Studies on the life cycle of the pseudophyllidean cestode Schistocephalus solidus
I am indebted to the Department of Scientific and Industrial Research and also to the University of Leeds for grants during these investigations. My thanks are due to Professor E. A. Spaul for encouragement and help, especially in the preparation of the manuscript, and to Dr. T. Kerr for constant advice and guidance.
- 1A study of host-parasite relationships has been made on the cestode Schistocephalus solidus Müller by examining conditions relating to the infection and growth of the plerocercoid in the stickleback, and- its transfer to other hosts for maturation; the infection of Cyclops by the coracidimn with its subsequent change to a procercoid; and the passage of the procercoid into the body cavity of the fish to become a plerocercoid. It has been possible, by establishing suitable techniques, to carry out the whole life cycle in the laboratory and so facilitate these studies.
- 2Eggs were obtained from plerocercoids and observations made on their development. The time of hatching of the coracidium varies considerably, both with temperature and with intrinsic factors. The average time is about three weeks but hatching may occur up to six months after the eggs have been shed.
- 3From such eggs, free coracidia and, subsequently, procercoids in Cyclops and nauplii, were secured and studied. The growth rate of procercoids was determined together with the effect of infection on the growth of nauplii.
- 4Infections in fish were secured from infected Cyclops but not in sufficient numbers to justify more than tentative conclusions about the growth rate of plerocercoids. The time required for penetration of the fish gut wall by the procercoid can be as short as two hours.
- 5Changes in morphology of the worm in its transformation from a coracidium to a plerocercoid were surveyed and it was shown how the formation of the musculature precedes the initiation of strobilation which occurs rapidly and completely, followed immediately by the formation of the genital rudiments.
- 6The excretory system of the plerocercoid could be displayed effectively by a method of embedding in transparent plastic.
- 7Infected fish from one source were collected throughout the year and the weights of the fish and of the worms within them were ascertained. It was found that in the autumn there was an increase in the number of large fish in the marginal zone–possibly related to an increase of plankton–but during the months from winter to summer the mean size of fish was less and remained relatively stable in this zone. The mean weights of plerocercoids however, increased during this period as did the temperature of the water. Some indications of the growth rate of the plerocercoid were observed from the collected data.
- 8An examination of the stomach contents of the fish showed a wide variety of diet with little or no change throughout the year and also indicated that Cyclops form an almost insignificant feature in this diet.