Seasonal variations in the incidence and development of the cestodeProteocephalus filicollis(rud. 1810) inGasterosteus aculeatus(L. 1766)

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Abstract
1. The incidence, intensity of infection, rate of growth and genital development inProteocephalus filicollishas been studied by the monthly examination of approximately 60Gasterosteus aculeatusover a period of 2 years.2. The existence of an annual cycle, commonly reported for cestode parasites of fish in a temperate climate, is clearly established. The most advanced stages commonly occurring are: plerocercoids (0·25–5mm.) July-November; strobilate worms with genital primordia (58mm.) December-April; mature worms (1 cm.) April-May; gravid (2 cm.) June-July.3. No tissue phase occurs between the time the procercoid is swallowed and the mature worm develops, but there is a migration from the rectum to the anterior part of the intestine.4. From a consideration of the seasonal variation in incidence it is deduced that the parasitic population is in dynamic balance and that approximately 1 % of worms present are lost daily.5. No evidence to support the existence of immunity was found. The lower incidence in 1+ than in 0+ fish was attributed to feeding habits.6. Other aspects discussed are the interrelationship of proglottid formation and appearance of genital rudiments; the structure of the egg-shell; and the intensity of infection ofCyclopswith procercoids as calculated from the numerical distribution of worms in the fish host.7. Reference is made to intercurrent helminth and protozoan infections and the effect of an epidemic ofIchthyophthiriussp. on the fish population.I wish to thank Prof. C. M. Yonge for granting every facility for this work. My thanks are also due to Mr Archer, Miss Bell, Mr Berrie, Miss Spence and Miss Wyllie for help in collecting fish, and to Mr Gray who showed me the lochan.
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