202 FLEAS, FLUKES AND CUCKOOS 



of sporocysts give rise to other sporocysts and cercariae. Sometimes the 

 mother sporocyst is lacking and a mother redia develops directly within 

 the miracidium. At times the cercaria encysts in the first intermediate 

 host, in the open, on vegetation or on inanimate objects. In this way 

 only one intermediate host is involved, but in other cases an extra, third 

 intermediate host, may be added. Despite these variations the cycle is 

 fundamentally the same and one can trace the Ggg, miracidium, 

 redia/sporocyst, cercaria and metacercaria stages before the sexually 

 mature adult is developed. This is one of the most mysterious aspects 

 of the digenetic Trematoda. As a group they are highly host-specific 

 with regard to the first intermediate host, which is almost always a 

 snail, a fact which has led to the widespread belief that they were 

 originally parasites of molluscs before the evolution of vertebrates. 

 Why have all these flukes followed this same path, and why has none 

 remained parasitic on molluscs in the sexual phase ? This appears to 

 be one of the most puzzling phenomena in the whole field of helmin- 

 thology. 



Cryptocotyle jejuna belongs to a large group of trematodes (Opisthor- 

 chioidea) which are all characterised by the same type of cercaria. 

 With one or two important exceptions they use fish — fresh water, salt 

 water or brackish water species — as the second intermediate host, and 

 the final host is thus, of necessity, a fish-eating animal. A related 

 species (C. lingua) parasitises the herring gull in Britain and many fish- 

 eating sea birds (Plate XXVIIa). The periwinkle serves as the first 

 host (Plate XXVIII) and various inshore fish like gobies [Gobius], 

 wrasse (Z^^rw^spp.), rockling {Onos spp.), blennies {Blennius spp.), and 

 butterfish {Pholis) as the second intermediate host (Fig 5). 



On the saltings it is a familiar sight to see large flocks of geese feeding 

 in the distance. With field-glasses one can sometimes identify brent 

 geese pulling at the eel grass stranded in the shallows — a plant which 

 constitutes one of their staple items of diet. A careful examination of 

 the long ribbon-like leaves reveals that they are often beaded with 

 small, dark, hemispherical pearl-like cysts. This is the metacercarial 

 stage of the trematode Catatropis verrucosum which as an adult worm is 

 found in the caecum of various geese and ducks, such as the barnacle- 

 goose, pink-foot, sheld-duck, merganser and so forth. The first inter- 

 mediate host is Hydrobia ulvae. When the cercaria escapes from the 

 mollusc it immediately settles on the shell of the snail or some in- 

 animate object nearby and pours out a secretion from specialised 



