206 CRUSTACEA CHAP. 
inhabit small pools; and also a great number of Cyclopidae. Of 
the other fresh-water families of Copepoda, viz. Centropagidae 
and Harpacticidae, inhabitants of small pieces of water are 
Diaptomus castor, as opposed to the other species of Diaptomus 
which are pelagic, and a number of Harpacticidae (Cantho- 
camptus), the members of this family living in the weed or mud 
of either small ponds or else on the shores of the larger lakes. 
The greater number of Ostracoda are found in similar situations. 
A district like the Broads of Norfolk, which consists partly 
of slowly-moving streams and partly of extensive stretches of 
shallow water, supports a Crustacean fauna intermediate in 
character between that found in small ponds and the truly ° 
pelagic fauna characteristic of deep lakes. A very complete list 
of the Crustacea of the Norfolk Broads, with an interesting 
commentary on their distribution, is given by Mr. Robert Gurney.’ 
We miss here the pelagic Cladocera, such as Leptodora, bytho- 
trephes, Holopedium, etc., which form so characteristic a feature 
of large lakes; at the same time, besides a rich development of 
the Cladocera, Cyclopidae, and Harpacticidae, which haunt the 
weeds and mud of shallow waters, we find such species as Poly- 
phemus pediculus and Bosmina longirostris among Cladocera, 
which are otherwise confined to large bodies of water, and a few 
pelagic Diaptomus, e.g. D. gracilis. The fauna is also complicated 
in this district by the proximity to the sea and the frequently 
high salinity of the water, which allows a number of typically 
marine Copepods to pass up the estuaries and intermingle with 
typically fresh-water species; such are Hurytemora affinis among 
the Centropagidae, and several species of Harpacticidae (see p. 62). 
The large lakes of the world, such as the continental lakes of 
Europe and America, or of our own Lake District, reproduce on a 
small scale the varied conditions which appertain to the ocean— 
as in the ocean, we can recognise in these lakes a littoral, a 
pelagic, and an abyssal region. Our knowledge of the physio- 
graphy of lakes is largely due to the classical work of Forel,’ and 
the following account of the physical conditions in the various 
regions is condensed from his book. 
The littoral region is sharply marked off from the others by 
the relative instability of its physical conditions, owing to the 
1 Trans. Norfolk and Norwich Nat. Soc. vii. 
2 Le Lac Leman, 3 vols., Lausanne, 1892. 
