It is intended that this review shall give an account of what is known of the physiology of the parasitic stages of nematodes parasitic in animals, with particular reference to those species which are found in the alimentary tract of their hosts. An account will also be given of attempts which have been made to keep the adult and larval phases of parasitic nematodes alive in vitro and of the bearing of the available physiological data upon this problem.

Up to the present very few authors have directed their attention to factors influencing the emergence of larval trematodes from their snail hosts. Cort (1922) has made a study of the escape of cercariae from their hosts and the writer (1931) has examined the effect of light and temperature on the emergence of three species of larval trematodes from three species of fresh-water molluscs. More recently, Giovannola (1936) has investigated the effect of a reversal of light and darkness on the periodicity of emission of cercariae from three species of host snails. All these studies have been made on freshwater molluscs. In the case of molluscs living between tide-marks on the shore, in addition to variation in temperature there exists another variable factor, namely, the salinity of the water. Many rock-pools situated in the intertidal zone are small and will therefore be subject to considerable variations in temperature and salinity at different times of the year and even for short periods of up to 12 hr. during individual days. In the summer months the temperature of the water in these pools may rise to a high degree and the high temperature would be maintained for a longer period near high-water mark than near low-water mark, where the period of exposure is less. Similarly, the decrease in temperature would be very marked during the cold winter months. As the water in the exposed pools evaporates the salinity increases and high up on the shore, between successive tides on hot summer days, it may become twice that of normal sea water. The pools are also exposed to rain and in the same period the salinity of the water may decrease considerably if the precipitation is heavy. The light factor would be the same for molluscs living in fresh and in salt water, but the reaction of various species of larval trematodes to the presence or absence of light varies. Some emerge from their hosts in larger numbers during the daytime, others during the night and some indiscriminately during periods of light and darkness.

Both species of tick which are described in the present paper are members of insular faunas and demonstrate remarkably well the tendency, shown by these parasites in geographical isolation, to species formation. In this connexion it is perhaps worth quoting what Cooley & Kohls (1945) say concerning this subject in their recent work on the genus Ixodes: ‘These studies’, they write on p. 1 of their introduction, ‘have revealed in considerable measure the more useful specific characters and the importance of a real appreciation of the significance of geographical range and variation in the differentiation of species.… In determination of species limits we have considered not only morphologic but also biologic factors such as host relationships and geographic distribution. No subspecies are recognized.’ There is little doubt but that geographical distribution will be found to play an almost equally important role in the genus Amblyomma.

1. Cyclical development of Spirochaeta duttoni in lice was observed in seven experiments. In the last four, the same strain was transmitted through four successive batches.

2. The resultant infections in animals behaved exactly like S. duttoni even after the fourth transmission through the abnormal host.

The comparative nutritive value of dried horse blood, highly milled wheat flour devoid of bran, a mixed diet of blood and wheat flour and that of blood and yeast for the larvae of Xenopsylla cheopis, X. brasiliensis and X. astia was ascertained experimentally. Pure blood proved inadequate, probably because it is deficient in accessory food factors. The growth of their larvae on wheat flour alone was erratic; only partial success was obtained, and the adults emerged after long and irregular intervals. This I attribute to the association of micro-organisms, possibly fungi, with this food. A mixture of blood and wheat flour quickened their larval development, but it was not a satisfactory larval diet. Blood and yeast form an ideal food for all flea larvae.

It is concluded that larval diets, containing blood or wheat proteins and vitamins of the B group, are essential for the successful rearing of these rat-fleas, and that the proper sclerotization of the adult is due to the presence of haemoglobin in the larval food. The available data on the effects of diverse diets on the growth of flea larvae lend strong support to the conclusion that successful development depends on the presence of these vitamins in the food; it also leads me to think that their source in nature may be the association of micro-organisms with the food.

The larval food appears to be an important factor that governs the distribution and host preferences of different species of flea. The larvae of X. astia require the most nutritive diet. If a rich larval food is present in a rodent burrow, X. astia flourishes, as in the burrows of Tatera indica and Bandicota malabarica, and even in those of the domestic rats in certain regions. In contrast, the nutritional requirements of the larvae of Xenopsylla cheopis and X. brasiliensis are simple; thus they prosper readily in a burrow of the domestic rat, even where the nutritive value of the larval food is very low. As the temperature tolerance of X. brasiliensis is the lowest, this species is confined to some of the cooler regions.

The irregular distribution of the three species of rat-fleas inside India may be related to differences in the nutritional value of the varied organic sub stances found in rat burrows in different plaoes. The fact that the distribution of X. cheopis and X. brasiliensis is wider than that of X. astia is attributed to the ability of the larvae of the first two species to grow better on flour alone; this possibly enables them to survive transport in grain, even without rats, to places far from their original home.

That quinine has little or no destructive action against gametocytes of Plasmodium Falciparum (crescents) has been known for a long time. Marchiafava & Bignami (1900) pointed out that quinine (grains xxx daily) over a period of time did not perceptibly affect the sexual parasites of P. falciparum. Darling (1914), working in the Panama Zone, showed that Anopheles albimanus readily became infected when fed upon carriers of M.T. crescents who were taking xxx grains of quinine daily. These and other experiments showed that quinine, even in large doses, has no direct destructive action on crescents, and does not prevent mosquitoes from becoming infected once the.crescents are formed and are present in the peripheral circulation. However, quinine given in heavy doses during the first day or two of a primary attack, restricts and reduces the asexual source of supply.