WORMS 199 



also very complicated, consisting of ramifying tubules with cilia 

 arranged at the terminal branches, which keep up a ceaseless flickering, 

 thus lashing the excretory products towards the bladder. These flame 

 cells are objects of great fascination — like candles twinkling on a 

 Christmas tree. On the other hand, the trematode's nervous system 

 is extremely simple and it has no blood circulatory system at all. Nor 

 does it possess a body cavity — so that the various organs lie embedded 

 in the fluke like currants in a cake. 



Excellent habitats for the study of bird flukes are mud-flats and 

 saltings. These generally consist of flat stretches of mud and water, the 

 no-man's land of the countryside, treeless, colourless and desolate 

 expanses, which belong neither to sea nor earth. To those curious people 

 who are attracted by lonely communion with nature, the bitter-sweet 

 quality of these tidal deserts affords the acme of pleasure. There they 

 can really get their fill of pale sunshine mixed with the nostalgic cry of 

 curlews, and oyster-catchers twinkling against a skyline where sky and 

 sea merge into a melancholy glassy waste. 



Saltings are a paradise for wild birds such as waders, ducks, geese 

 and gulls and for this reason they are also a paradise for flukes. The 

 conditions found in the pools on saltings are about as favourable as they 

 can be for these particular parasites; but the hazards of the trematode 

 life-cycle are so great that survival must always be problematical. The 

 future life for a larval flatworm is only the reward of one in a million. 



A familiar bird in these surroundings is the redshank; and this is the 

 host of Cryptocotyle jejuna which, in the adult stage, is located in the 

 bird's intestine. The flukes' eggs pass out with the droppings and in 

 this way become scattered over the mud and in the brackish water 

 pools. Another animal which abounds in this habitat is a small mollusc, 

 one of the spire shells [Hydrobia ulvae). It is sometimes found in concentra- 

 tions of 32,000 to the square yard. The eggs of this particular redshank 

 fluke hatch if they are eaten by the snail in question. Whether they are 

 accidentally ingested with other food or whether Hydrobia has a fatal 

 weakness for trematode eggs is not known. Once inside the mollusc's 

 alimentary canal the Q^g liberates the ciliated larva known as a mira- 

 cidium. This quickly bores its way into the tissues of the snail. In 

 many species of trematode the miracidium hatches in the water and 

 actively seeks the snail host. These microscopical larvae are pro- 

 vided with eye-spots as well as a boring spine and swim vigorously by 

 means of their covering of cilia. They are generally strongly host-specific 



