The survival characteristics of free-living cercarial populations of Transversotrema patialensis were described and shown to be age-dependent. The maximum life-span was found to be 44 h with a 50% survival at 26 h. Activity and infectivity of the larvae were also characterized by age-dependence, and were demonstrated to be closely correlated with one another. For individual cercariae, both activity and infectivity had dropped to extremely low levels many hours before death occurred. An attempt was made to interrelate activity and infectivity, in a theoretical manner, with the availability of energy reserves.

Conceptual understanding of the biological processes involved was aided by the formulation of simple mathematical models.

The behaviour which enables the cercariae of the ectoparasitic digenean, Transversotrema patialensis, to establish themselves on the surface of their fish hosts has been described experimentally using anaesthetized fish. Specialized regions of the cercarial tail, the arm processes, seem to be adapted for both specific recognition of, and rapid attachment to, the outer surface of a fish. Each arm process bears an array of nine ciliary sensory structures, the mammiform receptors, which have been tentatively identified as contact chemoreceptor organs involved in host recognition. Also present on each arm is an adhesive pad that mediates the initial attachment to the host. It is a differentiated region of the distal cytoplasm of the epidermal syncytium which invests the whole arm process. The cytoplasm of the pad region contains membrane-bounded adhesive granules, the contents of which are released during activation of the pad.

The ultrastructural and cytochemical changes accompanying intracellular digestion and cellular defecation in the haematin cell of Diclidophora merlangi have been described. Blood proteins of the host-fish are sequestered by endocytosis and degraded within an interconnecting network of channels that form an integral, but changing, part of the cell. The digestive enzymes involved originate in the granular endoplasmic reticulum and are packaged in the Golgi apparatus and transferred to the channels in Golgi vesicles. The rate of haemoglobin absorption and the activity of the Golgi, as judged by vesicle counts and staining intensities for thiamine pyrophosphatase activity, are stimulated by the introduction of host protein into the gut lumen. The haematin residues of digestion are extruded periodically into the lumen by exocytosis involving membrane fusion. The process is a continuous one and, in worms starved of food, can result in the complete evacuation of pigment from the cell. It is suggested that a lysosomal system operates in the digestive cycle of the haematin cell.

Second-instar larvae of the gypsy moth, Porthetria dispar (L.) were infected with Beauveria bassiana (Balsamo) Vuill. by allowing larvae to crawl over sporulating cultures of the fungus. Virulence of the pathogen was increased by inoculating larvae of the wax moth, Galleria melonella (L.), twice in serial succession. Observations were made on the infectivity and histopathology of the parasite in the host. Penetration of the cuticle occurred 24 h after inoculation and at 64 h post-inoculation the interior was completely filled with hyphae. Penetration of the gut wall was also observed.

Serum taken from mice 12–15 weeks after a primary infection of Sehistosoma mansoni transfers to normal recipients a partial resistance to subsequent schistosome challenge. The transfer of immunity is evident not oniy in the reduced recovery of mature parasites from the liver, but also in the diminished numbers of invading schistosomula recovered from the lungs. Since the recipients of immune serum exhibit a level of resistance equivalent on average to 47 % of that found in actively immunized animals, the results suggest that humoral factors play a major role in the effector mechanism of schistosome immunity.

Five species of parasitic copepods are described based on the material collected during cruise 16 of R/V Anton Bruun. They are: from the gill chambers of macrourids, Chondracanthodes deflexus Wilson, Lateracanthus quadripedis Kabata and Gusev and Brachiella annulata Markevitch; from the anal fin of a macrourid, Nudiclavella galapagoense gen.nov., sp.nov.; and from the gill filaments of brotulids, Naobranchia maxima sp.nov. All fishes were caught with FVSL (free vehicle set lines) in the deep sea off the Galápagos Islands, ranging from 1710 to 3330 m.

Methyl benzoquate at 20 ppm or clopidol at 250 ppm in the diet show true coccidiostatic activity in infections with Eimeria tenella in the chick. Lerbek, a synergistic mixture yielding 8·35 ppm methyl benzoquate and 100 ppm clopidol, produced a partially coccidiocidal effect; relapses following restricted periods of medication were less severe, and suppression of parasite development following delayed medication was quicker and more effective than with either component of the mixture used alone.

Ultrastructural observations of the surface of Diplocystis schneideri reveal a folded, multilaminate membrane, similar to that found in other eugregarines, covered with a layer of fibrous material. It is suggested that this fibrous material is a product of the haemocytes of the host, Periplaneta americana.

The ultrastructure of the intestinal-cloacal junction of the male nematode Heterakis gallinarum is described. A sphincter muscle seals the lumen of the alimentary tract at this point by squeezing on the posterior cells of the intestine. These posterior cells of the intestine are unusual in that they contain numerous microtubules, and are very different from the other cells of the intestine. Electron-dense granules lie at the base of these cells where they abut on to the portion of the sphincter muscle which contains the myofilaments. It is suggested that a build-up of pressure in the intestinal lumen, brought about by the pumping action of the pharynx and/or contraction of the muscle network around the intestine, is the most probable means of opening the sphincter at defecation. On the other hand, the structure of the posterior cells of the intestine suggests that they may act as a stretch receptor which communicates with the sphincter muscle by means of a pharmacologically active substance held in the electron-dense granules which lie in the basal region of these cells.

Immune serum accelerated the expulsion of Trichuris muris when transferred into normal mice on days 0 and 3 after infection, but had no effect when the recipient mice had been immunosuppressed by sublethal irradiation or by cortisone treatment. Delaying serum transfer until days 7 and 8 in normal mice failed to accelerate expulsion, although immune mesenteric lymph node cells (MLNC) accelerated expulsion whether transferred early or late in infection.

Expulsion from NIH mice, normally complete by 12 days, was prevented by sublethal irradiation given as late as 9 days after infection, but could be restored by subsequent transfer of immune MLNC or, to a lesser degree, non-immune MLNC. Immune MLNC were unable to restore worm expulsion in mice irradiated before infection.

These results are interpreted as showing that the immune expulsion of T. muris from mice during a primary infection requires the sequential activities of antibody-mediated and lymphoid cell-mediated components.

Three groups of chickens were fed small numbers of ova (3, 5 or 9 ova per bird) of Heterakis gallinarum in a quantitative study of the transovarial transmission of Parahistomonas wenrichi by the nematode. Subsequent incidence rates of Parahistomonas infections in recipient birds were analysed according to the methods of Chiang & Reeves (1962). The results indicated that at least 25 % of Heterakis ova, passed in the faeces of birds with concomitant Parahistomonas infection, carried the protozoon. The appearance of Parahistomonas in a bird which retained all its challenge nematodes indicated that the protozoon can be released by living worms. Data on the pre-patent periods of Parahistomonas infections are consistent with a hypothesis that the protozoon is typically released at the time of the final (fourth) moult of the nematode.