A quantitative assessment of the demand by the parasite Schistocephalus solidus (Miiller, 1776) upon its secondary host, GasteroJfeus aculentus L., was obtained by comparing the energy budgets for infected and non-infected fish. Observations on the energy transformations during feeding, assimilation, growth and respiration of fishes indicated some overall effects of the parasite upon host metabolism.Infection resulted in a greater depletion of host food reserws, shown by a marked increase in mortality of parasitized fish during starvation. When fed ad libitum upon Tubifex, differences were recorded in the feeding and assimilation rates of fish. By comparison, no significant differences were detected in the respiratory expenditure of infected and noninfected hosts. The computation of energy budgets indicated that fish bearing parasites characteristically exhibited a higher gross efficiency than did fish without parasites. However, subtraction of the calculated effect due to the presence of worms, suggested that the efficiency of infected fish alone, that is, without their parasites, was actually lower than the efficiency of uninfected fish. It is considered therefore that the apparent greater energy turnover in a parasitized fish is due to the parasite being more efficient in its energy transformations than is its host.

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Rates of oxygen consumption by parasitized and unparasitized sticklebacks were recorded at three levels of activity in February and August. Negative correlations were demonstrated between specific respiratory rates and fish body weights. At routine and maximum activity, infected fish consumed more oxygen than uninfected fish. At miniinum activity levels no significant differences in respiration rates were detected. Seasonal variation in respiration rates was attributed to acclimation. Difficulties of determining specific respiration rates for parasitized organisms are discussed and attempts are made to assess the physiological basis of the respiratory and behavioural characteristics of infected fish.

During extensive sampling of populations of sticklebacks for parasites, it became apparent that marked differences existed in the distribution of the 2 British freshwater species. Observation of the survival, respiratory rates and distribution of three-and nine-spined sticklebacks from a variety of habitats supports the suggestion that physiologically, the nine-spined sticklcback is much better adapted to life in closed, shallow, weedy, eutrophic waters, depleted in oxygen. By comparison, Custerosfeus ucuknfus favours more open well oxygenated water.

The experimental infection of rodents as potential, definitive hosts with Hymenolepis straminea indicated that hamsters and field mice were susceptible to infection. By comparison, laboratory mice exhibited a significantly lowered susceptibility. Artificially induced depression of the immune response enabled the parasite to be cultured in several strains of laboratory mice, and suggested that the thymus played a major role in rejection of the parasite.