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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.