The gregarine Stylocephalus mesomorphi has a diploid chromosome number of 8. The diploid stage is represented only by the zygote nucleus before the metagamic divisions start. The other stages in the entire life history of this gregarine are haploid.

The mitotic chromosomes are four in number, one being the longest, two of medium size and the fourth the smallest.

I am grateful to Professor J. C. Uttangi, Head of the Zoology Department, Karnatak Science College, Dharwar, for the encouragement he gave me throughout this work. I am also greatly indebted to my colleague Dr M. J. Devadhar, for having given me many valuable suggestions.

The biology and life cycle of Apatemon gracilis minor is briefly described.

The morphology of the cup–shaped forebody with the small lappets, the pear-shaped subcuticular cells and the very flexible finger–like adhesive organ lobes is considered in some detail.

An extensive description of the body musculature is given and the mode of attachment of the parasite to the host mucosa is studied.

The main results of the histochemical tests are: (a) a relative absence of enzymes in the adhesive organ and a greater activity in the lappets; (b) the presence of two types of esterase, a non-specific esterase of B-type in the caecal cells and a non-specific esterase of resistant type, possibly a non-specific cholinesterase, in the rest of the reacting sites in the parasite; (c) a relatively small activity for leucine aminopeptidase; (d) the presence of RNA at all sites of enzyme formation; and finally, (e), the presence of enzymes and carbohydrates in the cuticle.

The activities of the parasite result in a local disruption of the host intestinal mucosa at the site of attachment. It is suggested that this is a result of the activities of the enzymes secreted by the parasite.

The absorption of nutrients and the circulatory function of the excretory system are discussed.

I should like to express my sincere thanks to Dr D. A. Erasmus for helpful discussions and advice during the progress of the work and to Professor J. Brough for giving me the opportunity to work in his department. The study was conducted during the tenure of a grant from Svenska Vetenskapliga Centralrådet i Finland.

Two new, nine little known species and nine unnamed ‘species’ of Echeneibothrium Beneden, 1850, have been investigated and observations have been made on the ‘related’ genera, Discobothrium Beneden, 1870, and Pseudanthobothrium Baer, 1956. Brief descriptions of the species are given, but a discussion of the history of each differs greatly from most previous accounts in that few or no synonyms have been listed for the species and in that nine forms are left unnamed; this approach differs greatly from that in a large number of taxonomic papers in helminthology. A detailed discussion of the reasons for adopting this procedure is, therefore, given. A provisional key to nineteen ‘species’ of Echeneibothrium is included. E. minutum sp.nov. closely resembles E. variabile var. exiguum of Euzet (1959), described in this paper as ‘Echeneibothrium sp. of R. clavata’, but occurs only in R. batis and differs in its very small size, in having only about ten proglottids, long peduncles to the bothridia and a myzorhynchus which remains evaginated in fixed specimens. It differs from E. demeusiae Euzet, 1959, mainly in the number of proglottids, testes and loculi. E. elongatum sp.nov. resembles E. dubium from which it differs in the length and form of the strobila, in having only six clearly visible loculi in each bothridium, about twenty testes and in that it occurs in R. circularis. It has, however, more characters in common with a species referred to in this paper as ‘Echeneibothrium sp. from Raja naevus’ and which possesses eight loculi. A list is given of twenty-seven species wrongly allocated by various authors to the genus Echeneibothrium. The comparative morphology of some species of the genus, their ecology and speculations on their life-histories, are described.

It is shown for the first time that Echeneibothrium from species of Raja caught off the British Isles, and presumably from rays of other localities, can be separated into two distinct biological groups. One group includes species which possess shallow open bothridia covering comparatively large areas of the host's mucosa, a variable myzorhynchus which may be either rudimentary or very large and spherical eggs laid singly or in capsules; these occur mainly in rays from shallow waters and in which the mucosa of the intestine lacks prominent villi. The other group includes species in which each bothridium possesses a small opening adapted for attachment to a single villus, the myzorhynchus is consistently cylindrical, the eggs have long polar filaments and the species occur in deep water rays with well-defined villi on the intestinal mucosa. Studies on variation in the form of the eggs in different species of Echeneibothrium in relation to the behaviour and feeding habits of the various host-species suggest that the eggs are eaten by arthropods which are most likely to form part of the diet of the final host. A general trend towards an increase in the complexity of the bothridium and a decrease in the size of the myzorhynchus in Echeneibothrium is thought to be of considerable advantage to the species and an indication that Rhinebothrium, which is restricted to the Myliobatoidea, has evolved from a form like Echeneibothrium which is restricted to the Rajoidea. This supports the view that the Myliobatoidea have evolved from the Rajoidea. The results from this study of Echeneibothrium illustrate the following general rule which can be applied to the Tetraphyllidea, namely, those species which are abundant in any given host are very well adapted for attachment to the host's gut and cause little, if any, damage while rare species are often highly pathogenic.

This work would not have been possible without generous financial aid from the Department of Scientific and Industrial Research and various kinds of other aid and encouragement from a number of persons, in particular Dr Gwendolen Rees.

It was begun in 1958 at the University College of Wales, Aberystwyth, the British Museum (Natural History), University of Montpellier, France, University of Neuchatel, Switzerland, the Plymouth Marine Laboratory and the Aberdeen Marine Laboratory during my tenure of a D.S.I.R. Fellowship. I wish to record my gratitude to Dr Rees who has also suggested improvements to the manuscript and to the following persons: the late Professor T. A. Stephenson, F.R.S., Dr E. E. Watkin, Mr S. Prudhoe, Dr L. Euzet, Professor Jean G. Baer, The Director, Dr F. S. Russell and Staff, in particular Mr J. Green and Mr A. Mattacola, of the Plymouth Marine Laboratory, and the Director and staff, in particular Dr B. B. Rae and Dr Z. Kabata, of the Aberdeen Marine Laboratory.

The work was later continued at the University College, Cardiff, and I am most grateful to Professor James Brough for providing excellent research facilities and to Mr W. O'Grady for technical assistance.

At the Bureau of Helminthology, St Albans, much encouragement from Professor R. T. Leiper, C.M.G., F.R.S., enabled me to put the work together while having free access to his invaluable collection of books and reprints on helminths; it became unnecessary to search elsewhere for the literature. Mr G. Dimmock gave technical assistance for which I am most grateful.

The cercaria of Fasciola hepatica has four types of cystogenic cells which can be characterized by their position and by their histochemical properties. On the basis of their histochemical reactions, each type of cell was related to a specific layer in the cyst wall of the metacercaria.

The tanned protein cells occupy almost all of the ventral half of the body and contain large granules consisting of basic protein and phenols. Although the presence of a phenolase could not be demonstrated, it is considered that the granules are composed of tanned protein. Before the cercaria leaves the redia, the granules are secreted to the exterior to form a layer contained by a thin, bounding membrane. The tanned protein cells form layer I of the cyst wall.

The mucopolysaccharide cells are concentrated at the dorsal and dorso–lateral surface except for a few around the ventral sucker. They contain acid and neutral mucopolysaccharides and contribute to layers II and IIIb and c. Some of the granules are secreted to the exterior before the cercaria leaves the redia, in a similar manner to the tanned protein granules.

The mucoprotein cells are few in number and lie centrally between the ventral sucker and the pharynx. They give reactions for proteins and carbohydrate-protein complexes and probably help to form layers II and IIIa of the cyst wall.

The keratin cells fill almost the entire dorsal half of the body and contain numerous rod-like granules. These rods are composed of a protein containing sulphydryl groups and disulphide bonds and give rise to layer IV. After secretion of the tanned protein granules to the exterior, some of the keratin cells move to the ventral part of the body formerly occupied by the tanned protein cells.

The outer cyst wall exists in a preformed state in the free swimming cercaria.

I am grateful to Professor J. D. Smyth and Dr J. A. Clegg of this department for advice and encouragement during this investigation and in the preparation of the manuscript. The work was done during the tenure of a Commonwealth Post-Graduate Scholarship.

The ultrastructure of the parenchyma and interstitial material and the interrelations between them are described.

The interstitial material, composed of a granular matrix containing fibres, is shown to ramify as a network between parenchymal cells and around organ systems. It is postulated that this structure is a ‘skeleton’.

In certain localized areas the interstitial material between parenchymal cells is absent and at these points adjacent cells are united by attachment plaques (desmosomes) without tonofibrils.

The possible functions of the parenchyma are considered and it is suggested that these cells are responsible not only for secreting the interstitial material, but also store glycogen and act, in the absence of a circulatory system, as a transport system for metabolites and excretory products.

Arborocystis nov.nom. is proposed to replace Dendrocystis Rees, 1962, a junior homonym of Dendrocystis Bather, 1889.

The authors wish to thank Dr David L. Pawson, Division of Marine Invertebrates, Smithsonian Institution, Washington, D.C., for furnishing data from Barrande (1887).

Dark-adapted third-stage Trichonema larvae were orthokinetically stimulated by light. The response soon reached a maximum and then gradually decreased, the larvae then being inactive as long as they were kept in light. The rate of response was independent of intensity, and was a constant for any one age. Inactive larvae were activated by a mechanical stimulus, and were therefore only insensitive to light.

The larvae needed 3 h adaptation in darkness before responding fully to a further light stimulus. Dark-adaptation required continuous darkness; larvae exposed to a flashing light responded as if in continuous light. The presence of a light-sensitive mechanism is postulated.

I thank Professor B. G. Peters for discussion of this work, and for reading the manuscript.

Some callus tissues induced by 2,4–D and derived from plants resistant to plant-parasitic nematodes lost their resistance to the nematodes. Red clover callus supported a large population of A. ritzemabosi, whereas red clover seedlings did not. Six races of D. dipsaci were cultured on lucerne and red clover callus tissues and reproduced rapidly, although races usually multiplied more on callus from susceptible than from resistant plants. A. ritzemabosi, normally only a foliage parasite, reproduced equally well in stem and root callus. H. rostochiensis did not reproduce in callus culture.

Nematodes multiplied most in callus that grew fastest; both reproduction of A. ritzemabosi and the growth of callus were greatest with 0.125 mg/1 of 2,4–D. Reproduction was inhibited by 5.0 mg/100 ml of 2,4–D. 2,4–D influenced nematode reproduction indirectly by making callus, which provides a better environment for nematode feeding and reproduction.

Nematode extracts added to an agar medium caused callus formation on red clover seedlings and nematodes feeding on these tissues reproduced faster than on normal seedlings. Hence, the substances secreted by a nematode into a plant may act on the tissues in a manner similar to 2,4–D. The host–parasite relationship is probably partially controlled by the host's plant-growth substances and the effect on them of the nematode's secretions.

We thank Mr C. T. Drakes for assistance, and Mr K. Smith of Imperial Chemical Industries for advice with the initial callus culture.

Single infections of Trichinella spiralis of 48 h duration did not produce a significant degree of immunity in mice.

Single or triple light infections of 7 h duration which, it is suggested, were composed entirely of fourth-stage larvae, did not produce significant immunity.

Single infections of 3 or 4 days duration did produce immunity. It is suggested that the fourth-stage intestinal larvae and the fourth moult are not the most important sources of immunizing antigen in a T. spiralis infection.

I would like to express my gratitude to H.M. Treasury and to the Agricultural Research Council for financial support during the time that this work was being conducted. I would also like to thank Professor W. I. B. Beveridge, within whose department this work was conducted, for the facilities provided. The helpful advice of my colleagues R. M. Connan and R. S. Phillips was of great value during the time that this manuscript was being prepared for publication.

Besnoitia sauriana sp.nov. is described from the tissues and organs of Basiliscus vittatus, with the type locality Baking Pot, British Honduras. It is placed in a new family, Besnoitiidae in the order Toxoplasmea.

The proliferative stage is thought to take place in the alveoli of the lungs. The cystic stage begins as the greatly altered host cell, containing a single round or oval body. The host-cell membrane becomes greatly thickened, the nuclei multiply and enlarge and the cytoplasm assumes a granular texture. The parasites (‘spores’) multiply by binary fission, enclosed in a fine membrane, within this cell, which eventually they entirely fill.

The ‘spores’ easily contaminate the blood and may be mistaken for Schellackia or Toxoplasma in blood smears of infected lizards.

A cycloguanil-resistant strain of Plasmodium gallinaceum was produced relatively rapidly by passage through chicks treated with low but effective doses of the drug, the dose being increased as resistance developed.

The strain was cross-resistant to proguanil but not to pyrimethamine or chloroquine.

A strain highly resistant to proguanil was resistant to cycloguanil but only slightly resistant to pyrimethamine.

A strain highly resistant to pyrimethamine was resistant to proguanil and cycloguanil.

Passage for 20 months through birds treated with doses of cycloguanil which suppressed infection for relatively long periods failed to change the sensitivity of the strain to this drug or to proguanil. Although the relatively large dose did not eradicate the infection in any of the birds, subinoculations demonstrated that parasites were absent from the blood for a period in some of the birds, though infections finally developed.

I am indebted to Parke Davis and Company for the supply of cycloguanil, to Imperial Chemical Industries Ltd. for the proguanil hydrochloride and chloroquine phosphate and to Burroughs Wellcome and Company for the pyrimethamine base.

The histological anatomy of the male reproductive tract as well as the cytochemistry of the cells of different regions of the male tract in Aspiculuris tetraptera are described.

It is shown that there are at least three different regions of the vas deferens, each of which releases one or more substances into the lumen of the male system and thus contributes to the composition of the semen. The histochemical nature of these secretions is given and it is suggested that the secretions of the distal vas deferens are oxytocic.

My thanks are due to Professor J. D. Smyth of the Australian National University, Canberra, for the gift of RNA-ase (L. Light and Co.) and some other histochemical reagents, to Dr T. R. R. Mann, C.B.E., F.R.S., and Dr D. L. Lee, for helpful discussions during the course of this study and for reading the draft manuscript.

The brush border of the intestinal cells of the infective larva of Trichinella spiralis is shown to consist of microvilli. It is suggested that large vesicular complexes in association with Golgi complexes may be responsible for the production of the mucopolysaccharide or mucoprotein filling the lumen of the intestine. Indirect evidence of a latent intranuclear virus infection is presented. The cuticle lining the hind gut, although continuous with the external cuticle, is not similar in structure.

I would like to thank Dr D. L. Lee for instruction in all aspects of this work, Professor Boyd for the use of the Anatomy Department electron microscope and the Agricultural Research Council for financial support.

The genitalia of Diplectanum aequans (Wagener) Diesing are described and a functional interpretation is postulated.

Spermatophores are produced from a pair of diffuse glands lying laterally amidst the vitellaria. The secretion from these glands is stored in an anterior reservoir, across the centre of which is a viscous disk which delineates two internal chambers.

Each spermatophore has an elongated stalk and an ovoid head and is considered to be moulded in a muscular bulb and in the penis, prior to extrusion through the penis.

The penis is composed of two concentric tubes and it has a slight hook at its distal end. The vagina has a glandular base and muscular lips. Its internal cast corresponds to the shape of the penis tip.

Copulation in D. aequans is described.

The occurrence and biological significance of spermatophore production in animals are discussed.

I would like to acknowledge the help given to me by the Director and Staff of the Marine Biological Association Laboratory and also by the Proprietors of Cook and Sons Ltd., of Salcombe, Devon. I am most grateful to Dr J. Llewellyn for much helpful advice throughout the course of this study.

The work was conducted during the tenure of a D.S.I.R. Studentship and includes the results of special studies made possible by a grant from The Browne Fund of The Royal Society.

By culturing Galleria mellonella, Neoaplectana sp. (DD-136) and Achromobacter nematophilus separately under axenic conditions in the laboratory, it was possible to study the relationship between the bacterium and nematode during nematode parasitism of the insect.

The infective-stage juveniles of the nematode were able to penetrate and kill the insect host without the presence of A. nematophilus or any other bacterium. However, without accompanying bacteria the nematode was unable to reproduce. Only when A. nematophilus or a possible replacement, such as Pseudomonas aeruginosa, was added to the blood did reproduction occur.

The relationship between A. nematophilus and the nematode is considered a mutualistic one, since the bacterium lives and is protected inside the intestine of the free-living stage of the nematode and is transported and released by the nematode to the haemolymph of a host insect. The nematode, in turn, is dependent on the bacterium for reproduction.

Previous studies on variation in size in the immature stages of ixodid ticks have been based on the assumption that the progeny of single females constitute a homogeneous population.

From the present work on the immature stages of Hyalomma anatolicum anatolicum, and by following larvae and nymphs to the adult stage, it is clear that there are two size groups of larvae and nymphs which reflect their potential sexuality. Larvae and nymphs which give rise to males are smaller in size than those which produce females. This size difference of the immature stages has other implications, for larvae and nymphs destined to become males imbibe, on average, about two-thirds the quantity of nutrients of those which yield females. Because of these apparent morphological and behavioural feeding mechanisms, which appear to be genetically determined, differences in physiological patterns of activity in the immature stages of the two sexes cannot be overlooked and may prove useful in elucidating the transmission of pathogenic organisms by larvae and nymphs.

We are grateful to Dr R. P. Chaudhuri for providing us with the original ticks from which our culture was started, and one of us (K.S.) to the Science Research Council for the provision of a research grant.