1. A new genus of trematode (family Allocre-adiidae) from the intestine of freshwater eels is described, and named Stegodexamene anguillae.

2. The genus is related to Aephnidiogenes, Lepidapedon and Neophasis, but differs significantly from each of them.

3. The life cycle followed is from the adult in Anguilla dieffenbachii to the redia in Potamopyrgus antipodum (fresh-water gastropod). The cercaria encysts in the eloetrid fish Gobiomorphus gobioides.

4. Progenetic production of eggs and spermatozoa occurs in the metacercariae.

5. Metacercarial excretory spherules are composed of calcium carbonate.

The material which forms the basis of this paper was obtained from the birds shot at Lashio, Upper Burma, in October 1941. I am greatly indebted to Prof. F. J. Meggitt for his help at Rangoon, in spite of his busy time owing to active field service at that period in Burma, and also for the use of his extensive private library before the general evacuation of 1942.

During investigations of the helminth parasites of the British limpets Patella vulgata, P. intermedia and P. depressa in relation to a possible bearing of parasitism on mortality and sex proportions, three larval trematodes, Cercaria Patellae Lebour, 1911; an unidentified Xiphidiocercaria, Cercaria B; an unidentified Cercaria C; and one larval cestode were found. Of these, C. patellae was the commonest.

Each of the three trematodes occurs independently in the digestive gland, the cestode being encysted on the surface of the visceral mass. The Cercaria C is visible only in smear preparations or sections, but infestations with the other three parasites can usually be seen on removing the limpet's shell. The sporocysts of the Xiphidiocercaria, Cercaria B, are restricted to a limited portion of the digestive gland or, very rarely, of the kidney; the rediae of C. patellae are loosely distributed throughout the visceral mass. No sporocysts of the Cercaria C have been observed.

C. patellae is commonest in Patella depressa, but since in all three species it is more common in populations inhabiting rock pools the apparent preference for P. depressa may be because this species occurs in pools or at low water marks, whereas the other species frequent ‘dry’ habitats.

Most limpets infested with Cercaria Patellae are in the larger size (age) groups. For example, the percentage infestation in Patella depressa at Port St Mary, Isle of Man, is very low in specimens less than 40 mm. in length, increasing to 100% in those of more than 65 mm.

Cercaria Patellae escapes from the limpet by way of the veinlets in the mantle skirt. It is suggested that migration of cercariae within the haemocoele, and subsequent escape from the mantle, may be a common occurrence in other molluscs infested with larval trematodes.

The number of cercariae given off by the limpet in vitro is greatest immediately after immersion in water, the rate of escape then steadily falls and finally ceases after about 30 hr. of immersion. Under natural conditions there is probably a rhythm imposed by tidal fluctuations, the cercariae escaping only when the host is covered by the tide.

In natural grazing grounds, bovines affected with East Coast fever remain ambulatory until a few days or even a few hours before death, and larval and nymphal Rhipicephalus appendwulatus and R. evertsi, which have fed and engorged, will infest the pasture over a wide area. Subsequently, when susceptible cattle graze over this area small numbers of infected instars may attach at intervals and begin to feed. In this experiment an attempt was made to simulate these conditions as closely as possible.

In group A wnere one infected tick was fed on the 1st day, two on the 3rd day, and three on the 21st day, two out of six calves developed an unmistakable East Coast fever infection, with Koch's bodies present in gland smears. In one calf (no. 5724) the reaction was most severe and was undoubtedly due to the three ticks which attached on the 1st and 3rd days, while in another calf (no. 115) the reaction was less severe and was most probably due to the three ticks which attached on the 21st day.

In group B where one infected tick was fed on the 1st day, two on the 5th day, and three on the 21st day; five out of six calves gave a definite unmistakable East Coast fever reaction with Koch's bodies present in gland smears, four of the reactions being undoubtedly due to the three ticks which attached on the 1st and 5th days. There is as yet no reasonable explanation as to why the number of definite reactors should differ so markedly in the two groups.

It is noteworthy that all these reactions were mild and that recovery in each case was uneventful. The only death to occur was control calf no 5729, on which ten infected ticks had fed at one time, but from the results of the other controls it may be assumed that the strain of Theileria parva had by some means become attenuated or that the calves had increased tolerance or resistance.

It is difficult to understand why calf no. 109 failed to react in the first experiment but showed some susceptibility when challenged 96 days later, and why calves nos. 5711, 116, 5642 and 39 gave doubtful reactions. No explanation can be given at this stage, but it raises two practical questions. First, it is difficult to assess the minimum intensity of a reaction which is sufficient to give a durable immunity. The fact that calf no. 114, after what appeared to be a definite T. parva reaction, reacted a second time on exposure to infection 110 days later, proved that the immunity was not prolonged. It is true that the first reaction was only mild, but practically the same remarks could be applied to calves nos. 117 and 5725, yet these calves had not lost their immunity on challenge. Secondly, it is difficult to ensure, by observation of daily temperatures alone, that all calves are really fully susceptible before being used on a critical East Coast fever experiment, for if only a few infected ticks are accidentally introduced into stables or grazing paddocks, it may be assumed that mild subclinical infections will result. These calves may then show varying degrees of resistance to subsequent infections, and results such as those experienced in these experiments would be a normal sequelae.

The intensity of the leucopaenia appeared to bear no relationship to the number of days Koch's bodies were present in gland smears. It was present before the first rise in temperature in all the calves except no. 114, the only calf to show a second reaction on challenge.

This experiment showed that infections of Theileria parva were still viable in nymphae of Rhipicephalus appendictdatus 171 days old, but when nine nymphae were fed on calves at intervals over 22 days the reactions produced were most irregular.

In group A reactions varied from acute fatal type to mild, doubtful or no obvious reaction. None of the four animals which survived was protected from subsequent infection when grazed in a tick-infested paddock (Table 1). The two fatal reactions which developed in this paddock varied in intensity; calf no. 5717 showing a typical acute East Coast fever reaction, but the reaction of calf no. 167 showed several atypical features especially in the sporadic appearance of Koch's bodies.

In group B the reactions were again in marked contrast to those seen in group A, one fatal reaction developed and only mild transient reactions without the production of Koch's bodies were seen in the remaining five calves. These mild reactions, however, appeared to be sufficient to protect against subsequent re-infection in the tick-paddock (Table 4).

It is doubtful if the transient reactions seen in calves nos. 5734 (on the 12th day), 172 and 168 could be diagnosed clinically under field conditions, and further investigations as to the infectivity of these types of reactions are required.

In calves nos. 165 and 167 Koch's bodies were absent from all tissue smears at time of death, and diagnosis of the true cause of death by smears alone would therefore be difficult. Reactions such as those given by calves 133 and 139 cast further doubt on the ability to diagnose East Coast fever by gland-smear technique.

In general, the results of this experiment appear to indicate that if the original reaction is mild a second reaction may develop within a few weeks with or without the production of Koch's bodies. The development of a durable resistance to further attacks may not, therefore, depend on the presence of Koch's bodies in the peripheral glandular tissues in detectable numbers.

My thanks are due to the Director of Veterinary Services, Uganda, for his permission to carry out this work and to publish this paper.

1. The musculature and excretory and nervous systems of Cysticercus taeniae-taeniaeformis are described.

2. The nervous system consists often longitudinal nerve cords in the strobila and of a brain and complicated system of nerves in the scolex. The nervous system of the scolex shows several variations on the earlier description by Bartels (1902).

3. Minor abnormalities have been found in the form of multiplication of the lateral margins and duplication of the ventral excretory vessels.

4. Three specimens showed spiral torsion of the internal organs involving the musculature and excretory and nervous systems of the strobila. The number of turns of the spiral and the positions of the chiasmata are variable. It is suggested that the spiral twisting occurs before growth and strobilation in the neck region has begun, the chiasmata being carried back as new segments are formed.

A linguatulid, very similar to the nymphal ‘Pentastoma gracile’ from crocodiles described by Sambon, was found in faecal material from a European resident in Khartoum. There is no evidence concerning the method of infection.

I am indebted to Dr E. S. Horgan, until recently Director of the Stack Medical Research Laboratories, for his reply to my inquiries concerning linguatulid infections in the Sudan; and to Prof. J. J. C. Butley, of the London School of Hygiene and Tropical Medicine, for his valuable assistance in obtaining relevant literature.

The survival of fasting imagines of Hyalomma savignyi under various conditions of temperature and relative humidity was studied.

Within the range of experimental conditions low temperature and high humidity are shown to be most favourable to the survival of ticks: at 17·5° C. and 80–95% R.H. mean longevity approaches 1 year, and a few individuals were observed to survive for more than 2 years. Longevity shortens as temperature rises and relative humidity decreases. At a temperature of 37° C. (and under various conditions of humidity) average longevity does not exceed 1·5–3·5 months, while under conditions of extreme dryness, viz. 10–20% b.h. (and various temperatures) average longevity ranges from 1·5 to 9 months.

The longevity of fasting imagines is not as much affected by dryness as that of preimaginal stages.

1. The external structure of the capitulum of Ixodes hexagonus is described in detail, and the differences between it and the closely allied I. cookei are noted.

2. The capitulum is divisible into two portions, an extra-basis and basis regions. The former is divided into the hypostomal groove, the labrohypostomal groove and the labro-hypostomal gutter. Internally the basis is divided into dorsal and ventral chambers by the subcheliceral plate. The dorsal chamber houses the chelicerae while the pharynx and the salivary ducts occupy the ventral portion.

3. The so-called ‘stylet-like process’ is the anterior prolongation from the basal lobe of the labrum, and forms the mid-rib of a lamella which divides the hypostome into a dorsal salivary channel and a ventral food conduit.

4. There are rows of teeth on the internal faces of the anterior fifth of the pharynx. Two series of pharyngeal muscles are present: (i) those having a dorsal attachment to the floor of the salivary chamber, and (ii) those having similar attachments to the subcheliceral plates.

5. During feeding the first incision is made by the denticulate apices of the digits and into it the hypostome is then forced. Further cutting by the digits enables the hypostome to penetrate to its base. At the same time the palps splay out. The blood is then drawn up the food conduit by the action of the sucking pharynx. The lower surface of the labrum is produced into a tooth-like projection that fits into a groove of the lower wall of the pharynx. Presumably it functions as a valve to prevent regurgitation of blood. The pharyngeal teeth may function as a supplementary valvular apparatus or else serve for the breakdown of erythrocytes.

6. The tissue reaction of a stoat's skin to penetration by the capitulum of I. hexagonus is described.

1. Ixodes hexagonus is a three-host tick.

2. The length of the pre-oviposition period is fairly constant at 19° C. in a saturated atmosphere, varying between 12 and 14 days.

3. Oviposition takes place as in I. ricinus and, at room temperatures at 18–20° C., R.H. 100%, the number of eggs laid varies from 880 to 1449. Experiments on the effects of different temperatures and humidities on egg-laying are described.

4. Eggs hatch in 52–59 days under the conditions stated above.

5. The larvae gorge for 3–6 days, the majority becoming replete on the third and fourth days after attachment.

6. The metamorphosis of the larva is prolonged by cold conditions and is hastened during warm weather. In a saturated atmosphere at 15, 20, 22 and 25° C., the mean times for moulting are respectively 60, 33, 27 and 23 days.

7. The nymphs gorge for 5–7 days, the greater number dropping off between the sixth and seventh days.

8. At 18° C. and 100% R. H. moulting of the nymph takes place about 32 days after repletion.

9. After metamorphosis the adults rest for 8–12 days, and feeding lasts 8–16 days.

1. Recent studies by Gerichter have demonstrated that I have erred in referring the skrjabingylin lungworms to the family Trichostrongylidae.

2. On the basis of his data and a re-evaluation of my theories of metastrongylid evolution it is suggested that the Skrjabingylinae (except Dictyocaulus) are close to the Filaroidinae and that quite possibly the position of the vulva in the former is the consequence of a secondary shift from opening just anterior to the anus to opening in the mid-region of the body; the ‘trichostrongylid’ configuration of the ovejectoral apparatus would thus be convergent.

3. The genus Dictyocaulus is removed from the Skrjabingylinae and left in a subfamily Dictyocaulinae in the family Metastrongylidae, although its trichostrongylid affinities are very suggestive. If trichostrongylid, this genus is considered to exhibit convergence with the metastrongylids and not to represent an evolutionary link between the two families.

4. The nature of metastrongylid life cycles is discussed, and the compatibility of known data with the evolutionary scheme proposed for the family is pointed out. It is evident that in metastrongylids symbiotizing marine littoral and pelagic hosts (Pinnipedia and Odontoceti) larval development must rely upon new intermediate host groups if the basic pattern has been retained from ancestors in terrestrial hosts.

5. The possible importance of histological studies and of investigations on chromosome number and structure for a further understanding of the evolution of the suborder Strongylina is pointed out.

6. A revised scheme for the evolution of the Metastrongylidae is presented as Fig. 1. The family Metastrongylidae now includes six subfamilies: Metastrongylinae, Filaroidinae, Skrjabingylinae, Pseudaliinae, Protostrongylinae, and Dictyocaulinae. The basic premises previously expressed (Dougherty, 1949b) on the evolution of the family are retained in the amended scheme.

From various parts of the world I have recently received a number of mites taken from bats, and the present paper gives descriptions and figures of five new species—one new species of the genus Ancystropus; two new species of the genus Spinturnix; one in the genus Hirstesia and the last in the genus Foliomyobia.

Under the research programme of the U.S. Public Health Service, Communicable Disease Centre Activities, the parasitic fauna of rodents and insectivores of the United States are coming into the laboratories in great numbers, and it has been my good fortune to have had the opportunity to examine a fair number of the parasitic mites collected from a diversity of hosts.

A strain of Plasmodium gallinaceum has been treated for 10 weeks with sulphadiazine (5 mg./20 g.) in the presence of p-aminobenzoic acid in doses (0·5 mg./ 20 g.) inhibiting the sulphadiazine. No resistance to paludrine or to sulphadiazine has developed.

1. No resistance to paludrine or to sulphadiazine was obtained after treating latent infections of Plasmodium gallinaceum in chickens with twice daily doses of 20 mg./20 g. of sulphadiazine over periods of 171, 178 and 190 days.

2. No resistance to paludrine was obtained after treating a latent infection of P. relictum in a canary over a period of 1 year with doses of paludrine increasing from 0·05 mg./20 g. once daily to 1·0 mg./ 20 g. twice daily.

3. It is considered that if drug-resistance arises by mutation and selection, then resistance should arise more readily when a large number of rapidly multiplying parasites is treated with a drug than when the population treated is small, with a low reproduction rate, i.e. the failure to obtain resistant strains of malaria by prolonged treatment of latent infections with large amounts of drug, lends support to the theory of the origin of resistant strains of malaria by the selection of resistant mutants.

1. Rats infected with Spirillum, minus are strongly resistant to infection with Trypanosoma lewisi and T. equinum if inoculated with these parasites shortly after inoculation with Spirillum minus. The resistance is most highly developed for a period of from 2 to 4 weeks after inoculation with S. minus.

2. Simultaneous inoculation of rats with S. minus and either of the trypanosomes does not influence the course of the resulting trypanosome infection.

3. Rats inoculated with Trypanosoma equinum during the period of acute infection with Spirillum minus develop only a very slight transient primary infection with the trypanosome and may survive for many weeks, up to 49, whereas control rats die in 5–17 days. Usually the resistant rats ultimately die from a relapse of the Trypanosoma equinum infection.

4. Similarly, in rats inoculated with T. lewisi during the resistant phase due to an infection with Spirillum minus, the primary acute infection with the trypanosome is suppressed and the rats develop a slight or subpatent infection.

5. The non-specific resistance to trypanosomes conferred by the acute S. minus infection soon disappears and is replaced by a specific trypanosome resistance which is active only against reinoculation with the same species of trypanosome. Thus, in this phase of resistance, a rat infected with Trypanosoma equinum is refractory to reinoculation with T. equinum but has no resistance to superinoculation with T. lewisi. Conversely, a rat infected with T. lewisi is refractory to reinoculation with that species but it is as susceptible to superinoculation with T. equinum as a normal rat.

It has recently been shown (Gibbs, 1950), in the case of Crithidia familiaris, parasitic in the digestive tract of Cenaeus carnifex (Hemiptera), that resistant, infective bodies are produced in the hind-gut by an unequal division leptomonads. One of the two daughter-cells fails to grow a flagellum but remains attachedm, by a thread, to the flagellum of the other. Laterm the non-flagellate body becomes more compact in form and finnaly becomes invested in a tough, protective periplast. It then becomes detached from the parent flagellate. Large numbers of these bodies are scattered in the faeces, and it has been shown that they are capable of infecting fresh insects when ingested. The purpose of the attachment of the non-motile, immature infective body to an active flagellate is apparently to prevent its being prematurely swept from the gut.