Most of the work which will be described in this thesis is concerned with the structure and function of a small organ, the osphradium, present in freshwater snails of the Pulmonata, Gastropoda. It was hoped to compare the osphradium of the Pulmonata with the similarly named organ in other groups of the Mollusca.

The main part of the thesis is a description of the reproductive system of the opisthobranch molluscs Philine aperta, Acteonia cocksii, Alderia modesta, Archidoris britannica and Idulia fraRilis. Reference is made to ten other genera which differ from these only in detail. In all, the initial part of the reproductive duct, which corresponds to the gonadial and renal sections of the prosobranchs, shows uniformity of structure and histology and part functions as a vesicula seminalis. The pallial region of the reproductive duct of a prosobranch contains oviducal and vaginal channels, but with the development of hermaphroditism the opisthobranchs require an additional male pathway for outgoing sperm. In the tectibranchs (except the Pleurobranchidae) incomplete separation of oviducal, vaginal and seminal channels is achieved by means of septa; in all others three separate ducts are present, the arrangement of which conforms to one of three plans represented by the Sacoglossa, the dorids and Idulia respectively. All three types may be derived, independently, from tectibranch-like ancestors. The development of a penial sheath in the opisthobranchs may be correlated either with the loss of mantle cavity or with the retention of the penis at the anterior end of the body remote from this. Two vesicles are associated with the vaginal pathway -a receptaculum and a bursa copulatrix. The functions of these vesicles are compared with those of the corresponding structures in the prosobranchs. Since male and female gametes are present simultaneously in the reproductive ducts the mechanism by which self-fertilization may be prevented is discussed. The type of spawn and its formation are compared with those of prosobranchs.

The snail Helix aspersa Müller on the Great Orme's Head Llandudno is usually found among rocks, hidden by layers of long thick grasses, or in the crevices of the rocks. They are usually found in groups both in winter and summer except during wet weather. The animals use some limestone boulders regularly as hibernacula, other boulders are used irregularly during winter, and some are used almost equally throughout the year. Juveniles of Helix aspersa grow very rapidly during late Spring and early Summer and. they grow fastest when their height is between 15 and 20 mm. Juveniles pass through one or two hibernation periods before reaching maturity. The population size varies considerably and is highest during late Spring and Summer. The highest number was during Summer 1976 when the area experienced long dry spells. This increase was due to immigration. The estimated survival rates for Helix aspersa were too low considering that some of them were known to survive for several years. This low estimate of survival rate was due principally to emigration. The animals' movements are maximum during Spring and Summer. There is no indication that Helix aspersa in the area showed any directed seasonal movement, but this species shows homing within a limited range. Helix aspersa at Llandudno and Birkdale consume a wide range of green plant materials. The major plants consumed are grasses and dicotyledonous leaves which are common in their habitat: some selection is shown in the choice of food. - The food plants are assimilated very efficiently by the snails, and their assimilation is both temperature and substrate dependent. Growth of Helix aspersa is influenced by temperature, light regime and calcium. The animals grow faster at higher temperatures and in alternating light and dark,. and. animals reared with excess of chalk grow faster and have heavier shells than those without. The activities of Helix aspersa, Cepaea nemoralis, Hy romia striolata and Candidula intersecta are temperature dependent. They show maximum activity between 13 and 22 C. Helix aspersa is the least active of the four species, Hygromia striolata the most active. In addition, Hygromia striolata shows activity at lower and higher temperatures than the other species. Cepaea nemoralis is more tolerant to low temperatures than Helix aspersa but less tolerant to-high temperatures. Increase in density has an influence on growth, mortality and food consumption. Interactions between snails might be responsible for slowing the growth rates in high density populations. The presence of mucus trails of other snails also inhibits activity but their effect seems to be short-lived. Space has an adverse influence on growth and mortality of the animals if it is provided below their requirements.

The present investigation on the biology of Bithynia tentaculata has two aims. Firstly to report on the life cycle, growth, reproduction and other aspects of the biology of this snail, and secondly to identify the larval trematode infection of the snail and to report on the incidence, seasonal variation and the development of these infections. The study area was a small artificial pond in Sefton Park, Liverpool. The biology of the snail was investigated by regular sampling of the habitat for 22 months, the snails were measured and the breeding activity was assessed. Laboratory experiments were performed on the growth, reproduction and natural diet of the snail. The larval trematode . infection of the snail was investigated and experimental infections were made. A monthly survey of the incidence of these infections was made for 13 months. The following results were obtained on the biology of the snail; 1) Growth of the snails practically ceases in the winter and the wintering population resumes growth in the spring.. The new .'F generation of snails äppears in June and growth is rapid during the summer. 2) The snails begin to breed in mid-April and active breeding is limited to April-June, but sporadic egg-laying occurs in July and August. 3) There is a minimum breeding size of the snails which varied from one year to the next depending on the growth rate. In addition to size, there is a minimum period of five to six months for maturity (oviposition) of the snails. 4) The onset of egg-laying is controlled by temperature, and the snails did not oviposit at 10-120C. 5) The snails show optimal growth when fed on detritus, and algae, growth is retarded on a diet of detritus alone. 6) Winter and unfavourable conditions cause aggregation of " the snails, and organic pollution is detrimental to their distribution within the habitat. 7) The life span of Bithynia tentaculata is about 14 to 23 months, and possibly few survive to breed for a second season. Results obtained on trematode infections of Bithynia tentaculata show the following:. 1) Six species of cercariae infect the snail, a Monostome cercaria, three Gymnocephalous cercariae, a Xiphidiocercaria and a pharyngeal longifurcate monoctome Furcocercaria. ti 2) One of these cercariae (i. e, Cercaria helvetica XIX) is a first record in British freshwater. Existing descriptions of some of the cercariae are expanded, and new descriptions of some developmental stages. are added. 3) The life cycle of Notocotylus imbricatus is traced and metacercarial infectivity is demonstrated soon afer encystment. The development of Notocotylus imbricatus in the snail host is followed and descriptions of the developmental stales are presented. 4) Multiple cercariae infections are generally rare, yet , It certain combinations of double infections are more frequent than expected. 5) The total cercariae infection of the snail, as well as four individual species of cercariae, show biannual peaks of incidence. No biannual peaks of metacercariae infections were detected. 6) Many cercariae infections enter the new generation of snails in their first summer. Some of these infections may mature in the same season, but most of the infections are carried through the winter in an immature state. New infections enter the snails in the spring, but few infections enter the snails in their second summer of life.

This study examines the population behaviour of the ovo-viviparous mollusc Potamopyrgus jenkinsi in a number of freshwater habitats. Two generations were normally completed annually but a single annual cycle also occurred. Two main overwintering strategies were recorded involving either high mortality of all but small individuals and their subsequent attainment of maturity in late June, or high winter survival with the consequent ability of adult snails to begin significant young production in early May. Reasons for observed variations in the lifecycle are discussed. Large scale migrations from deep to shallow water were recorded in the spring. Reproductive rates in field and experimental populations were investigated on the basis of cage experiments, embryo number and ratio of developmental stages. Reproduction was continuous and was closely correlated with temperature. Growth and mortality rates were also investigated. Snail populations about to undergo heavy winter mortality were characterised by the presence of fully developed embryos only; adult mortality at other times was not accompanied by changes in reproductive condition. Evidence of density effects in the field are discussed. Onset of maturation was characterised by changes in shell configuration in which the aperture formed a small lip or shoulder, precluding further significant growth. Final (maturation) size varied seasonally due to different growth rates, and was greatest in May and J1me and least in August and September. The implications for reproductive rate and development are discussed. Maturation size was combined with a quantitative assessment of shell deposit to determine relative age and survival of adult snails. These indicated that regulation of breeding populations occurred both through sub-adult and adult mortality, the relative importance of these mechanisms varying at different times of year and between generations Records of the patchy and apparently aberrant distribution of P. jenkinsi may be partly attributable to its variable population behaviour but specific habitat requirements, particularly in relation to substratum, were also indicated. The sites examined were to some extent atypical, reaching extremely high densities and resembling estuarine populations of hydrobiids in this respect. A comparison with production levels attained by other freshwater, and marine, molluscs is carried out and related to the probable position of P. jenkinsi in freshwater habitats.

No description available.

No description available.

In a typical mollusc, the shell is the most important defensive mechanism. But the shell does not protect the mollusc from all predators, and it is for this reason that secondary defensive mechanisms have evolved in many species. In some cases, these secondary mechanisms become more important than the shell as a means of protection from predators, and where this occurs selection may favour the reduction and even the total loss of the shell. The shell has been lost independently in each of the nine orders of the Opisthobranchia except the Thecosomata, and in the Sacoglossa and the Nudibranchia it may have been lost two or three times. In all of these cases where the shell-less or nudibranch condition has evolved, the animal must be adequately protected by defensive mechanisms other than the shell. This thesis studies the defensive mechanisms of a number of nudibranch molluscs. In the Sacoglossa there is a series of forms from the primitive Arthessa, with a well-developed defensive shell, through Oxynoandeuml; and Lobiger, with reduced shells, to the nudibranch Stiliger on the one hand and to the bivalved Berthelinia on the other. The defensive mechanisms of Berthelinia and Stiliger are compared. It is shown that even in Berthelinia. which is protected by a tightly closing bivalved shell, there is a secondary defensive mechanism in the form of a secretion from the hypobranchial gland. In Stiliger, the defensive mechanisms are located in the cerata. Three types of gland are present of which at least one, and probably all, are defensive. In addition the cerata can be autotomised and regenerated, and this may also be of defensive importance. It is also shown that the ceratal glands of Stiliger and the hypobranchial gland of Berthelinia are muscle-operated. It is concluded that in both Stiliger and Berthelinia the defensive system involves two or three defensive mechanisms; and there is evidence that these mechanisms act in series just as do the defensive mechanisms of many tropical insects. The defensive mechanisms of the Doridacea are discussed. It is concluded that camouflage is of widespread occurrence, but that there is as yet no evidence for the occurrence of warning coloration. Many bright colours are deflection marks, but the importance of other bright colours is still not known. Dorsal papillae with a probable defensive function are present in some species of the Doridacea, and they are usually supplied with glands. The function of the caryphyllidia of certain species is not known. Autotomy of papillae has not been described, but autotomy of the mantle edge occurs in the Discodoridinae. Defensive glands are of widespread occurrence in the Doridacea. One type of defensive gland that has not been described before in dorids is the acid gland of Discodoris pusae and Anisodoris stellifera. Acid secretion is well known from certain other opisthobranch and prosobranch molluscs, and the discovery of its occurrence in dorids means that it has evolved independently at least four times in the Mollusca. In D.pusae and A.stellifera, the acid glands are large subepidermal pits with a mucous plug and a muscular sphincter. The acid is inorganic and contains sulphate ions. It may register pH 1 or 2 close to the skin of the mollusc. Another species of Discodoris was found to secrete acid of a similar pH. In this species acid-secretion is from the ordinary epidermal cells, and subepidermal glands are totally absent. In the light of these results, the evolution of acid-secretion is discussed. In all four suborders of the Nudibranchia and in the Sacoglossa, species with dorsal papillae or cerata have evolved. In all cases where these have been studied, they have been found to be of defensive importance. Cerata usually contain glands, are mobile, can be autotomised without harm to the mollusc, and may have yet other defensive mechanisms. The Eolidacea are the best known of these groups with defensive dorsal papillae. In the eolids, some species are camouflaged, but none has been shown to possess warning coloration. Cerata may be brightly coloured to direct attacks away from the head, and the behaviour of the mollusc supports this view. Cerata can be autotomised and regenerated, but the ease with which autotomy occurs varies between different species, Cerata are especially sensitive at their tips, and it is shown that this is because there is a high concentration of neurosensory cilia in this region. Stimulation, by touch, of these cilia can cause ejection of nematocysts or of glandular secretions as a defensive response. Ceratal glands are described for a number of species of solid. The role of the cellules spéciales is discussed, and it is concluded that these are not of defensive importance. Their content of RNA and of protein suggests that they synthesize protein, and their high glycogen content suggests that they are storage cells. They may have a duct in some species, and could thus be excretory. Simple, unicellular mucous glands occur in probably all eolids, but they may be epidermal or subepidermal in different species. They probably protect the animal from abrasion. They may also be responsible for the mucin cuticle, or for the presence of a mucous film just outside the cuticle which is continuously being carried off the tips of the cerata by ciliary action. Mucous glands are the only glands present in some species, for example Eolis M and Tergipes despectus. In other acleioproct eolids there are glands which have a defensive functions the species of Catriona studied have two types of defensive gland, whilst the eubranchids studied have three and possibly five types of defensive gland. These defensive glands are usually concentrated at the tips of the cerata and are exuded when the animal is violently disturbed. Some of them are muscle-operated and are proteinaceous, others contain mucopolysaccharides or mucoproteins. In the smaller species of both Catriona and Eubranchus, the surface area of ceratal epithelium is limited, and the defensive glands are packed so as to utilize all available space. The method of packing the glands is different in the two genera, suggesting that it is a case of parallel evolution. It is further suggested that the defensive glands of Calma, Catriona and the Eubranchidae are convergent. In the cleioprocts studied, no such concentration of glands at the ceras tip was found, but all species possess two or more types of ceratal gland. There is evidence that some of these may be defensive in function. Nematocysts are used in defence by many eolids, but for them to be effective, they must be ejected from the cnidosac and an appreciable percentage of them must explode. It is shown that whilst nematocysts are frequently ejected and exploded as a defensive reaction, there is considerable variation between different species. In Calma, glands are of considerable defensive value, but nematocysts are totally absent; whilst in Tergipes despectus, defensive glands are absent and nematocysts are important in defence. Other species utilize both defensive mechanisms, but to varying extents. In the Cleioprocta generally, and particularly in the Aeolidiidae, glands are not of great importance in defence whilst nematocysts are; but in the Eubranchidae and in Catriona glands are well-developed and of considerable defensive importance. There is even variation within one genus. Thus Catriona tina rarely uses glands but frequently uses nematocysts in defence, whilst C.perca rarely uses nematocysts but frequently uses glands. This interspecific variation in behaviour is probably related to the different predators which different species of eolid are likely to encounter. The predators of nudibranchs in the sea are not well known, but laboratory experiments have demonstrated that many nudibranchs are relatively unpalatable to many species of fish. However, some species of fish are not deterred from eating eolids by nematocysts, and it is likely that it is with relation to these predators that glandular defensive mechanisms have evolved. Thus nematocysts may be protective against one set of predators whilst glands are protective against another set of predators. Nematocysts may be damaging not only to potential predators of eolids, but also to the eolid itself when it crawls over coelenterates. The mechanism by which eolids escape damage from nematocysts is not known, but there is evidence that the vesicular structure of the epidermis is involved. It is concluded that in many nudibranchs the defensive system involves several distinct mechanisms which come into action in series. There is some evidence that certain mechanisms are adapted to specific predators. The eolidiform condition is a particularly efficient defensive adaptation since it concentrates several mechanisms into that part of the mollusc which is expendible, and which is the first to be encountered by a potential predator. Three papers are included in the appendix. The first describes the occurrence of Polycera elegans (Bergh) in Britain, and discusses its taxonomy. The second gives notice of a new species of bivalved gastropod from Jamaica, and the third is a description of this animal.

No description available.

No description available.