1. Cerfontaine's (1896) description of the egg is confirmed. The eggs are not retained on the gills during development.2. Maturation of the oocyte takes place after egg formation. The haploid number of chromosomes is 9.3. The onset of embryonic development is related to the time of egg formation or sperm entry. Cleavage is total and slightly unequal, leading to the formation of a morula in which there are no germ layers. The larval gut is formed by epiboly round a mass of yolk which becomes incorporated in mid-body. Larval organs and a partially ciliated epithelium are differentiated in situ.4. During embryonic development yolk is broken down and absorbed, some being incorporated in the body.5. Hatching does not seem to result from any special stimulus. Mechanical pressure by the larva forces off the operculum, but secretions may weaken the suture.6. The free-swimming larva is gyrodactyloid with a winged haptor bearing four pairs of lateral, one pair of postero-lateral and one pair of median hooks. Four pairs of flame cells are present and one pair of longitudinal, lateral, excretory ducts, which open antero-laterally. The larva swims by ciliary action and does not respond to jarring, light and shade stimuli, or, when vigorous, to the presence of the excised gill of the host. Tired larvae are doubtfully attracted by host gill tissue. Larval haptorial type is thought to be determined phylogenetically rather than as a result of adaptation to the type of host tissue to which the larva attaches itself.7. The larva attaches itself to the host's gill and sloughs off the ciliated coat. Growth follows with the development of the adult clamps in pairs, starting posteriorly, each pair replacing a pair of lateral, larval hooks. Primordia of the reproductive organs are laid down in the larval stage, but are only finally elaborated when the adult form has been attained. Maturity is not reached at a definite size and the ovary and testes mature simultaneously.8. Feeding is at least partly on the host's blood.9. The longest time which may elapse between formation and laying of an egg is about 5 hr. The average time taken for embryonic development is 18·3 days at 14·25° C., but temperature affects the developmental rate. Ciliated larvae survive about 24 hr. in the absence of a host. The second larval stage is reached between 5 and 13 days after hatching, the third larval stage after more than 38 days. The immature stage is thought to be reached within 3 months and the adult stage within 6 months.10. Reproduction occurs throughout the year. The animal is functionally hermaphrodite at all times once maturity is reached.11. A certain degree of osmo-regulation is possible in adults, but this does not extend to ciliated larvae.12. Adults rest along a gill filament on its inner face and do not move about. Larvae are more ready to move.13. The average number of flukes per host is 2·24. The 1st and 2nd pairs of gills carry three times as many flukes as the 3rd and 4th pairs.14. The host suffers no apparent harm from the presence of the flukes.15. Premunition of Gadus virens against Diclidophora denticulata does not occur. Age resistance is probably developed to the extent that larvae cannot establish themselves on older fish and is probably due to mechanical difficulties and changed habits of the host. The ability of this fluke to parasitize species of Gadus does not extend to G. callarias.16. D. denticulata is recorded for the first time from St Andrews Bay and the Firth of Forth.

1. Moles were collected from four main areas, Suffolk, Cheshire-Staffordshire, Westmorland-Yorkshire, and south-west Scotland, and examined for the presence of flukes. Three species were found: Itygonimus lorum from all the areas (and in one mole from Oxford); I ocreatus from Suffolk; Omphalometra flexuosa from Suffolk and Cheshire-Staffordshire.2. The sex of the host has no effect on the incidence of parasitism with flukes.3. I. lorum is almost exclusively a parasite of the lower half of the intestine, whereas I.ocreatus and O. flexuosa are predominantly parasites of the upper half of the intestine.4. The commonest number of flukes of any of the three species to find in a mole is one, though two individuals is almost as common a number in the case of I. lorum. Nevertheless, large populations of I. ocreatus or O. flexuosa and moderate numbers of I. lorum are on occasion found in one mole.5. Premunition does not occur with infestations of I. lorum and probably not with I. ocreatus and O. flexuosa.6. The distribution of each species even within an area where it occurs is very patchy and local. This is probably bound up with restricted movement of the hosts, whether final or intermediate, and may also be associated with the influence of soil conditions on the intermediate hosts.7. There appears to be no seasonal variation in the presence of flukes in moles.8. Simultaneous infestation with more than one species of fluke is rare. It is possible for O. flexuosa to live in company with I. lorum or I. Ocreatus, but the two Itygonimus species were never found together in the same mole.9. The presence of flukes in a mole has no effect on the rest of the intestinal helminth population of the host.

An inquiry has been made to determine whether shearling rams sire more good-quality offspring than would be expected from the proportion of them used in Swaledale flocks in northern England, 1951–3.Quality was assessed at two levels, both concerning shearling rams: those passed for registration in the Flock Book, and those making £50 or more at Kirkby Stephen ram sale.The expected percentage of lambs, and hence good-quality offspring, by shearling rams was estimated from the practice on a sample group of farms, using the formulawhere E is the estimated percentage of lambs by shearlings, x the proportion of shearlings to all rams in use, and y the fractional value in numbers of ewes served where aged rams are regarded as having unit value.The estimated percentage of lambs by shearlings is 28·7, with s.e. = 2·444.The actual percentage of registered shearlings sired by shearlings in 1951–3 was 35·7 with s.e. = 0·764, and the actual percentage of shearlings sold for £50 or more in the same years was 32·8 with s.e. = 3·313.That shearlings sire a slightly, but significantly, higher number of registered shearlings than would be expected is probably due to the transmission of earlymaturing qualities, since shearlings of high quality are consistently used in early-maturing, and therefore shearling-producing, flocks. Those likely to sire ‘£50’ shearlings often remain in early-maturing flocks throughout life and, by continuing to sire this high-quality progeny, depress the percentage of such shearlings sired by shearlings to a figure not significantly higher than the ‘expected’ figure.

C. pygocytophora emerges from P. carinatus predominantly in the dark, independently of the time of day, and in the absence of temperature variation.C. Pygocytophora shows strong negative phototaxis.