only in the ocean. 
A considerable number of what we may popularly call " worms 
are internal parasites, and so fall outside the scope of our inquiry. 
Of more highly organised forms, the Polychasta are all but entirely 
marine, while the Oligochaeta, with a few marine and many terrestrial 
types, yet includes a number (such as Nais and Tuhifex) which are 
characteristic of fresh water. The leeches are for the most part 
terrestrial and fresh- water, though some forms inhabit the sea; and 
the Nemertinea, on the other hand, are principally marine forms, 
although a few are known from fresh water. Amongst the flat-worms 
