Rational approaches to anthelmintic discovery include the design of screens for compounds directed at specific proteins in helminths that are pharmacologically distinguishable from their vertebrate homologues. The existence of several anthelmintics that selectively target the neuromusculature of helminths (e.g. levamisole, ivermectin, praziquantel, metrifonate), together with recent basic research in helminth physiology, have contributed to the recognition that neurobiology distinguishes these organisms from their vertebrate hosts. In this survey, we focus on mechanism-based screening and its application to anthelmintic discovery, with particular emphasis on targets in the neuromusculature of helminths. Few of these proteins have been exploited in chemotherapy. However, recent studies in comparative pharmacology and molecular biology, including the C. elegans genome project, have provided insights on potential new targets and, in some cases, molecular probes useful for their incorporation in mechanism-based screens.

Trypanosoma brucei is the causative agent of African sleeping sickness in humans and one of several pathogens that cause the related veterinary disease Nagana. A complex co-evolution has occurred between these parasites and primates that led to the emergence of trypanosome-specific defences and counter-measures. The first line of defence in humans and several other catarrhine primates is the trypanolytic protein apolipoprotein-L1 (APOL1) found within two serum protein complexes, trypanosome lytic factor 1 and 2 (TLF-1 and TLF-2). Two sub-species of T. brucei have evolved specific mechanisms to overcome this innate resistance, Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense. In T. b. rhodesiense, the presence of the serum resistance associated (SRA) gene, a truncated variable surface glycoprotein (VSG), is sufficient to confer resistance to lysis. The resistance mechanism of T. b. gambiense is more complex, involving multiple components: reduction in binding affinity of a receptor for TLF, increased cysteine protease activity and the presence of the truncated VSG, T. b. gambiense-specific glycoprotein (TgsGP). In a striking example of co-evolution, evidence is emerging that primates are responding to challenge by T. b. gambiense and T. b. rhodesiense, with several populations of humans and primates displaying resistance to infection by these two sub-species.

Metacyclic forms of Leishmania major and putative metacyclics of L. mexicana mexicana were found to occur in abundance in stationary phase cultures. These forms have been compared in several ways with promastigotes from mid-log phase cultures and, in the case of L. m. mexicana, amastigotes. Metacyclics are smaller, contain less protein and appear more active than other promastigotes. Both forms of promastigote respire at a high rate in the absence of exogenous substrate. The free amino-acid contents of the various forms of the two species have been analysed. They differ in detail but alanine was the major amino acid in all cases. The isoenzyme content of the different forms differed significantly. That of the putative metacyclics of L. m. mexicana was in several respects more similar to amastigotes than promastigotes, suggesting that the form is pre-adapted for life in a mammal. Metacyclics of L. major apparently did not divide in culture but transformed back over a period of 48 h to mid-log phase cells. The results provide further detail of the molecular differences between mid-log phase and metacyclic promastigotes and confirm that metacyclics are a distinct form in the life-cycle.

The Dactylogyrus fauna was studied from the gills of 293 roach between February 1988 and April 1989. Roach were caught from three interconnected lakes in Central Finland. Nine Dactylogyrus species were found. Of these the seven most abundant were used for studying the abundance of species on different gill-arches, niche breadth and niche overlap between the species. It was found that all species had a species-specific preference for certain gill-arches. The total abundances of all the species went through distinct seasonal changes, peaking during the spring and early summer. Some seasonal variation was also exhibited in gill-arch preferences. Niche breadth and species overlap were mostly higher during the period of peak occurrence of the various species, but in the case of some species they appeared to be unrelated to abundance, indicating possible niche restriction by other species. Most significant was the restriction of D. micracanthus to the fourth gill-arch during the period of peak occurrence of D. similis. Experimental work is needed to verify the reasons for variations in the microhabitat distribution of certain Dactylogyrus species.

Parasitic helminths belong to 3 separate phyla and there is always the danger of over-generalization. The various routes of anaerobic carbohydrate breakdown in parasitic helminth differ in their efficiencies and in their power output. The choice of end-product represents a compromise between these two conflicting forces. In addition, anaerobic pathways must satisfy the redox requirements of the tissues and provide a source of intermediates for synthetic reactions. Other considerations include the metabolic cost of excretion and the effect of end-products on protein structure and function. The different end-products may fulfil additional functions such as pH control, nitrogenous excretion, osmotic regulation, intracellular signalling and the suppression of host responses.

A complicating factor in parasitic helminths is the existence of strains with different biochemical characteristics, including marked variation in end-product formation. The various tissues of the same parasite can also produce different end-products and the pattern of end-product formation is influenced by a variety of extrinsic and intrinsic factors such as age, sex, length of incubation, pO2 and availability of substrates. The catabolic pathways of helminths thus show considerable functional adaptation.

There is, as yet, no satisfactory explanation as to why helminths do not make the maximum use of any oxygen available to them; and the contribution of oxidative processes to the overall energy balance of parasites probably varies from species to species.

The catabolic pathways of adult helminths are derived from the anaerobic pathways present in their free-living relatives. Two main trends are evident, homolactic fermentation and carbon dioxide fixation, the latter involving a partial reverse tricarboxylic acid cycle. In general, homolactic fermentation is found in blood and tissue parasites, carbon dioxide fixation in gut parasites. These two types of metabolism are, of course, in no way absolute, most homolactic fermentors fix carbon dioxide to a certain extent and many parasites which fix carbon dioxide also produce lactate. Parasitic helminths possess a wide range of different catabolic pathways, superimposed upon which is a high degree of functional plasticity.

Studies incorporating the ecology of clinical and sub-clinical disease in wild populations of conservation concern are rare. Here we examine sub-clinical infection by Trichomonas gallinae in a declining population of free-living European Turtle Doves and suggest caseous lesions cause mortality in adults and nestlings through subsequent starvation and/or suffocation. We found a 100% infection rate by T. gallinae in adult and nestling Turtle Doves (n = 25) and observed clinical signs in three adults and four nestlings (28%). Adults with clinical signs displayed no differences in any skeletal measures of size but had a mean 3·7% reduction in wing length, with no overlap compared to those without clinical signs. We also identified T. gallinae as the suggested cause of mortality in one Red-legged Partridge although disease presentation was different. A minimum of four strains of T. gallinae, characterized at the ITS/5·8S/ITS2 ribosomal region, were isolated from Turtle Doves. However, all birds with clinical signs (Turtle Doves and the Red-legged Partridge) carried a single strain of T. gallinae, suggesting that parasite spill over between Columbidae and Galliformes is a possibility that should be further investigated. Overall, we highlight the importance of monitoring populations for sub-clinical infection rather than just clinical disease.

This paper summarises the progress towards vaccine development against the major blood-feeding nematodes of man and livestock, the hookworms and Haemonchus contortus, respectively. The impact of the diseases and the drivers for vaccine development are summarized as well as the anticipated impact of the host immune response on vaccine design. The performance requirements are discussed and progress towards these objectives using defined larval and adult antigens, many of these being shared between species. Specific examples include the Ancylostoma secreted proteins and homologues in Haemonchus as well as proteases used for digestion of the blood meal. This discussion shows that many of the major vaccine candidates are shared between these blood-feeding species, not only those from the blood-feeding stages but also those expressed by infective L3s in the early stages of infection. Challenges for the future include: exploiting the expanding genome information for antigen discovery, use of different recombinant protein expression systems, formulation with new adjuvants, and novel methods of field testing vaccine efficacy.

Modelling wildlife disease poses some unique challenges. Wildlife disease systems are data poor in comparison with human or livestock disease systems, and the impact of disease on population size is often the key question of interest. This review concentrates specifically on the application of dynamic models to evaluate and guide management strategies. Models have proved useful particularly in two areas. They have been widely used to evaluate vaccination strategies, both for protecting endangered species and for preventing spillover from wildlife to humans or livestock. They have also been extensively used to evaluate culling strategies, again both for diseases in species of conservation interest and to prevent spillover. In addition, models are important to evaluate the potential of parasites and pathogens as biological control agents. The review concludes by identifying some key research gaps, which are further development of models of macroparasites, deciding on appropriate levels of complexity, modelling genetic management and connecting models to data.

Mathematical models predict that the future of epidemics of drug-resistant pathogens depends in part on the competitive fitness of drug-resistant strains. Considering metacyclogenesis (differentiation process essential for infectivity) as a major contributor to the fitness of Leishmania donovani, we tested its relationship with pentavalent antimony (SbV) resistance in clinical lines. Different methods for the assessment of metacyclogenesis were cross-validated: gene expression profiling (META1 and SHERP), morphometry (microscopy and FACS), in vitro infectivity to macrophages and resistance to complement lysis. This was done on a model constituted by 2 pairs of reference strains cloned from a SbV-resistant and -sensitive isolate. We selected the most adequate parameter and extended the analysis of metacyclogenesis diversity to a sample of 20 clinical lines with different in vitro susceptibility to the drug. The capacity of metacyclogenesis, as measured by the complement lysis test, was shown to be significantly higher in SbV-resistant clinical lines of L. donovani than in SbV-sensitive lines. Together with other lines of evidence, it is concluded that L. donovani constitutes a unique example and model of drug-resistant pathogens with traits of increased fitness. These findings raise a fundamental question about the potential risks of selecting more virulent pathogens through massive chemotherapeutic interventions.

As a consequence of comprehensive transcriptome analysis followed by sequencing and draft assembly of the genome, the emphasis of schistosome research is shifting from the identification of genes to the characterization of their functions and interactions. Developmental biologists have long used whole mount in situ hybridization (WISH) to determine gene expression patterns, as a vital tool for formulating and testing hypotheses about function. This paper describes the application of WISH to the study of gene expression in larval and adult schistosomes. Fixed worms were permeablized by proteinase K treatment for hybridization with digoxygenin-labelled RNA probes, with binding being detected by alkaline phosphatase-coupled anti-digoxygenin antibodies, and BM Purple substrate. Discrete staining patterns for the transcripts of the molecules Sm29, cathepsin L, antigen 10.3 and chorion were observed in the tegument cell bodies, gut epithelium, oesophageal gland and vitelline lobules, respectively, of adult worms. Transcripts of the molecules SGTP4, GP18-22 and cathepsin L were localized to tegument cell bodies and embryonic gut, respectively, of lung schistosomula. We also showed that Fast Red TR fluorescent substrate can refine the pattern of localization permitting use of confocal microscopy. We believe that method of WISH will find broad application, in synergy with other emerging post-genomic techniques, such as RNA interference, to studies focused at increasing our molecular understanding of schistosomes.

The habitat of patients with L. minor infection in Trinidad, Tobago and Surinam is described. Three species of Lagochilascaris are redescribed and compared. A new species from Felis concolor is described and a synopsis, with a key to the genus is provided.

The development of L. minor is described from eggs and larval stages obtained from neck lesions of patients. The developmental pattern through third- and fourth-stage larvae was found to conform closely to that of other ascaridoid species, but the tail of the second-stage larva showed a unique conformation.

The earliest stage of development found in human lesions was the third stage, the smallest larva being 1·8 mm in length. The third moult was observed at about 5 mm and the fourth moult at 8 mm. Adult worms were observed with a minimum length of 5 mm in the male and 6 mm in the female. Eggs were present in some females of 8 mm length.

The observations are discussed in relation to previous concepts concerning the affinities, host specificity and evolution of this group of nematodes.

The writer takes pleasure in acknowledging the considerable help he received in this work from several colleagues and friends: Professor J. J. C. Buckley for sending specimens of L. minor and L. buckleyi from the Department of Helminthology, London School of Hygiene and Tropical Medicine; Drs A. G. Chabaud and Marie-Claude Durette from the Museum national d'Histoire naturelle, Paris for sending specimens of L. major; Dr P. Araujo from the Universidade de São Paulo, Faculdade de Farm´cia e Bioquímica for sending specimens of L. minor; Dr J. F. Teixeira de Freitas of the Instituto Oswaldo Cruz, Rio de Janeiro for sending specimens of L. turgida; Drs M. D. Little and P. C. Beaver from the Department of Tropical Medicine, Tulane University New Orleans, for sending specimens of L. minor and notes on their observations relating to L. minor. Grateful acknowledgement is also due to Dr O. H. Siung of the Ministry of Health, Port of Spain, Trinidad, and Dr Cox, Chief Medical Officer, Tobago, Dr A. H. Jonkers and his colleagues at the Trinidad Regional Virus Laboratory, and to His Excellency Dr B. F. J. Oostburg, Minister of Health of Surinam, for their considerable help and co-operation which enabled the writer to observe patients and examine specimens in Trinidad and Surinam. The able technical assistance of Miss Mary Cremin, Mrs A. McKeown and Mrs A. Green is gratefully acknowledged. Financial assistance for this study was provided under grants from the Australian Research Grants Committee, the U.S. Dept. of Health, Education and Welfare no. A. 107023–02, and the University of Queensland Research Grant.

Little is known of the prevalence and life-cycle of trypanosomes in mammals native to Australia. Native Australian trypanosomes have previously been identified in marsupials in the eastern states of Australia, with one recent report in brush-tailed bettongs (Bettongia penicillata), or woylie in Western Australia in 2008. This study reports a novel Trypanosoma sp. identified in blood smears, from 7 critically endangered Gilbert's potoroos (Potorous gilbertii) and 3 quokkas (Setonix brachyurus) in Western Australia. Trypanosomes were successfully cultured in vitro and showed morphological characteristics similar to members of the subgenus Herpetosoma. Phylogenetic analysis of 18S rRNA gene sequences identified 2 different novel genotypes A and B that are closely related to trypanosomes previously isolated from a common wombat (Vombatus ursinus) in Victoria, Australia. The new species is proposed to be named Trypanosoma copemani n. sp.

The migration in mice of 20, 50 and 90 krad. 60Co-irradiated Schistosoma mansoni larvae, biosynthetically radio-isotope labelled with [75Se]-selenomethionine, was evaluated by autoradiography of compressed tissues and compared to the migration of non-irradiated 75Se-labelled larvae. The migration of 20 krad. -irradiated schistosomula between skin and lungs was slightly delayed but otherwise paralleled the migration of normal, non-irradiated schistosomula during the first 8 days following exposure. By day 8 over 90% of both non-irradiated and 20 krad. -irradiated organises were located in the lungs. In contrast to non-irradiated organisms, however, only a small proportion of 20 krad. organisms migrated to the liver. The delay in migration between skin and lungs was more pronounced with 50 krad. -irradiated schistosomula. Nevertheless, 45–93% of 50 krad. -irradiated organisms migrated to the lungs by 8 days post-exposure. Over 90% of the 50 krad. larvae detected in the mouse on day 21 were in the lungs; no more than an occasional 50 krad.-irradiated organism was ever detected in the liver. In three experiments, over 85% of the 90 krad. -irradiated organisms were retained in the skin; in a fourth experiment about half of the 90 krad. -irradiated organisms migrated as far as the lungs. As with 50 krad. organisms, only an occasional 90 krad. organism was ever detected in the liver. Removal of the skin exposure site within the first 4 days of immunization with either 50 or 90 krad. -irradiated cercariae completely blocked the induction of resistance. Removal between the 4th and 6th days gave variable results. Mice had to be in contact with the irradiated larvae for a minimum of 8–11 days to stimulate a level of resistance comparable to that of mice whose immunization site was not removed.

Observations were made on the histology of tissues from the first 6–10 m of the small intestine from both sheep infected with Trichostrongylus colubriformis and from uninfected controls. In uninfected animals, and in distal samples from infected lambs, villi were tall and covered by tall columnar epithelium, the intestinal crypts were convoluted and the lamina propria was only moderately cellular. In tissues with sub-total villus atrophy and a convoluted surface, the superficial epithelium was usually low columnar, often with an indistinct brush border. More severely affected mucosa was flat, with low surface epithelium and protruding crypt openings. The flat surfaces frequently had leaks of eosinophilic material and polymorphonuclear cells between enterocytes, or through erosions in the epithelium. Such defects were smaller and less frequent in mucosa with a convoluted surface. There were increased mitoses in intestinal crypts and a heavy inflammatory cell infiltrate in the lamina propria. Mast cell, globule leucocyte and theliolymphocyte numbers were not increased in infected sheep.

Fascioliasis pathogenesis depends on fluke burden. In human hyperendemic zones, individual infection intensities reach very high levels and the majority of infected subjects should be in the advanced chronic phase. The rat model offers a useful approach for pathological research in the advanced chronic period. The influence of infection intensity per rat on fluke development, pre-patent period and egg shedding (eggs/g faeces/worm) was analysed in 3 groups (I: 1–3 worms/rat; II: 4–6; III: 7–9). Ontogenetic trajectories of fluke body measures followed a logistic model. Results showed that when the burden increases, the maximum values of fluke measures decrease. The crowding effect is manifest when fluke measures approximate their maximums in the advanced chronic stage. The pre-patent period and egg production decrease when the burden increases. This means that measurements of eggs per gramme of faeces tend to underestimate the fluke burden. The present study demonstrates how to quantify the fascioliasis experimental rat model crowding effect on adult growth, pre-patent period and egg production. This quantification may be of great interest in epidemiological studies and in experimental research on the in vivo actions of different anthelminthic drugs and vaccines, pathology, immunology and resistance studies.

Two stocks of Trypanosoma congolense were established in culture at 28 °C using trypanosomes from the proboscides of infective Glossina morsitans. Successful primary cultures were initiated by placing an infected tsetse proboscis beside a bovine dermal collagen explant in Eagle's minimum essential medium supplemented with foetal calf serum. The trypanosomes multiplied rapidly in the medium and also gradually formed an adherent layer on the plastic surface of the culture vessel. Three primary cultures produced organisms infective for mice from 14, 20 and 35 days after initiation and thereafter continuously until days 76, 76 and 52 when they were discarded. Four attempts to initiate infective cultures using infected tsetse proboscides in medium without dermal explants were unsuccessful. When trypanosomes from primary cultures were placed in culture medium with proboscides from uninfected tsetse flies, the parasites multiplied, formed an adherent layer in the culture flasks and were seen in the proboscides within 24 h. A line of 1 stock was serially sub-passaged in this way 4 times during a period of 215 days. Infectivity titrations in mice indicated that primary and sub-passaged cultures each contained similar numbers of infective organisms. Another line of the same stock was also sub-passaged 4 times in medium alone over a period of 186 days. These sub-cultures again retained infectivity for mice, but titrations showed a decrease in infective organism production in the 4th sub-culture. Primary and sub-passaged cultures all included a variety of morphologically different developmental forms of T. congolense, closely resembling those described in the labrum and hypopharynx of Glossina by previous workers. Short metacyclic-like trypanosomes and organisms with proteinaceous surface coats were present in infective cultures. Cultures were successfully re-established after cryopreservation at −196 °C and retained the ability to produce infective organisms.

An experiment was conducted to determine the rate and maximum percentage of sporulation of Eimeria acervulina oocysts at various environmental conditions relating to temperature (21 versus 33 °C) and relative humidity (RH) (40 versus 80%). Measurements were made during 44 h after excretion of oocysts in 3 substrates: dry litter, clammy litter and pure faeces respectively. Maximum sporulation percentage in both dry (22·6%) and clammy litter (19·5%) was higher (P < 0·005) than in pure faeces (11·6%). Neither temperature nor RH had a significant influence on percentage of oocysts that sporulated. Under these simulated practical conditions approximately 25% of all oocysts sporulated, whereas sporulation under optimal conditions (29 °C, aeration, 2% K2Cr2O7) showed a higher (68%) sporulation ability of oocysts. At 33 °C sporulation proceeded at a faster pace than at 21 °C (P < 0·005). With respect to RH and substrate, once sporulation started, the rate of increase to maximum percentage was not different. Time of onset of sporulation was influenced by temperature (P < 0·0001) and RH (P < 0·001). Time of onset occurred 15 h later at 21 °C compared with 33 °C and 5 h later at 40% RH compared with 80%. Also, an interaction effect (P < 0·01) was found with effect of RH being stronger at 21 °C compared with 33 °C. It was concluded that the most important aspect in the epidemiology of E. acervulina during a flock cycle is the time of onset of sporulation with temperature being the most important factor.

Three groups of lambs, aged 3, 5 and 6 months respectively, were infected daily with 5000 infective larvae of Trichostrongylus colubriformis for periods of 15–20 weeks. Some of the lambs also received a single challenge infection at the end of week 16. The course of the infections was monitored by means of faecal egg counts, measurement of live-weight gains and regular post mortem worm counts.

There was an initial rapid increase in egg counts soon after patency, which was followed by an exponential fall in the counts in most lambs. This fall seemed to precede the expulsion of worms by a few weeks and was associated with reduced fecundity in the female worms. Furthermore, although the trend of the egg counts of some lambs suggested that a self-cure reaction had taken place, in a number of these lambs the low faecal egg counts did not reflect the size of their worm burden.

Heavy worm burdens were usually associated with clinical symptoms of trichostrongylosis, including anorexia, diarrhoea and loss of weight. As the lambs developed resistance to reinfection and expelled their worm burden a rapid clinical recovery occurred, except in a few lambs which appeared to have suffered irreversible damage from the earlier heavy worm burden.

The worm burden in most lambs appeared to be cumulative during the first 4–8 weeks before they developed a strong resistance to reinfection. There was considerable loss of the adult worm burden by 8–15 weeks after initial infection and some evidence of stunting in the remaining parasites.

The immunological control of T. colubriformis infections in these lambs was achieved by means of the development of resistance to reinfection, inhibition of the ovulation of the female worms, stunting of adult worms and expulsion of adult worms by resistant lambs. These may be separate immune phenomena requiring different thresholds of parasitic material in individual animals, depending on their age, the rate and duration of infection and their innate ability to develop resistance to the parasite.

1. The second ecdyses experimentally produced in 1 in 20 and other watery dilutions of Milton hypochlorite are described. These occur without the detachment of a typical cap. The larva is passively propelled part of the way out through a hole at the anterior tip of the sheath, and completes its emergence by its own movements.

The explanation of this is that the chlorine in the Milton solution alters the chemical composition of the sheath with two results: (a) the permeability of the sheath is altered so that increased internal pressure results, by which the larva is propelled out of the hole at the anterior tip, and (b) the sheath is rendered soluble in the NaOH in the Milton solution so that it is subsequently dissolved. The larva is expelled through a hole at the anterior tip before the whole sheath is dissolved because the sheath gives way here first to the combined effects of the increased internal pressure and the solvent action of the NaOH, this being its weakest point, where the remains of the mouth of the second larva are.

The larva is only rarely propelled completely out by the raised internal pressure, namely, when the hole at the anterior tip is big enough to allow its widest part, at the base of its oesophageal region, to pass; or, alternatively, when the internal pressure is high enough to force this part of it through the hole. Usually it requires the help of its own movements to force this widest part of it through the hole.

2. Neither chlorine, oxygen, nor caustic soda alone can cause ecdyses, although the latter may dissolve the sheaths of larvae kept long in cultures or exposed long upon pastures, so that a chemical change in their sheaths has occurred similar to that just described. Caustic soda may then also so weaken and thin the sheaths that larvae, if they are active enough, may break their way out.

Neither carbonic nor hydrochloric acids, nor solutions of sodium chloride, could alone cause ecdyses; nor could solutions of sodium carbonate, although these acted upon “older” larvae in much the same way as solutions of caustic soda, which they must contain.

3. Larvae, however, whether they were recently mature or had been long kept in cultures, showed, when they were treated with chlorine water or with hypochlorite solutions acidified with excess of either HCl or acetic acids, effects which indicated that the conclusion described in (a) above was correct. They never exsheathed, but their sheaths were blistered and thinned and frequently distended also, with the exception of the cap which is detached at the ecdysis; this was usually neither blistered, thinned nor distended. This distension was especially marked when these larvae were transferred, after this treatment with chlorine, to solutions hypotonic to them. In hypertonic solutions, on the other hand, the larvae were shrunken. This perhaps explains why larvae are often shrunken away from their sheaths in old cultures from which much evaporation has occurred or when they have been exposed upon pastures.

Their sheaths were, moreover, rendered soluble in 0·4 or 0·8 per cent. NaOH by this treatment with chlorine, so that treatment with chlorine, followed by immersion in weak NaOH, will exsheath the larvae, whether they are active or not, by dissolving off their sheaths. This probably explains why larvae exposed on pastures or kept long in cultures, exsheath sometimes in alkaline media such as artificial pancreatic juice, or in the saliva of sheep.

4. Observation of ecdyses that occurred at 38° C. in about 8 days in mixtures of 0·5 per cent. solutions of HCl and 0·5 per cent. NaOH or in 0·5 per cent. HCl followed by 0·5 or 0·8 per cent. NaOH, showed that these always occurred by the detachment of a typical cap, and that the process was essentially the same as that described above. An alteration of the permeability of the sheath occurred by which the pressure inside it was increased, so that the sheath was often distended. The sheath was at the same time rendered soluble in NaOH.

The cap which was detached at the ecdysis resisted, however, this distension, perhaps because it had a slightly different chemical composition, so that tension existed at the base of this more rigid cap, between it and the more flexible and elastic part of the sheath behind it. The tension at this point was further observed to be increased by specific rotatory movements of the oral end of the larva, which incidentally also determined the length of the cap, and by movements of the more flexible and elastic part of the sheath behind the cap, by which it could be repeatedly rolled up on to the cap and back again, as rubber tubing may be rolled up on to a tap. All these movements, especially when they acted upon a distended sheath, ruptured first the inner layer of the sheath at the base of the cap, and then the much stronger outer layer.

The larva then emerged by its own movements; or sometimes it was propelled out passively part of the way by the release of the high internal pressure.

Both the rotatory movements of the oral end of the larva before the detachment of the cap, and the bending and swimming movements of the whole larva by which it extricates itself from the sheath after the cap has been detached, are essential to successful and normal ecdyses. The rotatory movements of the oral end are probably most effective when the oral tip of the sheath is fixed against some resistant object, as skin-penetrating larvae may, for example, fix it against the skin. This fixation of the oral end is not, however, necessary, because most of the ecdyses here recorded occurred in this way in drops quite free of any solid or semi-solid matter.

5. Some experiments are briefly described which indicate that mechanical damage to the sheaths, or friction applied to them, cannot be more than secondary factors which influence only the ecdyses of so-called “older” larvae, whose sheaths are already chemically altered by climatic factors or long life in cultures, with results similar to those which follow treatment with chlorine.

6. It is suggested that factors similar to those indicated above may be present in the stomach and duodenum of the host and may govern any second ecdyses that occur in these situations; and that changes in the sheaths, similar to those produced by chlorine in the experiments here described, may be produced outside the body of the host, by climatic factors, so that the sheaths become soluble in the saliva or other alkaline digestive juices.

7. All attempts failed to induce the third ecdysis of larvae exsheathed and sterilised by 1 in 20 dilutions of Milton with water, and kept in sterile media until they were ready for this ecdysis. These larvae also, like the infective larvae, were susceptible to changes in the tonicity of the media in which they were kept, so that their sheaths could be alternately distended or collapsed upon them, and the larvae themselves could be shrunken or restored to normal by alterations in the concentration of the media. These third (or first parasitic) larvae showed a remarkable power of resistance to such treatment and seemed to be more resistant to it than infective larvae are.

8. Further investigations of the second ecdyses by biophysical methods may bring all the ecdyses of parasitic nematodes under experimental control and so make possible the artificial cultivation of the adult parasites. They may also suggest methods of altering the permeability of the cuticle of adult nematodes, so that anthelmintics may be found which will penetrate this more readily and at the same time will not be toxic to the host.

Four species of Eimeria from fowls were able to develop in chicken embryos following the inoculation of sporozoites. Three species (E. brunetti, E. mivati and E. tenella) were able to complete the whole of their endogenous cycle in embryos. E. necatrix developed to late schizogony, E. acervulina and E. maxima did not develop.

The life-cycles appeared to be delayed, and in the case of E. tenella embryo mortality was associated with heavy parasitism of the chorioallantoic membranes. Oocysts recovered from urate deposits in the allantoic cavity sporulated normally and induced infections in chickens.

Attempts to infect turkey and quail embryos with E. tenella or chicken and turkey embryos with E. stiedae were unsuccessful.

The implications of the findings are discussed.

I wish to thank Dr C. Horton-Smith for his encouragement and guidance with this work; Dr M. E. Rose for help in the preparation of the manuscript and Mr B. J. Millard for careful technical assistance.