194 FLEAS, FLUKES AND CUCKOOS 



found in Britain in the carrion-crow and the rook, and in the U.S.A., in 

 the eastern crow and the fish-crow {Corvus brachyrhynchos and C. ossifragus) . 

 From the same genus A. nymphaea occurs in the common curlew in 

 Britain and in the Esquimo curlew [Numenius borealis) in the U.S.A., and 

 A. arionis in sandpipers ( Tringa ochropus, T. stagnatilis and Actitis hypoleucos) 

 in Britain and in the yellowshanks ( T, jiavipes and T. melanoleuca) in 

 North America. 



In another large genus Hymenolepis, which occurs in mammals as 

 well as birds, the same phenomenon can be observed, for species like 

 H. himantopodis occurs in Britain in the black-winged stilt {Himantopus 

 himantopus) and in the U.S.A. in the black-necked stilt (//. mexicanus). 

 Two other species of cestodes, Acoleus vaginatus and Diplophallus 

 polyniorphus are also shared by these two waders on both side of the 

 Atlantic. 



The most impressive examples of this type are not, however, found 

 among the British fauna. A separate order has been erected for the 

 monstrously aberrant genus, Nematoparataema, which contains only two 

 species. One is found in the Australian black swan {Chenopis atrata) and 

 the other in the mute swan {Cygnus olor) in Sweden. The highly special- 

 ised genus Amabilia contains one species which is found in both African 

 and South American flamingoes. We have already noted on p. 145 

 that the African ostriches and South American rheas share the same 

 species of tapeworm, Houttuynia struthiocameli. It does not, in these 

 cases, seem unreasonable to suppose that when these hosts diverged 

 from a single stock they already harboured the tapeworms which are 

 still common to them both to-day. In the interval they themselves 

 have developed or evolved along different lines. 



The Class is generally divided into two sub-classes, one of which is 

 reserved for the primitive Cestodaria from fish. The other, the Cestoda, 

 contains three orders, of which two infest birds. The first, the Pseudo- 

 phyllidea, develop as a so-called procercoid larva in the body cavity of 

 Crustacea, such as copepods, and in the plerocercoid stage in the 

 muscles and coelom offish. The sexual stage is found in aquatic and 

 fish-eating mammals, birds and reptiles. In the genus Diphyllobothrium 

 the final hosts include man, the cat, arctic fox, various seals, gulls, and 

 terns. 



The great mass of bird cestodes are found in the second order, the 

 CyclophylHdea (Plate XXVIIc). These tapeworms are characterised 

 by the possession of four cup-shaped suckers on the scolex (see 



