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SUMMARYLake Tanganyika harbours the most diverse endemic cichlid fish assemblage of Africa, but its monogenean fish parasites have not been investigated. Here we report, for the first time, on the Gyrodactylus parasites in this hotspot of fish biodiversity. Haptor morphometrics and nuclear ribosomal DNA sequences revealed 3 new species on Zambian Simochromis diagramma: Gyrodactylus sturmbaueri n. sp., G. thysi n. sp. and G. zimbae n. sp. Their distinct morphology and strong genetic differentiation suggest that they belong to distant lineages within the genus Gyrodactylus, and phylogenetic reconstructions suggest affinities with other genera of gyrodactylids. Additional U-shaped haptoral plates in G. thysi n. sp. and a second large spine-like structure in the male copulatory organ of G. zimbae seem to represent new features for the genus. Such large diversity on a single host species can probably be explained by host-switching events during the course of evolution, in agreement with the generally accepted concept that ecological transfer is an important aspect of gyrodactylid speciation. Additional parasitological surveys on other host species, covering a broader phylogenetic and geographical range, should clarify the evolutionary history of Gyrodactylidae on cichlids in the African Great Lake and other parts of Africa.

SUMMARYThis review deals with the problems faced by those monogenean (platyhelminth) parasites that attach themselves to fish skin. The structure of the skin and the ways in which the posterior hook-bearing haptor achieves virtually permanent attachment to the skin are considered. Small marginal hooklets are specialized for attachment to superficial host epidermal cells, finding anchorage in the terminal web of keratinous tonofilaments, while large hooks (hamuli) may penetrate into and lodge in the collagenous dermis. The complementary roles of suction and sticky secretions in haptor attachment and the role of the pharynx in temporary attachment during feeding are also considered. During leech-like locomotion the haptor is briefly detached and, at this critical time, the anterior end is strongly fixed to the wet, current-swept and possibly slimy skin by a sticky secretion. This secretion is deployed on paired pads or discs, the latter sometimes backed up by suction. After attachment by the haptor is re-established, the special tegument covering the anterior adhesive areas may be instrumental in their instant release. The role of fish skin in the phenomenon of host specificity and in the generation of a defensive response against monogeneans is considered and site-specificity of parasites on the host's body is discussed. Possible selection pressures exerted by predatory ‘cleaner’ organisms are briefly evaluated.

SUMMARYThe eggs and oncomiracidia of two species of the capsalid genus Encotyllabe are described. These parasites were identified tentatively as E. caballeroi Velasquez, 1977 and E. caranxi Lebedev, 1967. If these identifications are correct, then E. caballeroi is recorded from two new hosts, Gymnocranius audleyi and the nemipterid Scolopsis monogramma, and E. caranxi from a new host, Pseudocaranx dentex, and a new locality, Heron Island, Queensland, Australia. Encotyllabines have not previously been recorded from fishes of the family Nemipteridae. The eggs of the two parasites failed to hatch spontaneously and did not hatch when exposed to a variety of potential hatching stimuli, but the oncomiracidia within survived for many weeks. Oncomiracidia expelled from eggs by cover-slip pressure are unciliated and possess a saucer-shaped haptor like that of other capsalids with three pairs of median sclerites and 14 marginal hooklets. The paths of tendons associated with the accessory sclerites and the presence of haptoral loculi suggest that encotyllabines are related to the trochopodines. Observations on a single juvenile specimen of E. caballeroi show that the accessory sclerites and the tendons are lost early in development and that one pair of hamuli (probably the posterior pair) ceases to grow early in post-oncomiracidial life. The loculi persist a little longer but also disappear before full sexual maturity is reached.

SUMMARYThe infection biology of Branchotenthes octohamatus (Monogenea: Hexabothriidae) from the gills of the southern fiddler ray, Trygonorrhina fasciata (Rhinobatidae), was studied using the fluorescent dye, 5(6)-carboxyfluorescein diacetate N-succinimidyl ester (CFSE). This is the first use of this technique on a monogenean species with unciliated larvae and the first for any monogenean larva infecting an elasmobranch host. CFSE-labelled post-larvae were recovered from gills of T. fasciata within 30 min of exposure to the host, providing strong evidence that larvae invade host gills directly and do not migrate after initial attachment elsewhere. The rapidity with which larvae settled suggests that the mode of infection may deliver larvae directly to the gills via the host's inhalant respiratory current. The specificity of B. octohamatus was investigated by exposing a sympatric rhinobatid host species, the western shovelnose ray, Aptychotrema vincentiana, to B. octohamatus larvae newly emerged from eggs laid by adult parasites from gills of T. fasciata. Experimental exposure of A. vincentiana to freshly hatched B. octohamatus larvae resulted in a persistent infection, indicating that B. octohamatus may not be strictly host specific. Post-larval development charted on these experimentally infected A. vincentiana specimens was slow. Parasites appeared to be sexually mature at 91 days at 21–25°C. Branchotenthes octohamatus larvae bear only 4 pairs of hooklets on the haptor whereas all other hexabothriid larvae described so far have 5 hooklet pairs. Ontogenetic changes to the haptor revealed that it is probably hooklet pair III that is lost from B. octohamatus prior to larval development.

An indirect immunocytochemical technique combined with confocal scanning laser microscopy has been used to demonstrate immunoreactivities to the nonapeptide, RPPGFSPFR (bradykinin, BK) and the endogenous flatworm regulatory peptide, GYIRFamide in the nervous system of the monogenean, Diclidophora merlangi. In addition, a simultaneous double-labelling technique was employed to examine possible co-localization of GYIRFamide- and neuropeptide F (NPF) immunoreactivities, using antisera to the C-terminal nonapeptide-amide of NPF (Moniezia expansa, FAIIGRPRF.NH2). BK immunostaining was restricted to a small population of nerve cells and associated fibres within the ventral nerve cords and to 2 pairs of nerve cells innervating the cirrus and the pharynx, respectively. No immunopositive nerve cells and fibres were identified within the brain or in association with the female reproductive apparatus. In contrast, GYIRFamide staining was abundant throughout the central and peripheral nervous systems, and appeared similar to the staining pattern revealed using an FMRFamide antiserum. GYIRFamide immunoreactivity was localized to nerve cells and fibres within the paired cerebral ganglia and the longitudinal ventral, dorsal and lateral nerve cords and their numerous interconnecting transverse commissures. The plexuses of the buccal suckers, pharynx and clamps of the haptor were strongly immunopositive for GYIRFamide, as were nerve cells innervating the ootype, the oviduct and the vitelline reservoir of the reproductive apparatus. Double-labelling experiments indicated an apparent co-localization of GYIRFamide and NPF immunoreactivities.

SummaryOögyrodactylus farlowellaegen. et sp.nov., an oviparous monogenean, is described from the skin and fins ofFarlowella amazonum, a South American loricarid catfish. The Oögyrodactylidae fam.nov., containingO. farlowellae and Phanerothecium caballeroiKritsky and Thatcher, 1977, is placed with the Gyrodactylidae in the Gyrodactylidea. Although oviparous, the Oögyrodactylids are closely related to the viviparous gyrodactylids. Both families share a combination of characters, including haptor morphology, articulated marginal hooks and spike sensilla, which is not seen in other monogeneans.O. farlowellaeis similar to a hypothetical ancestor of the gyrodactylids and shows that the family arose from monopisthocotylean rather than polyopisthocotylean stock. There is a close resemblance between the reproductive system of the immatureOögyrodactylusand that of the matureGyrodactylus, suggesting that progenesis, involving the precocious maturation of oocytes, may have been important in the evolution of the viviparous genera.O. farlowellaeretains eggs individually in the ootype until embryonation is comparatively advanced. After oviposition the eggs adhere to the substrate and hatch to give a crawling, unciliated larva. These are adaptations to a host which inhabits fast-flowing streams, where the eggs and infective larvae are exposed to a constant, uni-directional water current. The life-cycle (egg to adult) is completed in 11–13 days at 27 °C.

A comparison of the oncomiracidia of the monogenean skin parasitesEntobdella hippoglossifrom the halibut (Hippoglossus hippoglossus), E. diademafrom the sting ray (Dasyatis pastinaca) andE. soleaefrom the skin of the common sole (Solea solea) has revealed the presence of ‘body gland cells’, all of which have openings on the ventral or lateral surfaces of the larvae. In all three species there are two pairs of glands lying in the posterior region of the body proper with long ducts extending into the haptor and opening on its ventral surface. In addition to these two pairs of glands there are seven other pairs of body glands inE. hippoglossi, six pairs inE. diademaand two pairs inE. soleae.InE. soleaeall the ‘body gland cells’ are poorly developed in the oncomiracidium and are more conspicuous in post-oncomiracidia. The fate of the ‘body gland cells’ during post-oncomiracidial development has been studied inE. soleae, and their possible functions are discussed. The glands in the head region (anterior median, posterior median and lateral head glands) are similar in all three species. Gland cells associated with the alimentary system are less easy to observe but differences between the three species appear to be small.In the oncomiracidia of all three species the number of flame cells and their distribution are the same, but the oncomiracidia ofE. hippoglossiandE. diademahave loops in the excretory ducts anterior to the bladder and the bladders ofE. diademacontain refringent bodies.The oncomiracidial eyes ofE. hippoglossiandE. soleaehave lenses but the lenses are greatly reduced in the oncomiracidia ofE. diadema.In the three larvae there are small but characteristic differences in the shapes of the accessory sclerites.The oncomiracidium ofE. hippoglossihas not previously been described.

SUMMARYCholinesterase activity has been demonstrated histochemically in the nervous system, tegumental and sub-tegumental musculature of the haptor and in the pharyngeal bulb of the oncomiracidium ofPricea multae. The presence of cholinesterase in the nervous system is attributed to neurotransmission and its presence in the tegumental and sub-tegumental musculature of the haptor and pharyngeal bulb is discussed. The nervous system of the oncomiracidium with respect to cholinesterase activity is described and is compared with the nervous system of the oncomiracidium ofProtopolystoma xenopodis.

SUMMARYA description is given of some features of the embryonic development, the structure of the oncomiracidium and the postlarval development of R. emarginata, a hexabothriid parasite on the gills of Raia clavata.There are three phases in the larval development of the haptor of Rajonchocotyle: an oncomiracidial or marginal hook stage, a hamulus stage and a sucker stage. Neither the embryo nor the oncomiracidium ever possesses more than five pairs of marginal hooks and marginal hooks I (the posterior-most pair) are considered to be missing. The marginal hooks develop within distinct binucleate oncoblasts. During the early stages of post-oncomiracidial growth the secondary attachment organs of the haptor are formed and it is only after the completion of the development of the haptor that the reproductive organs begin to appear. Firstly, a pair of hamuli is acquired, followed by four pairs of suckers, which form in posterior-anterior succession at the site of marginal hooks III-VI. The first pair of suckers remains unarmed while the other three pairs acquire hooked sclerites and become the main functional attachment organs of the adult. As the first pair of suckers appears the posterior part of the haptor lengthens to form a caudal appendage. Hexabothriids are considered to show closer affinity to the 8-suckered chimaericolids and diclidophorids than to the 6-suckered polysto-matids, the unarmed suckers representing a simplification, by loss of the sclerite and reduction in sucker size, of an originally armed sucker.I would like to express my thanks to Dr J. Llewellyn for his interest and for much helpful discussion.Thanks are also due to the Director and Staff of the Marine Biological Association at Plymouth, in particular Mr J. E. Green for material and assistance, and to Dr A.j Brinkmann, of the Department of Zoology, University of Bergen, for supplying specimens of Squalonchocotyle borealis.

The muscular saucer-shaped haptor of the monogenean parasite Entobdella soleae is attached to the skin of its host Solea solea by means of a suction pressure generated within the sea-water-filled cavity enclosed between the cup-shaped haptor and the skin of the fish. The suction pressure is produced by the action of a pair of extrinsic muscles which are situated in the body of the parasite. Each extrinsic muscle communicates with the haptor by means of a long tendon which passes through a fair-lead in a prop-like accessory sclerite and is inserted on the end of a girder-like anterior hamulus embedded in the roof of the concavity of the haptor. The pull exerted by the muscles lifts the girders and the roof of the suction cup in which they are embedded relative to the props, thereby producing a suction pressure.An electron microscope was used to investigate the ultrastructure of the tendon, which was found to consist largely of unbranched banded fibrils which differed from the collagen fibrils of vertebrate tendon in their diameter and periodicity.I would like to express my thanks to Dr J. Llewellyn for suggesting the problem and for much helpful discussion. I am also grateful to Dr M. P. Osborne for tuition on the cutting of sections for electron microscopy. The work was conducted during the tenure of a Fishery Research Training Grant from the Development Commission.

Opisthogynidae fam.nov. is erected for a group of Gastrocotyloidea Price (1959), in which the gastrocotylid clamps are limited to the four primary pairs, the ovarian field is posterior to the main testicular field, and the cuticle is finely plicated, producing forward-pointing serrations along the margins of the hind body. Opisthogyninae subfam.nov. have a slightly asymmetrical haptor due to unequal lengths of clamps and their pedicels, or to one row being sessile on glandular pads which may be confluent as a thick shelf on one side only. The clamps have an incipient internal asymmetry, the adaxial sector being the more developed. Opisthogyne keralae gen. et sp.nov., on Sphyraena acutipennis: with subequal clamps unequally pedunculated, retains the protohaptor with three pairs of anchors; the muscular penis opens in a lateral atrium, and the median dorsal vagina is unarmed. Gemmaecaputia corrugata Tripathi, 1959, and G. brinkmannii sp.nov., both on Sphyraena obtusata, have each a very conspicuous pre-oral gland-mass. They have unequally disposed and asymmetrical clamps on stalks but lack a protohaptor and anchors. The genital terminalis is similar. The genotype Gemmaecaputia corrugata is redescribed. The new species G. brinkmannii differs in having some post-ovarial testes, in the muscular penis being less heavily cuticularized and the egg being smaller; yet it is nearly twice the size of the genotype, and its pre-oral glands are in two groups of two rows, instead of a single continuous row. Both Allodiscocotyla chorinemi Yamag., 1953 and A. diacanthi sp.nov., occur on Chorinemus sanctipetri. In both of these, unlike the last two genera, the asymmetrical clamps have ribs in the walls. One row of clamps is sessile, on a shelf, or on separate pads, and the other is short-stalked. The protohaptor is retained by the adult. There is a median spiny eversible cirrus and a single lateral unarmed vagina. The genotype is redescribed from a new host. The new species differs in its four separate clamppads on one side, the opposite row being subsessile. There is only one pair of anchors (the largest) instead of three pairs; there are no par-ovarial testes (all are preovarial), and the internal details of the clamps differ. Pseudomazocraes Caballero & Bravo Hollis, 1955, is compared with and included in the Opisthogyninae. Pentatrinae subfam.nov. is erected for Pentatres sphyraenae Euzet & Razarihelisoa, 1959, since its gastrocotylid clamps are modified like those of Priceinae (Gastrocotylidae), but are internally symmetrical and disposed in an asymmetrical fringing-arc round the hind body; the three larger clamps are pedunculated. The muscular penis and atrium are sublateral. Vallisiinae Price, 1943, is emended to include only Vallisia and Vallisiopsis, with highly developed somatic asymmetry. Slight asymmetry is seen in the short-stalked clamps which are usually shifted backwards from the sessile row, the clamps of which are either near three marginal tabs separated by glandular clefts, or on four pads. The clamps themselves are symmetrical and gastrocotylid with ribs in the walls, but without extra effective sclerites. The protohaptor is broad and has one pair of anchors. In all there is a long median spiny eversible cirrus. Vallisia indica sp.nov. from Chorinemus sanctipetri, has sessile clamps on four separate pads and is nearest to Vallisia chorinemi Yamag. This latter, however, has three marginal tabs for the sessile clamps and the combs in the two lateral vaginae are different. V. indica has only half the number of testes and lacks oesophageal diverticula.Protomicrocotyloidea sup.fam.nov. is created and defined to include aberrant discocotyline forms in which the protohaptor is secondarily developed as a main holdfast. This is a transverse musculo-glandular organ retaining the three dissimilar pairs of larval anchors. The other main holdfast is the unique posteriorlateral glandular pit with projecting muscular lips, just anterior-dorsal to the relatively feeble clamps, which may be gastrocotylid or discocotylid, some always being inhibited. Carangixenus gen.nov. erected for Protomicrocotyle celebensis Yamag., 1953, shares Protomicrocotylinae with Protomicrocotyle mirabilis (MacCallum), since both have gastrocotylid clamps, but in the former they have additional sclerites and an asymmetry approaching that of Gotocotyla spp. (Gastrocotylidae). In Abortipedinae subfam.nov. the muscular protohaptor has anchor-like lobes at each end as in Protomicrocotylinae, but the four clamps on the uninhibited side of the euhaptor are simple discocotylid and without ribs in the capsule walls. The genital terminalia are similar, though the testes, always mainly pre-ovarial, are far more numerous. Abortipedia indica gen. et sp.nov., from the gills of Caranx hippos, differs from the second species Abortipedia pacifica (Meserve) comb.nov. in details of the clamps, in the lack of oesophageal diverticula, and in its uniquely convoluted ovary. The spines of the lateral genital atrium and penis, and those of the lateral vaginal plaque also, are different, and so are the hosts and geographical locations. Lethacotylinae subfam.nov. has been erected for Lethacotyle Manter & Prince, 1953, in which all the clamps are inhibited, and Bilateracotyle Chauhan, 1945, in which only the anterior pair of the four primary pairs is inhibited. In the latter, the clamps are simple and discocotylid. In both genera, the protohaptor is a large wide hemispherical musculo-glandular organ constricted off from the hind body, and is provided with a plicated cuticle, and bears the three characteristic pairs of anchors; and again in both genera, the posterior-lateral funnel-like organ is particularly prominent. The Protomicrocotyloidea are unique in the higher Monogenoidea in using parts of the body as holdfasts which therefore tend to be substitutes, functionally, for the clamps; they are unique with the doubtful exception of Osphyobothrus Yamag., 1958, in showing a secondary development of the protohaptor.I would like to thank, sincerely, Miss Nora G. Sproston for maintaining our correspondence and for the exchange of ideas on the present subject and particularly for her collaboration in the last section ‘Relationships of the Protomicrocotyloidea’; Dr N. K. Panikkar for providing facilities for my investigations at the Central Marine Fisheries Research Station, Mandapam Camp, and Dr C. C. John, for permission to collect material from the Marine Biological Laboratory of the University of Kerala, Trivandrum.

Adult specimens ofTrochopus piniattach themselves by folding the disk-shaped haptor around the edge of the host's primary gill lamella. They are able to move from one place to another with the aid of disk-shaped pads on the head region.Ducts from the posterior median gland cells, which are extensive in the head region of the adult, open on the periphery of each disk and exude an adhesive secretion.The oncomiracidium ofT. piniis described and evidence is offered in support of direct invasion of the gills by these free-swimming larvae. Special attention is paid to the development of the head region of the parasite. Newly attached larvae lose their eye lenses and possibly their anterior median gland cells; their lateral head glands are well developed and most probably serve for attachment of the head region, since the posterior median gland cells are poorly developed and appear to have no ducts. Later the posterior median gland cells and their ducts increase in numbers and, in association with the pads on the head region, take over from the lateral glands the job of attaching the head region.The paths of the tendons associated with the accessory sclerites on the haptor ofT. piniindicate that this gill parasite has evolved relatively recently from the ancestors of benedeniid skin parasites.I am most grateful to Commander J. Pareira, the Director of the Aquario Vasco da Gama in Lisbon where most of this work was done, and to the Staff of the Aquario and especially to Mr A. Ferreira. I am indebted to the Calouste Gulbenkian Foundation for a generous grant which made this visit possible. I am grateful also to the Directors and Staff of the Laboratoire Arago, Banyuls, France, the Station Biologique of the University of Bordeaux at Arcachon in France, and the Laboratory of the Marine Biological Association of the United Kingdom at Plymouth, England. I would like to thank the Royal Society for a grant from the Browne Fund which enabled me to work at Banyuls in 1962 and for a Pergamon Travel Award which made it possible to visit Arcachon in 1967.

A description is given of the morphology and behaviour of the oncomiracidium of the monogenean D. sagittata. The larva has many features in common with the oncomiracidium of Diplozoon paradoxum but its development is less precocious than the latter. Suggestions are made as to the possible relationships of a pair of degenerate hooks on the haptor.

Both individuals of a pair of Diplozoon paradoxum Nord., 1832, when parasitizing the gills of Rutilus rutilus, are normally attached to the same hemibranch but never to the same surface of one primary lamella. A preference is shown for attachment to certain gills and also to particular locations on the gills.Diplozoon paradoxum displays asymmetry of its haptor when attached to the gills of the roach but the haptor assumes bilateral symmetry when unattached. As in Discocotyle, this impermanent asymmetry is facultative to the left or right, dependent upon the incidence of the gill-ventilating current, so that the body of a worm will present the least amount of resistance to the water as it passes across the gills. It is probable that impermanent, facultative asymmetry occurs in other species of the genus Diplozoon, and the usefulness of the asymmetrical nature of the haptor as a taxonomic criterion is therefore extremely limited.It is a pleasure to record my thanks to Dr F. G. Rees, University College of Wales, Aberystwyth, for her encouragement and guidance and to Dr J. Llewellyn, University of Birmingham, for his valuable help and critical reading of the manuscript. I am indebted to Mr D. Haigh for supplying me with fishes. This work was done while receiving a grant from the Department of Scientific and Industrial Research.

The adhesive organs of diplectanid, tetraonchid and dactylogyrid monogeneans develop in very similar ways indicating that all these gill parasites are closely related to each other. The oncomiracidia make initial contact with the general body skin or the lining of the buccal cavity to which they attach themselves by 14 or 16 ventrally orientated marginal hooklets. The growing parasites then find their way to the gills which call for a different kind of adhesive apparatus. In order to anchor itself between two of the host's secondary gill lamellae, two pairs of the parasite's marginal hooklets migrate to the dorsal surface of the haptor and two of the four developing hamuli rotate so that they also point dorsally.In diplectanids the hamuli are assisted by accessory friction pads (squamodiscs) which develop on the surface of the body anterior to the hooks.In dactylogyrids the ventral hamuli do not develop and are represented in the adult only by small spicules. In contrast the marginal hooklets grow during larval development and become important adult attachment organs, together with the dorsal hamuli.Adults of Tetraonchus monenteron feed on the epidermis covering the gills.I am most grateful to the Director and Staff of the Laboratory of the Marine Biological Association at Plymouth and particularly to Mr J. E. Green for help and hospitality. I would also like to express my thanks to the East Suffolk and Norfolk River Authority and particularly to the Fishery Superintendent, Mr E. Q. Bitton, and to Mr D. Leary, for supplying me with freshwater fishes.

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.