2o6 CRUSTACEA 



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 

 Diaptomios castor, as opposed to the other species of Diaptomus 

 which are pelagic, and a number of Harpactieidae (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 lilce 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. Eobert Gurney.'^ 

 We miss here the pelagic Cladocera, such as Leptodora, Bytho- 

 trephes, Holopcdmm, etc., which form so characteristic a feature 

 of large lakes ; at the same time, besides a rich development of 

 the Cladocera, Cyclopidae, and Harpactieidae, which haunt the 

 weeds and mud of shallow waters, we find such species as Poly- 

 pihermis 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 Eurytemora affinis among 

 the Centropagidae, and several species of Harpactieidae (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 



^ Trans. Norfolk and Norwich Nat. Sac. vii. 

 ^ Le Lac Leman, 3 vols., Lausanne, 1892. 



