Thin sections of Trypanosoma cruzi in tissue cultures and from blood agar medium have been examined with the electron microscope.

In the leishmania and crithidia forms of the parasite a neat separation of kinetonucleus and blepharoplast has been obtained and these structures are described. The blepharoplast is the row of basal corpuscles which give origin to the axial fibres of the flagellum.

Division of the parasite seems to begin in the basal corpuscles; it is followed by binary division of the kinetonucleus and only later by fission of nucleus and cytoplasm.

During the development of the leishmania into the crithidia form, the kinetonucleus and basal bodies are dislocated towards the posterior half of the trypanosome. The migration of these two structures and the consequent lengthening of the flagellum causes the surface membrane of the flagellum to be pushed inwards to form a deep invagination, so that the flagellum comes to the surface of the body and is separated from it along its whole length by its own sheath.

In the adult form of the parasite a great vacuole is formed near the posterior tip of the trypanosome. It has no definite structure in the electron microscope, is transparent and seems to be filled with a liquid content. The basal corpuscles are situated in this region near the wall of the vacuole. The typical structure of the kinetonucleus could not be identified with certainty in these forms; it seems to be altered by the formation of the vacuole.

The vacuole is crossed by a fibre system which comes from the body of the trypanosome and is often dislodged by the size of the vacuole. This fibre system ends in a sharply pointed process of varying length.

The cost of reproduction of figures was defrayed by the Instituto de Biofísica da Universidade do Brasil.

In the last twenty-five years, since it was realized that the control of flock infections was more important than the treatment of individuals with patent infections, there has been an increasing interest in the use of anthelmintic drugs. As might be expected there has been a wide search for the most effective chemicals and considerable research into the best methods of administration, and as a consequence of this many workers have been engaged in devising methods of testing the efficiency of various compounds.

1. The genus Meteterakis is reconstituted with eight species: M. govindi, M. louisi, M. baylisi, M. longispiculata, M. cophotis, M. japonica, M. mabuyi and M. triaculeata.

2. M. cophotis, M. japonica and M. longispiculata were previously placed in the genus Spinicauda.

3. M. mabuyi was previously in the genus Africana.

4. M. triaculeata was previously in the genus Ganguleterakis.

5. M. baylisi and M. louisi are new species.

6. Spinicauda bufonis and, probably, Africana varani are considered synonyms of M. govindi.

7. Africana howardi is considered a synonym of M. japonica.

8. A list of the remaining species in the genera Africana and Spinicauda is given.

In a previous paper (Crofton, 1954) it was shown that the rise in egg counts occurring in ewe flocks in spring was related to the time of lambing. It was further shown that the rise in individuals occurred about 6 weeks after parturition and was normally of short duration. While the mean time of increased egg count for each individual was about 2 weeks, the increased egg count in the flock was spread over a period equivalent to the length of the lambing period. It is clear that the apparent rise in the flock as a whole will be greatest when all the flock lambs within a short period. With greater scatter of times of parturition the flock rise will be smaller but will extend over a greater period. This association of rise in egg count with time of parturition is obviously one of considerable selective advantage, in so far as an increase in infective stages will occur at a time when the susceptible lambs have a reduced milk intake and an increased grass intake. The synchronization of availability of infective stages and the occurrence of susceptible individuals is further emphasized by the relation of the period of increase to the time of birth of the lambs, the duration of the period of increase in the flock being directly proportional to the duration of the lambing period. This type of ‘escapement’ control is seen in so many other host-parasite relationships that it makes considerable appeal on general grounds. Other workers, Morgan, Parnell & Rayski (1951), have, however, related the rise in egg counts to climatological factors, but climatic control would not give the close synchronization described above.

1. An examination of the cuticle of Oxyuris equi has revealed the presence of previously unidentified structures. The layers of the cuticle of this nematode have been renamed.

2. The cuticle of Strongylus equinus has been examined in a similar fashion and, whilst the basic nomenclature of the various layers has been retained, previous descriptions of the individual layers have been modified and extended.

3. Further evidence is put forward in favour of the presence of pore canals in the cuticle of Ascaris lumbricoides.

It is usual, when considering the trichostrongyle infections of sheep, to refer to parasitic stages in the host and free-living stages on pasture as two separate entities with ecological, structural and physiological differences emphasized by their temporal relationship. They are, however, interlinked phases in a continuous exchange process in which the host acts as a vehicle. The role of the host as a mediator in this exchange has received little attention until recently and has been mainly restricted to a more dynamic approach to problems of immunity. The behaviour of the host, as well as that of the parasite, is obviously of great importance, but there has been little work on the behaviour of sheep in relation to patterns of helminth infections, although Taylor (1954) discussed the problem in general terms.

1. Infestation of the kidney of M. sowerbii and A. reticulatus by unencysted brachylaemid metacercariae led to a widespread necrosis of the organ. The necrotic debris was ingested into the gut crura of the parasites.

2. M. sowerbii is considered to be a natural intermediate host of the parasite and A. reticulatus to be an unnatural host, because in A. reticulatus parasitization led to a host mortality closely related to the degree of infestation, whereas M. sowerbii withstood the damaging effects of parasitization until the parasites reached maturity.

3. In A. reticulatus the rate of ingestion of food and the rate of egg production were reduced during parasitization.

4. No further effects were observed in either host species.

5. The possibilities and difficulties of using unencysted metacercariae to control pests are discussed.

The following new species of Ixodes from eastern Africa are described and figured: I. rotundatus, I. auriculaelongae, I. nchisiensis, I. latus, I. loveridgei, I. spinae, and I. euplecti.

The female of I. oldi is redescribed and the hitherto unknown male and nymph are described and figured. Some consideration is given to two specimens from Uganda which differ in certain particulars from typical specimens from East and West Africa.

The life cycle and pathogenicity of a strain of Eimeria isolated in Great Britain from turkey poults by single cell inoculation are described and, using the criteria laid down by Tyzzer, the species is identified as E. adenoeides.

The life cycle is of the same general pattern as in other Eimeria species, consisting of two asexual and one sexual generations.

The organism is highly pathogenic for young poults, a dose of 200,000 oocysts producing 100 % mortality in 3-week-old birds and smaller doses causing reduced weight gain. Birds 11 weeks old resisted a dose of 3 million oocysts.

The gross and microscopic pathology of the infection is described. No changes were found in the blood picture.

1. Cucumaricola notabilis, a new genus and species of Copepoda, occurs in cysts in the coelom of Cucumaria frauenfeldi Ludwig collected at Sea Point, Blaauwberg Strand and Dalebrook in the Cape Peninsula.

2. An account is given of the features of the sexually dimorphic males and females.

3. Fertilized eggs and nauplius larvae are also contained in the cysts and, from observations on living material, it has been established that, after escaping from the cysts, the larvae metamorphose within a short period into the copepodid stage. There are two copepodid stages, the first an active swimming form and the second a quiescent phase from which the juvenile parasite develops.

4. Although of uncertain systematic position, this parasite seems to be nearly related to the Chondracanthidae. This conclusion is based on the similarity of the trunk appendages of the adult female to those of Acanthochondria as described by Oakley (1930), and also on the general agreement of the ontogeny with that of Acanthochondria cornuta investigated by Heegaard in 1947. It is further suggested that it may be assigned to the key devised by Delamare Deboutteville & Nunes-Ruivo (1955) for certain copepod parasites having some affinity with the Chondracanthidae.

The occurrence of Globidium gilruthi in sheep and goats in Egypt is recorded. Preliminary investigation showed that 18 % of 250 sheep and 28 % of 150 goats were infected. The lesions caused by G. gilruthi are described.

1. A short note is given on Pterygosoma hystrix Lawrence, 1935, amplifying the original description and extending the known range of variation.

2. The hitherto undescribed male, larva, protonymph and deutonymph of Pterygosoma gerrhosauri Law, 1935, are described and figured.

3. The protonymph and deutonymph of Pterygosoma bicolor Law, 1935, are described and figured, together with a record of variation in the female.

This paper gives an account of the mature eggs and the miracidium of Paryphostomum mehrai obtained from experimentally infected white rats, which were fed with the cysts of Cercaria mehrai obtained from naturally infected as well as artificially infected snails of the species Indoplanorbis exustus. The metacercariae develop into the adults in the intestine of the host in a period of 20 days, after which the adults begin to lay their eggs, which are passed out with the faeces of the host. The molluscan phase of the life cycle involves the gradual development of the miracidium larva after its penetration into the molluscan host into a mother rediae. The mother rediae, not more than three to five in number, were observed in the preoesophageal region of the snail host. They produce large numbers of daughter rediae, which were found migrating towards the liver mass. The daughter rediae, after fixing themselves in the liver mass, undergo further development and produce large numbers of cercariae. The entire process, from the entry of the miracidium until the cercariae are matured takes about 8 weeks.

The stages of spermatogenesis and the structure of the primary oocyte of Sphaerostoma bramae were studied in material fixed in Bouin and Flemming's fluid and in preparations treated according to the Kolatchev and the Feulgen techniques.

Chromosome counts of primary and secondary spermatocytes indicate that the diploid number is twenty-four.

The stages of spermatogenesis, in general, conform to the usual pattern of this process in digenetic trematodes. The spermatogonia, spermatocytes and early spermatids are not connected together by central strands, nor by a central disk, as is claimed for some other species. A study of sections stained in haematoxylin and of Feulgen preparations showed that the spermatozoon is composed of an elongate nucleus and a tail. It was not possible, with the methods employed during the present investigation, to determine the fine structure of the tail.

The primary ovarian oocytes are in the interphase or in early prophase of the first maturation division. There is evidence that material is extruded from the nucleus to the cytoplasm.

The Golgi elements of the male germ-cells are revealed in Kolatchev preparations as short rods and filaments. The Golgi elements of the spermatid are eliminated in the residual cytoplasm. Mitochondria, in the form of granular bodies and short rods, were visible in spermatogonia, spermatocytes and spermatids. Those of the spermatid remain in the residual cytoplasm.

Short, rod-like Golgi bodies are present in the primary oocytes. In the young cells they form a compact mass situated close to or in contact with the nuclear membrane. Later, the elements spread out through the cell. Granular and rod-shaped mitochondria are concentrated in a single mass at one pole of the nucleus or in two masses at opposite sides of the nucleus. A few mitochondria are scattered through the cytoplasm.

1. The parasitization of the slugs Milax sowerbii and Agriolimax reticulatus by a brachylaemid trematode in an area in Durham, during the period 1949–53, is described. Evidence is given that M. sowerbii is a natural intermediate host of the parasite.

2. Seasonal fluctuations in the incidence of infestation are described, including the fluctuations in the incidence of the four developmental stages of the metacercariae. Primary (spring) and secondary (autumnal) infestations of the molluscs are described.

3. The implications of an unexplained decline in parasite density during 1953 are noted.

4. A life history for the parasite, involving a small mammal as the vertebrate host, is postulated.

1. Methods are described for the isolation and maintenance of a pure strain of Eimeria mitis.

2. The prepatent period of the infection in chicks was 101 hr. The mean dimensions of the oocysts were 15·38 × 13·83μ, and sporulation occurred after 48 hr. at 60° F. (15·5° C.). The production of oocysts following infection with known numbers of oocysts is recorded.

3. When 500,000 or more oocysts of E. mitis were given to chicks aged 6, 17 or 26 days the growth rate of the birds was markedly reduced. Young chicks were most susceptible and a few deaths occurred when 6-day-old chicks received 2·5 million oocysts. E. mitis is unlikely to be of pathological importance under normal conditions of good husbandry practice.

4. Pathological lesions due to this species of Eimeria were not found in sections of intestinal tissues.

5. Developmental stages of this parasite were found throughout the whole of the small intestine.

6. The growth-retarding effects and oocyst production by E. mitis were controlled when 0·2% sodium sulphadimidine was given in the drinking water.

1. Cercaria X (Baylis & Taylor, 1930) and its sporocyst have been redescribed with some additions to the original descriptions.

2. The details of the development of the cercaria into a metacercaria of the diplostomulum type have been obtained and the anatomy of the mature metacercaria is described in detail for the first time. It is proposed to name the metacercaria Diplostomulum X. Development of the cercaria into a metacercaria takes from 40 to 90 days and the metacercariae have been shown to survive for a period of 544 days after the time of infection. This development has been obtained in the lenses of rats, frogs and a wide variety of fish. Development is not completed in any other region of the host's body.

3.The effect of the sporocyst, migration of the cercariae, and the metacercaria on the appropriate host tissues has been considered. All these stages produce extensive changes in the host tissues.

4. Histochemical tests have shown the presence of alkaline phosphatase activity in the cuticle of the sporocyst. The glycogen storage regions of the cercaria are described and it has been demonstrated that the caudal bodies in the tail-stem contain large amounts of glycogen which are utilized during the free-swimming period.

5. Attempts at the completion of the life cycle were unsuccessful.

1. A brief outline of the life cycle of a species of Nosema parasitic in Saturniid moths is given. It is of the normal type.

2. The structure of the spore is described, and from electron microscopy evidence is presented which supports the theory that the sporoplasm is injected into the host's tissue.

3. The volume of the spore is affected by the ambient medium, by the pretreatments of fixation and of desiccation, by the site of development within the host and by the age of the host.

4. These results have been discussed with reference to the use of spore size as a diagnostic character in the classification of the Microsporidia. It is concluded that spore size is not a reliable character for taxonomic purposes.

A male and a female of a species of Syngamus were found in copula in the small intestine, about 1 in. from the gizzard, of a fieldfare (Turdus pilaris) which had been shot accidentally. The nematodes were well preserved and were undamaged. Eighty-four small tapeworms, of two different genera, were also present in the intestine. Neither the tapeworms nor the nematodes were attached to the intestinal wall.

1. Crepidostomum farionis (Müller, 1784) and C. metoecus (Braun, 1900) were found to occur together in the same host fish, Salmo trutta L., S. salar L. and Anguilla anguilla (L.) in mid-Wales. It is believed that the latter species constituted an accidental host.

2. C. metoecus is recorded in Britain for the first time.

3. There was some evidence of a habitat isolation in individual fish as C. metoecus occurred predominantly within the pyloric caeca, while C. farionis usually occupied a more posterior station in the intestine.

4. A detailed description is given of G. metoecus and the salient features of this species are compared with those of C. farionis.

5. The two species of Crepidostomum commonly occurred in large numbers in their fish hosts, individual fish harbouring up to 157 worms. No lesions attributable to Crepidostomum were, however, detected and there was no evidence of pathogenicity.

6. It would appear that the eel is physiologically immune to both species of Crepidostomum and that the salmon parr is partially resistant.

7. The degree of infestation of trout and parr with both the Crepidostomum species is at a maximum during the winter months and at a minimum in the summer months. This seasonal variation can be attributed to a periodicity in the swarming of the cercariae.

8. There is no evidence of age resistance or acquired immunity of the trout to infestation by Crepidostomum species.