WORMS 207 



have penetrated the skin they work their way through the flesh until 

 they reach the blood circulating system. Here they migrate along the 

 vessels until they reach the lens of the eye, where their peregrination 

 ends. They do not encyst but remain as so-called " Diplostomulum " 

 larvae which are free in the tissues of the host and there undergo further 

 development. The pressure of these larvae often causes blindness in 

 the infected fish and this probably furthers their chances of reaching 

 the final host. 



A number of small passerine birds such as wagtails and pipits are 

 regularly seen on the saltings and although their flukes are principally 

 found encysted in insects, some of their characteristic trematodes are 

 essentially part of the brackish water fauna. 



These few examples of the flukes found in a small number of repre- 

 sentative birds in a restricted habitat scarcely touch the fringe of the 

 subject, for trematodes are found in almost every species of birds and 

 in almost every organ of the bird's body — ranging from the eyeball and 

 frontal sinus to the air sacs, the kidneys, the stomach and the skin. In 

 their life-cycle they utilise a vast network of animals — molluscs, leeches, 

 worms, Crustacea, insects, amphibians, fish and even small mammals. 



At first sight it appears that adult trematodes are not a suitable 

 group for studying the parallel evolution of host and parasite. It is, 

 of course, obvious that the links between certain hosts which harbour 

 similar worms, are their similar feeding habits, not hidden relationships. 

 The cat, bass, osprey and man are all parasitised by the superfamily 

 Opisthorchioidea — because they all eat fish. Frogs, bats and swallows, 

 because of their predilection for insects, are the victims of the Plagior- 

 chioidea. Nevertheless, once flukes become established within a certain 

 group of animals they begin to evolve parallel with their hosts and in 

 many cases it has been found that particular families of birds are 

 parasitised by certain sub-families and genera of worms. It is certain 

 that this particular line of research among trematodes will prove most 

 fruitful and that more profound studies will reveal far greater host 

 specificity, segregation and parallel evolution with the host, than is 

 suspected at present. 



The solving of life-cycles, however, is probably the most interesting 

 and rewarding research in Helminthology to-day. The fluke living 

 under the eyelids of carrion-crows is a peculiarly interesting species — 

 but how does it get there ? That is considerably more interesting. In 

 order to solve this question one has to inquire into the habits of the 



