WORMS 201 



resemble minims on a line of music. The body is a semi-transparent 

 colourless oval and the tail long and thin, and held aloft. The micro- 

 scope reveals that this tail is provided with undulating frilly fin-folds. 

 The cercariae of the redshank fluke can live in the water about eight 

 hours. They swim strongly for a few seconds and then stop suddenly 

 and sink slowly downwards in the characteristic "minim" attitude. 

 Then, with equal suddenness, they begin to swim again. These cer- 

 cariae, which have eye-spots, are extremely sensitive to change in light 

 and shadow — a quality which no doubt assists them in reacting to the 

 presence of second intermediate hosts, which in this case are fish — the 

 gobies. If a cercaria accidentally comes into close contact with one of 

 these fish it immediately attaches itself by means of the anterior spines, 

 casts off its tail, pours out the contents of the penetration glands which 

 soften the skin of the fish and quickly bores its way inside. It soon 

 comes to rest a Httle way beneath the scales and there forms a trans- 

 parent cyst. Within the cyst it undergoes further development in the 

 direction of the adult fluke, and larval specialisations, such as the eye- 

 spots and boring spine, are lost. In some species of goby the presence of 

 the metacercaria stimulates the host to produce pigment granules in 

 the skin. The hideous black spots covering the specimen on Plate VI lb 

 each mark the site of one cyst of Cryptocotyle. 



As we have seen the numbers of these larvae emerging from a 

 Hydrobia which has eaten one trematode ^gg may run into several 

 thousands. Occasionally a fish swims into a large swarm and the 

 simultaneous penetration of great numbers of cercariae kills it. But 

 even in small pools which favour high infection rates — for then eggs, 

 larvae and hosts are all present in a small area and are more hkely to 

 make contact with one another — it is usual to find that individual fish 

 harbour only a few cysts. When one of these fish is eaten by a redshank 

 — and contrary to general behef redshank are very fond of small fish — 

 the digestive juices dissolve away the cyst wall and the young fluke is 

 liberated and continues its development in the intestine of the bird. 

 Thus the life-cycle is completed. 



In this hfe-cycle there are at least seven distinct phases : o^gg, mira- 

 cidium, mother sporocyst, redia, free-swimming cercaria, encapsu- 

 lated metacercaria and sexually mature adult. This is characteristic of 

 most bird flukes although in some species variations occur. The 

 miracidium can have a free-swimming stage while in others it has none. 

 Again the redial generations may be missing and instead a succession 



