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An account is given of the forms of Chthamalus depressus (Poli). An extreme hypobiotic form common in caves is described; it is shown that all forms may be found in very restricted localities (even on a single boulder) according to different micro-habitats. Some biometrical data on the species and regressions of the several variables are given; they indicate that thickness tends to increase with exposure. The effect of exposure on growth-form is considered and compared with similar data on the laying down of the shell in molluscs. Thickening of the shell is believed to be due to withdrawal of the mantle tissue under wave action. The effect of light on the penetration of algae is discussed; animals growing in darkness have no endolithic algae. Erosion is considered to be due to mechanical forces acting on shells much weakened by endolithic algae. The changes on transplantation to Scotland have been observed, and measurements of the growth rate made; growth is slow, but somewhat greater under conditions of total immersion. The reproductive and moulting cycles, fertility, and general behaviour have been determined and the results compared with similar data on C. stellatus (Poli). The results are discussed in relation to the ecology of C. depressus and, in particular, relative to possible competition with C. stellatus.

The effect of changes in salinity and temperatures on the survival and osmotic behaviour of Verruca stroemia (O.F. Muller) have been investigated. The species is an ‘osmoconformer’ over a wide range of salinities, but the haemolymph is maintained slightly hypertonic to the medium. The species survives over a wide range of salinities. The ecological implications of the observations are discussed.

A summary is given of the distribution of Balanus balanoides (L.) and of earlier work on differences — racial and otherwise. The variation in the date at which different populations are fertilized is considered. Data are given on the time required for the development of the embryos of this species in vitro at 10 °C, with eggs from a number of populations from the eastern and western Atlantic and examined soon after collection, and also after transplantation and maintenance under ambient laboratory conditions over the next breeding season. Both aspects of reproduction are under genetic control. There is no correlation between the date of fertilization and the time required for development. Oocyte and egg size appear to be largely controlled by temperature. On both sides of the Atlantic there is a general tendency for the time of development to decrease with decreasing latitude. The population at Millport, Scotland appears unusual. The southernmost populations on European shores resemble those at the extreme south in North America. A study of the effect of temperature has shown that at lower temperatures the time of development of the embryos in vitro from all populations tends to be the same; the difference lies in the Q 10 values. The results are discussed relative to palaeontological data and the existence of clines in other features.

The structure of the calcitic shell of extreme hypobiotic Chthamalus depressus (Poli) using untreated, treated, and polished material has been investigated by scanning electron microscopy, and supplemented by observations on normal forms and C. Stellatus (Poli). The laterale has been largely used. The plate is made up of prisms consisting of stacks of units ≈ 5 μm high; the prisms are separated by matrix and intraprismatic organic matter may also be present. The inner side of the plate is penetrated by holes. The structure of the wall plate is reflected in that of the sheath. Epicuticular lamina run across the wall plates separating the stacks of prisms. Attention is drawn to the possible implications with respect to the recent controversy regarding the form of chitin in crustacean endocuticle. The various structures give the surface a ‘banded’ appearance. From the available data on growth rate, shape, and moulting frequency, estimates are made of the expected size of these bands; the results agree reasonably well with the observations. The frequency of the smaller bands would seem to be endogenously determined by processes taking place within an intermoult period.

The protein, chitin, calcium, and magnesium present in the shells of several species of cirripedes have been determined; the powdered shell material was also examined by differential thermal analysis (DTA). Chthamalus sp., in which lamina of epicuticular matter are present within the shell, show the greatest organic content. When the organic material is separated into its protein and chitin components the latter is present in similar quantities in all species; this reflects a similar quantity of the chitin-protein matrix. Reduction in the protein content of heavily eroded C. depressus seems to be due to the removal of protein by endolithic algae. The Ca Mg ratio increases from extreme hypobiotic to exposed C. depressus. There is a marked correlation between the Ca Mg ratio and the organic content and it is suggested that the magnesium is largely associated with protein rather than with the calcitic lattice. This receives support from the thermal analyses. Small quantities of quartz were detected by X-ray analysis.

A study has been made at the ultrastructural level of the formation of the cuticle in some cirripede larvae. The exoskeleton of both the stage I and stage II nauplius is laid down during the later stages of the embryonic development. In both stages it consists of a trilaminate epicuticle and a laminated but otherwise uniform procuticle which is not further differentiated. In stage I the epicuticle bears on its surface hemispherical ‘blebs’; prior to the deposition of the main part of the stage II cuticle the procuticle of stage I is largely absorbed so that only stage I epicuticle is present on hatching. The epicuticle of newly developed stage II appendages tends to resemble that of stage I. The functional significance of the form and variations in exoskeletal structure in the two nauplius stages are discussed.

The changes taking place in the oviducal sac of Pollicipes cornucopia Leach as the oocytes enter are described. Fluid may first be forced through the pores before their closure, but as the sac is distended a granular secretion is forced into them as the inner reticulum becomes disorganized. On the basis of these observations it seems that spermatozoa must enter the sac before it is grossly distended if they are to pass through the pores. Transmission electron micrographs are given showing spermatozoa in the pores and in the lumen of the sac of Balanus balanus (L.). The pore size is adequate for the spermatozoa only after the accessory droplet has been lost. Many spermatozoa within the lumen appear to have disintegrated, the membranes having been lost and the axonemes separated. Some calculations of the time required for penetration and the number of spermatozoa able to enter the sac, assuming one passes through each pore, are given. The origin of the various secretions is discussed and their possible relation to the motility of the spermatozoa considered. The advantages of pseudo-copulation are stressed.

The possible modes of breakdown of the oviducal sac which comes to surround the egg lamellae in cirripedes, has been investigated in a variety of common species. Bacterial action appears to be unimportant. The effect of protease on the sac has been investigated. The rate of breakdown is unrelated to season when it is considered relative to the stage of embryonic development. Apart from Verruca stroemia Muller which is not attacked, the initial rate of ‘dissolution’ is rapid, reaches a minimum, and then increases. Although the time course is similar in all the species examined, there are quantitative differences between species. The differences may only reflect differences in the protein composition of the sac and their resistance to protease. Under the action of its own enzyme species may show much less differences. It is considered that the results indicate that the enzyme necessary for proteolysis is produced by the embryos themselves at a certain stage in their development. The results are discussed relative to other known hatching enzymes.

That the absence of breeding in natural populations of Verruca stroemia (O. F. Muller) during the autumn and early winter is due to the absence of food has been confirmed; animals brought into the laboratory and held under ambient light and temperature conditions produce viable egg masses throughout this period when liberally fed. Breeding is not inhibited in either constant light or dark. A temperature of 20°C is lethal even over moderate periods. There is no increase of moulting frequency between 10° and 15°C.

The structure of the oviducal sac of Pollicipes cornucopia Leach and of Balanus balanus (L.) has been examined by scanning and transmission electron microscopy. The sac in various species has been given a variety of names and the nomenclature is considered. The outermost part of the sac i.e. , that exterior after extrusion, consists in Pollicipes of uniformly electron-dense material with an even denser boundary layer. Pores lead from the outside to the interior of the sac and they contain remnants of the cytoplasmic filaments. The pores are continued towards the lumen in dense reticulate material projecting in a stalactite-like form towards the inside of the sac. In addition to this dense reticulum, finer reticular material is present which has been shown to be ‘liquid’ before fixation. Numerous fine ‘holes’ — as distinct from true pores — are present in the dense outer layer. The cytoplasmic filaments contain microtubules (or rods), and small vesicles on the cell membrane seem to aggregate to form the coarse reticulum. Similar structures are present in Balanus balanus , but the reticulum is less dense and less concentrated round the pores. Proximally, i.e. , towards the entrance to the oviduct, the pores gradually become reduced in number eventually being virtually absent, and the wall is less organized.

The optical density of the shell-material and that of the underlying pigmented epithelium has been determined for a number of species — littoral, sublittoral, operculate, and pedunculate. In all the species except Lepas fascicularis Ellis & Solander there is a marked change in the attenuation coefficient at a thickness of ≈ 0.20 mm; above and below this thickness log (100 I I 0 ) is linearly related to thickness i.e., the attenuation coefficient is constant. In Balanus crenatus Brug. the optical density appears to reach a constant value independent of thickness. The shell material transmits more at longer wavelengths but the pigmented epithelium acts virtually as a neutral density filter above ≈ 400 nm. Some properties of the pigment are noted and its development relative to the degree of insolation was investigated. The bearing of the results on the light-limitation of endolithic algae and the response of the photoreceptors to light are discussed.

The effect of hypo- and hypersaline media on the development of the embryos of the common cirripedes, Balanus balanoides , (L.), B. crenatus Brug., B. balanus (L.), B. perforatus Brug., Chthamalus stellatus (Poli), and Elminius modestus Darwin has been examined. These species represent littoral and sublittoral, boreo-arctic and warm-temperate, relatively steno- and euryhaline, and steno- and eurythermal forms. The range of salinity over which the embryos would develop to give free-swimming stage I nauplius larvae, no matter at what stage of development the cultures were started, was similar in all species, namely 15–20 to 40%. Low-salinity media cause cytolysis while high salinity media give grossly abnormal embryos which may, however, develop further. There seems little direct relation between salinity tolerance and that of the habitat in which the animals are found; a capacity for acclimation may be more important. The causes underlying the production of abnormalities are discussed.

Because of the small amount of pigmentation the difficulties associated with earlier histological work on Chthamalus stellatus (Poli) are absent in extreme hypobiotic C. depressus (Poli). The formation of the wall plates has been investigated by normal histological techniques and supported by scanning electron microscopy. The wall plates are covered by epicuticle which in its continuity resembles that of Acasta spongites ; the epicuticle is strongly ribbed and the plate has more or less regularly spaced, volcano-like outgrowths. The various cell-types and the part they play in the expansion of the basis and growth of the wall plate are described. A series of ‘bridging’ cells are present and lie between the lower part of the last completely formed ‘scale’ and the basal cells. Growth is considered from the evolutionary point of view and in relation to the forms assumed under natural conditions, in particular to the capacity of the animal to adapt to changes in the rock surface.

New records for Elminus modestus Darwin, and a single one for Balanus eburneus Gould are given for the western coast of France. The local population of B. balanoides (L.) at Arcachon (Gironde) has increased in extent and population density over the past few years; a curious growth form of this species — probably resulting from intense competition with Chthamalus stellatus (Poli) — is described.

The various terms which have been used to describe some quantitative parameters of considerable importance in the reproductive cycle of marine invertebrates are discussed; egg number, fecundity and the metabolic efficiency of egg production are defined in a way applicable to animals such as many crustaceans in which the number of eggs produced is a function of adult size. The variation with tidal level and latitude of egg numbers as well as their changes from one season to another are described and discussed for a number of common cirripedes. Local factors may often obscure latitudinal variations. There is no evidence that egg numbers or fecundity are greatest under poor conditions or at the limits of distribution; indeed the reverse seems to be the case. The metabolic efficiency of egg production, namely, the weight of egg tissue produced per unit time per standard increase in body weight is relatively constant for many operculate species even when they are of different adult or different egg size. It follows that fecundity, the number of eggs produced per unit time per standard increase in body size, is largely a function of egg size. The importance of these factors to the ecology of the animals and to their distribution is discussed.

The general morphology and detailed histology of the penis of two common boreo-arctic cirripedes, Balanus balanoides (L.) and B. balanus (L.) have been investigated. The penis is a highly extensible, annulated organ beset with four rows of sensory setae. The paired vesiculae seminales unite within the pedicel of the penis to give the single ductus. Distally, the exoskeleton is invaginated into this ductus. Circular muscles are present in the vesiculae seminales but do not continue into the penis. The histology of the ductus epithelium indicates a secretory nature as does that of a specialized group of cells, termed the ‘cushion’, towards the distal end: this group of cells is surrounded by circular muscle bands. Longitudinal muscles extend virtually the whole length of the penis; they give off fibres which are inserted at the junctions of the annulations. The muscles of the pedicel are described. Paired nerves in the pedicel give rise to four in the penis. The sensory innervation of the setae is described. The possible functional relations of the structure to the activities of the penis at copulation and during the emission of semen is discussed.

The QO2 of three common cirripedes has been measured at four temperatures and on each of four occasions during a year: dissected bodies were used. Statistical analysis indicates that only infrequently is the Q10 weight dependent. Such dependence seems to be related to the nutritional conditions in the environment rather than to the internal state or species habitat. Q10 varies widely even in a given species. In Balanus balanoides there is a marked homeostasis at certain seasons as there is in Chthamalus stellatus at other seasons: Balanus balanus shows somewhat less marked homeostasis. A low Q10 of basal metabolic rate in littoral invertebrates has been observed by others in a variety of animals. The results of some relevant previous work are critically examined and compared with those of the present investigation.

Simultaneous observations with echo sounders and underwater television have been made both in and out of deep scattering layers. All the evidence indicates that, with type of sonic gear used, micro plankton was not responsible for either the observed deep scattering or for detectable echo sequences on the short range equipment. It was possible to “see” for the first time the sound scatters responsible for the typical echo sequences from single animals and to confirm the deductions of Johnson et al. (1956) that individual small fishes give rise to the sort of echo sequences that have been seen in deep scattering layers. The type of echo record and its relation to the number of fish scattering the sound was observed.

The effect of starvation for various periods and also at various times during the development of the ovaries of Balanus balanoides (L.), followed by feeding, has been investigated. The total feeding time to full maturation shows little variation. The time at which the egg masses are laid down can, however, be to some extent manipulated by controlling the starvation period.

Further records of the occurrence of Elminius modestus and changes in its population density at two localities in France, namely, the Baie de St Malo and the St Jean-de-Luz area are given. In the latter region the species is now well-established, although there is some evidence that colonization took place perhaps only a few years ago. The Dordonne estuary may have been the primary site of colonization from which it has been carried southwards as far as Hendaye Plage. The coast between St Quay Portrieux and Cancale now has abundant Elminius in all places where there are suitable habitats: it seems from the available records that this coast had for a long time remained uncolonized. Northward from Granville on the western side of the Cotentin peninsula there is still a scarcity of Elminius. Small populations may be established each summer only to be largely destroyed in the winter. The findings are discussed in relation to previous surveys.

An account is given of some observations on Elminius modestus Darwin along the coasts of south and south-west Europe including northern France, Spain and Portugal. The consolidation and extension of the immigrant are discussed.

The structure and innervation of the sensory setae which are present in large numbers on the penis of Balanus balanoides (L.) have been established by scanning and transmission electron microscopy. Each seta contains a small number of sensory dendrites surrounded by an extracellular supporting tube which is presumed to be secreted by the enclosing sheath cell. The dendrites, which distally extend beyond both the sheath cell and supporting tube, terminate at the tip of the seta within a pore-like invagination of the cuticle and thus are in direct contact with the environment. Proximally bundles of dendrites pass into the penis tissue where they are surrounded by several sheath cells. The supporting tube terminates at a point within the body of the penis where a series of intracellular rods arise. The ciliary character of the dendrites is evident in this region, the microtubules being organized into the (9 × 2) +2 pattern. It is deduced that the sensilla are chemosensory; their structure is compared with that of other crustacean sensilla which are presumed to be chemosensory.