Showing papers in "Biological Reviews in 1973"

The structure of fish gills in relation to their respiratory function

Abstract: Summary 1. The general structure of the gills of different fishes is compared and it is concluded that, though essentially the same, there are certain differences by which they can be recognized. Possible ways in which they may have evolved from one another are considered. 2. A detailed account is given of the structure of the secondary lamellae, where gaseous exchange takes place, and it is shown that two epithelial sheets are separated by a vascular axis mainly composed of pillar cells overlain by a basement membrane on each side. Blood pathways through the gills are discussed in relation to their respiratory function. 3. The embryonic development of gills is described and evidence regarding homo-logies of different structures, particularly the pillar cells, is reviewed. 4. The gills of fish having different modes of life show variations in (a) the number of arches, (b) the number and length of the gill filaments, and (c) the size and frequency of the secondary lamellae. Ways in which measurements of gill area may be carried out and some of the complications involved are reviewed and a summary given of measurements made for a wide variety of species. Measurements of the thickness of the water-blood barrier are also discussed; the more active fish generally have thinner water-blood barriers and larger gill areas. 5. The different mechanisms of gill ventilation are summarized and characteristics of gill resistance in elasmobranchs and teleosts are compared. Gas exchange is discussed in relation to available techniques and the current terminology and symbols, and to indicate the value of analogies between gill exchangers and systems studied by engineers. 6. It is outlined how studies of the functioning of gills during coughing, parasitic infection, and in polluted waters add to knowledge of their role in respiration.

The chitin system

Abstract: SUMMARY 1The view is supported that chitin is not found in Deuterostomia because of the absence of chitin synthetase, and is not found in higher plants because of the absence of glucosamine. In Fungi, control mechanisms are present affecting the synthesis of glucosamine; chitin is often present, but when it is absent this probably results from a failure to synthesize glucosamine. 2A review of conformation maps for cellulose and chitin indicates the possibility of a slightly right-handed twist in small groups of chitin chains. 3The occurrence of α, β and γ-forms of chitin in the peritrophic membranes of various insects is described. Gamma chitin seems to be the commonest form. 4In several beetles, optical and electron-microscope studies trace the formation of chitinous cocoon fibres from larval peritrophic membrane and define the discrete ribbon-like nature of the, β chitin produced in the mid-gut. 5By studying apodemes it is found that orthopteroid insects are most varied, different molecular structures being present in levator, depressor and pretarsal tendons. By contrast, Hymenoptera and Coleoptera show very similar structures in all three apodemes as well as in other parts of the cuticle. Apodemes are regarded as sampling the cuticle at their varying points of origin; they provide especially favour able material for diffraction studies. 6In arthropod cuticles there is evidence for the widespread occurrence of α chitin micelles which are three chains thick in the direction of the c axis. This is compared with the structure of γ chitin where the chains repeat in groups of three along the c axis. 7Changes in the diffraction pattern are related to the series of proteins defined by Hackman. The chitin-protein complex is not affected by water or neutral salt extrac tion, but is disrupted by treatment in urea. 8Electron microscopy defines the unit of structure as a composite microfibril: a core of chitin surrounded by adsorbed proteins. This consists of ‘primary’ protein (often repeating as regular units along the fibrils) and a quantity of ‘satellite’ protein which obscures the imaging of the regularly arranged ‘primary’ protein. There are apparent ‘bridges' between the microfibrils. 9New diffraction data give information about the size and arrangement of micro-fibrils. These fibrils may be arranged in layers of ‘rods’, or as an hexagonal arrange ment of ‘rods’.

Concepts in fungal nutrition and the origin of biotrophy

Abstract: Summary 1. Since use of the terms, symbiosis and obligate parasitism, is ambiguous, an attempt is made to re-impose precision by a re-evaluation of basic concepts. 2. For symbiosis, there should be a return to a concept closer to that of de Bary's original, embracing those parasitic and mutualistic associations which involve permanent, intimate contact. 3. Culturability should be abandoned as a criterion for classifying fungal behaviour. 4. Instead of culturability, emphasis should be placed on the ecological and nutritional behaviour of fungi, permitting the recognition of five groups: obligate saprotrophs, facultative necrotrophs, obligate necrotrophs, facultative biotrophs and obligate biotrophs. 5. Since environmental conditions can determine whether the nutrition of a fungus is biotrophic or necrotrophic, a scheme, speculative but amenable to experimental test, for the origin of biotrophy is proposed. This involves the interplay of alterations to patterns of translocation produced by fungally-induced changes in hormonal balance in infected plants and catabolite repression of degradative enzymes of the fungus. 6. The dependence of mutualistic symbiosis on the maintenance of biotrophy is stressed. 7. Based on the nutrition of the diverse kinds of fungi involved in mycorrhizas, a re-alignment of groupings of mycorrhizal associations is proposed. This directs research towards seeking generalizations within, and differences between, four clearly defined groups (sheathing, vesicular-arbuscular, orchidaceous and ericaceous) instead of, as at present, within and between two (variously termed ectotrophic and endotrophic or ectomycorrhiza and endomycorrhiza), the second of which is highly artificial. Earlier drafts of the paper were read by Professors J. L. Harley, L. B. Thrower and S. D. Garrett, and Drs M. J. Carlile, B. C. Clifford, G. Hadley, P. M. Holligan, D. M. Losel and D. C. Smith. I am most grateful for their advice on specific points. Many, but not all, of their comments have been incorporated and I must stress that some views expressed, particularly with regard to the classification of mycorrhizas, are at variance with those of some of these colleagues.

Tidal rhythms: the clock control of the rhythmic physiology of marine organisms

Abstract: Summary 1. A great number of vital processes are rhythmic and the rhythms quite often persist in constant conditions. The best-known rhythms are circadian; much less is known about circalunadian rhythms, and this review was prepared in an attempt to rectify this deficiency. All through the article comparisons are drawn between circalunadian and circacian rhythms. 2. Activity rhythms. (a) The activity patterns of 28 intertidal animals are discussed. All describe a periodicity with a basic component of 24.8 hours, and this approximate period persists in the laboratory in constant light and temperature and in the absence of the tides. The duration of persistence ranges from a few cycles to months, and is a function of the species studied, the conditions imposed, and individual tenacity. (b) In those few cases where relatively long-term observations have been made, there is a trend for the period of the rhythm to become circatidal, or better, circalunadian. (c) The ‘desired’ phase relationship between rhythm and tidal cycle is species-specific. Geographical translocation experiments have shown that the phase is set by the local tides. (d) In some cases the amplitude of the persistent rhythm mimics the semidiurnal inequality of the tides. (e) In about a third of the species discussed, a circadian component has been found combined with the tidal component. Many of the other studies were of such short duration that a low-amplitude circadian component would have gone unnoticed. (f) The tidal rhythm is innate. However, the rhythm is (i) sometimes lacking in organisms living in non-tidal habitats, or (ii) fades after a spell of incarceration in constant conditions. Various treatments — some aperiodic — can induce the expression of the missing tidal rhythm. (g) In the green crab, removal of the eyestalks destroys the activity rhythm. 3. Vertical migration rhythms. (a) A rather surprisingly large number of intertidal animals have been found to undergo migration rhythms between the upper layers of the substratum and its surface. The movements are synchronized with the tides in nature, but most species have either been shown to be diurnal in constant conditions, or in cases where adequate testing has not been done, suspected of being so. (b) In only one species has confirming work shown that the fundamental frequency is truly tidal. This finding is especially important as it shows that tidal rhythms need only the single-cell level of organization for expression. Even at this level there appears to be a dictatorial override by a circadian clock. 4. Colour change. Low-amplitude tidal rhythms in colour change — superimposed on a more dominant circadian change — have been reported to be intrinsic in four species and inducible in a fifth. 5. Oxygen consumption. Tidal rhythms in oxygen consumption have been described for seven invertebrates and one alga; six of the species have superimposed solar-day rhythmic components also. 6. Translocation. A total of five geographical translocation experiments, in which the organisms were maintained in constant conditions throughout, have been tried. Unequivocally in one case, and possibly in a second, the test organisms rephased spontaneously to the times commensurate with local tidal conditions. In two other cases, the pretranslocation phase was retained. The fifth experiment has not been reproducible. 7. Determination of phase. (a) The tidal cycle on the home shoreline sets the phase of the inhabitant's rhythms. Even the location of a crab's burrow on the beach incline can play a determining role. (b) Paradoxically, the periodic wetting by inundation is not an important entraining factor for most intertidal organisms. Instead, the effective portions of the tidal cycle include one or more of the following. (i) Mechanical agitation, especially for animals living in an uprush zone where they are periodically subjected to the pounding surf, (ii) Temperature cycles, though they have not yet been systematically investigated, have very pronounced entraining roles in crabs. (iii) Pressure is probably not a generally important entraining agent for most intertidal organisms, but it is so for the green crab. (c) Light-dark cycles in general, whether daily or tidal in length, have no effect on the entrainment or phase setting of many tidal rhythms. There are two exceptions: (i) a 24-hour light-dark cycle is known to keep a tidal locomotor rhythm (one that becomes circalunadian in constant conditions) at a strict tidal frequency. (ii) In rhythms with both daily and tidal components, when the former is shifted by light stimuli, the latter is affected in a nearly identical manner. 8. Temperature. (a) The role of temperature on tidal rhythms is compared with its role on circadian rhythms. (b) The effects of different constant temperatures have so far been studied on only four tidal rhythms. All studies indicate a lack of any permanent change in period, which is not so with most circadian rhythms; the latter having temperature coefficients around 1.1. In two of the studies the rhythms under test temperatures were followed for less than a day, and a third study cannot be repeated. (c) Short exposure to very cold temperature pulses produced a response that may be interpreted as a temporary stoppage of the clock. Exposure to relatively less-cold pulses appear simply to reset the hands of the clock. The same responses have been demonstrated with circadian rhythms. (d) In the case of green crabs, which had become arrhythmic during prolongued captivity in the laboratory, a tidal rhythm could be reinitiated by a single short cold treatment. The cold pulse also set the phase of the rhythm. (e) A few superficial studies employing temperature steps or pulses have produced results which suggest that a phase-change sensitivity rhythm — just like that found associated with circadian rhythms — may underlie tidal rhythms. Certainly a determined search for this rhythm should be made in the near future. 9. Clock control of rhythms. (a) An argument is constructed claiming that tidal rhythms have a basic period of about 24–8 hours rather than the more expected tidal interval of 12.4 hours. In constant conditions, a circalunadian period is usually displayed. (b) After speculating that a frequency-transforming coupler may function between the clock and the overt rhythm, reasons are given that lead to the further speculation that both circadian and circalunadian rhythms could be generated by a single clock, via specific coupling mechanisms. (c) Two current hypotheses concerning the nature of the clockworks are reviewed and discussed. (d) Suggestions are made for future investigations.

Protein and nucleic acid metabolism in insect fat body

Abstract: Summary 1. The appearance of larval fat body as seen under the light or electron microscope depends on the nutritional state of the larva and on the stage of larval development at which the fat body is observed. 2. Early in the last larval instar the cells usually possess a well-developed endo-plasmic reticulum rich in ribosomes, numerous mitochondria, glycogen granules, a Golgi complex and fat droplets, while later in the instar the endoplasmic reticulum is much reduced and mitochondria are few, but glycogen and fat droplets are present in greater amount together with the appearance of large numbers of proteinaceous spheres. 3. Early in the last instar the fat body synthesizes proteins and exports them into the blood, while later in the instar proteins are sequestered from the blood into the fat body. 4. The rate of protein synthesis by the fat body is high in the early to mid part of the last instar, but then falls off rapidly to a low level, at which it remains until the larva pupates. In diapausing pupae, protein synthesis remains at this low level. 5. The similarity between the electrophoretic patterns of proteins from the fat body and those from the blood provides strong evidence that the fat body is the site of synthesis of many of the blood proteins. 6. Some of the blood proteins have been shown to possess enzymic properties, while others are thought to play a role in the transportation of various types of compounds. 7. Ecdysone and juvenile hormone both stimulate the rate of protein synthesis by larval fat body. Protein synthesis in fat body from diapausing pupae is stimulated after injury to the pupae. 8. The appearance of adult fat body and the amount of protein it contains is often closely linked with the nutritional and reproductive states of the insect. 9. An important role of the fat body in the adult female insect is the synthesis of yolk proteins, which are released into the blood and then taken up by the developing oocytes. This synthesis and uptake are under the control of hormones secreted by the corpora allata and by the median neurosecretory cells of the pars intercerebralis. 10. The RNA content of fat body in final-instar larvae is not constant throughout the instar. In some larvae it is at its highest level early in the instar, falling to a low level as the instar progresses, while in other larvae (e.g. Calliphora) the level of RNA in fat body does not decrease as the instar progresses. 11. In some dipterous insects the base composition of total RNA is DNA-like in that the guanine + cytosine content is low, accounting for 40 % of the bases. A similar composition is seen in rapidly labelled RNA isolated from insects of other orders (Coleoptera and Lepidoptera), but the base content of total RNA from these latter insects resembles ribosomal RNA from vertebrate tissues in that it has a high (ca. 60 %) guanine + cytosine content. 12. The RNA/DNA ratios in blowfly larval tissues are high compared with those found in any vertebrate tissue. 13. In larval fat body, RNA synthesis is low at the time of a moult, increases during the early and mid-instar period and subsequently falls during the latter part of the instar. During the pupal period, especially during pupal diapause, the rate of RNA synthesis is very low and then increases during the subsequent development of the pharate adult. Injury to diapausing pupae results in an increased rate of RNA synthesis in most of their tissues. 14. Ecdysone and juvenile hormone both stimulate RNA and DNA synthesis in larval and adult fat body and in other tissues, although there is evidence that in some tissues these two hormones may act antagonistically to each other. The insecticide DDT also has been shown to stimulate RNA synthesis in tissues of adult insects.

Autorotation, self-stability, and structure of single-winged fruits and seeds (samaras) with comparative remarks on animal flight

Abstract: SUMMARY 1 A samara is a winged fruit or seed that autorotates when falling, thereby reducing the sinking speed of the diaspore and increasing the distance it may be transported by winds. Samaras have evolved independently in a large number of plants. 2 Aerodynamical, mechanical, and structural properties crucial for the inherent self-stability are analysed, and formulae for calculation of performance data are given. 3 The momentum theorem is applied to samaras to calculate induced air velocities. As a basis for blade element analysis, and for directional stability analysis, various velocity components are put together into resultant relative air velocities normal to the blade's span axis for a samara in vertical autorotation and also in autorotation with side-slip. 4 When falling, a samara is free to move in any sense, but in autorotation it possesses static and dynamic stability. Mainly qualitative aspects on static stability are pre sented. Simple experiments on flat plates at Reynolds numbers about 2000 as in samaras, showed that pitch stability prevails when the C. M. (centre of mass) is located 27–35 % of the chord behind the leading edge. The aerodynamic c.p. (centre of pressure) moves forward upon a decrease of the angle of attack, backward upon an increase. In samara blades the c.m. lies ca. one-third chord behind the leading edge, and hence the aerodynamic and centrifugal forces interact so as to give pitch stability, involving stability of the angles of attack and gliding angles. 5 Photographs show that the centre of rotation of the samara approximately coincides with its c.m. 6 The coning angle (blade angle to tip path plane) taken up by the samara is determined by opposing moments set up by the centrifugal and aerodynamic forces. It is essentially the centrifugal moment (being a tangent function of the coning angle, which is small) that changes upon a change of coning angle, until the centrifugal and aerodynamic moments cancel out at the equilibrium coning angle. 7 Directional stability is maintained by keeping the tip path plane horizontal whereby a vertical descent path relative to the ambient air is maintained. Tilting of the tip path plane results in side-slip. Side-slip leads to an increased relative air speed at the blade when advancing, a reduced speed when retreating. The correspondingly fluctuating aerodynamic force and the gyroscopic action of the samara lead to restoring moments that bring the tip path plane back to the horizontal. 8 Entrance into autorotation is due to interaction between aerodynamic forces, the force of gravity, and inertial forces (when the blade accelerates towards a trailing position behind the c.m. of the samara). 9 The mass distribution must be such that the c.m. lies 0–30 % of the span from one end. In Acer and Plcea samaras the C.M. lies 10–20% from one end, thereby making the disk area swept by the blade large and the sinking speed low. 10 The blade plan-form is discussed in relation to aerodynamics. The width is largest far out on the blade where the relative air velocities are large. The large width of the blade contributes to a high Re number and thus probably to a better L/D (lift/drag) ratio and a slower descent. 11 The concentration of vascular bundles at the leading edge of the blade and the tapering of the blade thickness towards the trailing edge are essential for a proper chord wise mass distribution. 12 Data are given for samaras of Acer and Plcea, and calculations of performance are made by means of the formulae given in the paper. Some figures for an Acer samara are: sinking speed 0.9 m/sec, tip path inclination 15°, average total force coefficient 1.7 (which is discussed), and a L/D ratio of the blade approximately 3. 13 The performances of samaras are compared with those of insects, birds, bats, a flat plate, and a parachute. They show the samara to be a relatively very efficient structure in braking the sinking speed of the diaspore. 14 In samaras the mass, aerodynamic, and torsion axes coincide, whereas in insect wings the torsicn axis often lies ahead of the other two. Location of the torsion axis in front of the aerodynamic axis in insects tends towards passive wing twisting and passive adjustment of the angles of attack relative to the incident air stream, the direction of which varies along the wing because of wing flapping. 15 Location of the mass axis behind the torsion axis may lead to unfavourable

The production of hormones in higher plants

Abstract: SUMMARY 1Although much is known about the effects of plant hormones and their role in the control of growth and differentiation, little is known about the way in which hormone production is itself controlled or about the cellular sites of hormone synthesis The literature on hormone production is discussed in this review in an attempt to shed some light on these problems 2The natural auxin of plants, indol-3yl-acetic acid (IAA) is produced by a wide variety of living organisms In animals, fungi and bacteria it is formed as a minor by-product of tryptophan degradation The pathways of its production involve either the transamination or the decarboxylation of tryptophan The transaminase route is the more important 3In higher plants auxin is also produced as a minor breakdown product of trypto phan, largely via transamination In some species decarboxylation may occur but is of minor importance Tryptophan can also be degraded by spontaneous reaction with oxidation products of certain phenols 4The unspecific nature of the enzymes involved in IAA production and the probable importance of spontaneous, nonenzymic reactions in the degradation of tryp to phan make it unlikely that auxin production from tryptophan can be regulated with any precision at the enzymic level The limiting factor for auxin production is the availability of tryptophan, which in most cells is present in insufficient quantities for its degradation to occur to a significant extent Tryptophan levels are, however, considerably elevated in cells in which net protein breakdown is taking place as a result of autolysis 5An indole compound, glucobrassicin, occurs in Brassica and a number of other genera It breaks down readily to form a variety of products including indole aceto-nitrile, which can give rise to IAA There is, however, no evidence to indicate that glucobrassicin is a precursor of auxin in vivo 6Conjugates of IAA, eg IAA-aspartic acid and IAA-glucose, are formed when IAA is supplied in unphysiologically high amounts to plant tissues These and other IAA conjugates occur naturally in developing seeds and fruits There is no persuasive evidence for the natural occurrence of IAA-protein complexes 7Tissues autolysing during prolonged extraction with ether produce IAA from tryptophan released by proteolysis IAA is produced in considerable quantities by autolysing tissues in vitro 8During the senescence of leaves proteolysis results in elevated levels of trypto phan Large amounts of auxin are produced by senescent leaves 9Coleoptile tips have a vicarious auxin economy which depends on a supply of IAA, IAA esters and other compounds closely related to IAA from the seed These move acropetally in the xylem and accumulate at the coleoptile tip The production of auxin in coleoptile tips involves the hydrolysis of IAA esters and the conversion of labile, as yet unidentified compounds, to IAA There is no evidence for the de novo synthesis of IAA in coleoptiles 10Practically all the other sites of auxin production are sites of both meristematic activity and cell death The production of auxin in developing anthers and fertilized ovaries takes place in the regressing nutritive tissues (tapetum, nucellus, endosperm) as the cells break down In shoot tips, developing leaves, secondarily thickening stems, roots and developing fruits auxin is produced as a consequence of vascular differ entiation; the differentiation of xylem cells and most fibres involves a complete auto-lysis of the cell contents; the differentiation of sieve tubes involves a partial autolysis There is no evidence that meristematic cells produce auxin 11The lysis and digestion of cells infected with fungi and bacteria results in elevated tryptophan levels and the production of auxin Viral infections reduce the levels of tryptophan and are asSociated with reduced levels of auxin 12Crown-gall tissues produce auxin It is suggested that the crown-gall disease may involve at any given time the death of a minority of the cells which produce auxin and other hormones as they autolyse; the other cells grow and divide in response to these hormones 13Auxin is produced in soils, particularly those rich in decaying organic matter, by micro-organisms This environmental auxin may be important for the growth of roots 14There is no convincing evidence that auxin is a hormone in non-vascular plants The induction of rhizoids in liverworts by low concentrations of auxin can be ex plained as a response to environmental auxin 15Abscisic acid is synthesized from mevalonic acid in living cells It is possible that under certain circumstances, abscisic acid or closely related compounds are formed by the oxidation of carotenoids 16The sites of gibberellin production are sites of cell death It is possible that precursors of gibberellins, such as kaurene, are oxidized to gibberellins when cells die 17Cytokinins are present in transfer-RNA (tRNA) of animals, fungi, bacteria and higher plants They are probably formed in plants by the hydrolysis of tRNA in autolysing cells There is evidence that they are also formed in living cells in root tips 18Ethylene is produced in senescent, dying or damaged cells by the breakdown of methionine 19It was shown many years ago that wounded and damaged cells produced sub stances which stimulate cell division It now seems likely that the production of wound hormones and the normal production of hormones as a consequence of cell death are two aspects of the same phenomenon Wounded cells can produce auxin, gibberellins, cytokinins and ethylene 20The control of hormone production in living cells is a biochemical problem which remains unsolved The control of production of hormones formed as a con sequence of cell death depends on the control of cell death itself Cell death is con trolled by hormones which are themselves produced as a consequence of cell death 21In spite of the fact that dying cells are present in all vascular plants, in all wounded and infected tissues, in certain differentiating tissues in animals, in cancerous tumours and in developing animal embryos, the biochemistry of cell death is a subject which has been almost completely ignored Dying cells are an important source of hormones in plants; some of the many substances released by dying cells may also be of physiological significance in animals

Evolution or revolution of ammonoids at mesozoic system boundaries

Abstract: Summary 1. Biological revolutions at major stratigraphical boundaries have been given numerous explanations involving endogenous biological, exogenous ecological, physical, and cosmic, as well as sedimentary or chemical factors. In an attempt to elucidate the true nature of these faunal revolutions and to assess the possible influence of biological and/or physical factors, the evolution of ammonites at the boundaries of Mesozoic stratigraphical Systems is reviewed. It is believed that the more detailed data now available can give a clearer impression of evolutionary events at these boundaries. 2. It can be demonstrated that there is neither an abrupt and world-wide extinction, nor a spontaneous replacement by new elements at these caesuras as had been generally supposed to have occurred at the Triassic-Jurassic boundary, for example. Instead, one can recognize three distinct phases in the sequence of events: (1) a continuous disappearance of the ‘antique’ faunal elements; (2) a similarly continuous, gradual, and largely synchronous appearance of, or substitution by, qualitatively distinguishable ‘modern’ elements in small populations, yet in various parallel lineages (mosaic evolution); (3) a quite revolutionary, and quantitatively very sudden, diversification of these new elements, occurring at or with some delay above the boundary. 3. Thus one can demonstrate both continuous evolution of the modern faunas (‘preadaptational phase’), as well as ‘discontinuous’ spontaneous revolution, which does not produce qualitatively new characters and must be explained by diversification or adaptive radiation. This means that no further explanation by internal factors or by higher mutation rates resulting from the impact of cosmic rays becomes necessary. It is believed that, preceded by high extinction rates, world-wide ecological factors promoting higher niche diversity suffice to explain these adaptive radiations. The high degree of provincialism, endemism and specialization of the ‘antique’ faunas and the constant survival of smooth oxycones — regarded as inhabitants of a deep-sea environment — demonstrate that marine regressions and transgressions were the most effective ecological factors. 4. If there is not too much time involved between the two events, the caesura (Faunenschnitt) between final extinction of the old faunas and the radiation of the new is the most appropriate point by which to define System boundaries.

The heterostracan fishes

Abstract: Summary 1. The heterostracan fishes were jawless, microphagous, devoid of paired fins and encased in a bony armour. The classification is based on the arrangement of the plates of the carapace — the primitive forms possessed a tessellated armour made up of numerous small polygonal plates. Several genera had a tessellated carapace ventrally but large discrete plates dorsally. All further groups are characterized by distinct patterns of plates which remain constant within each order. The proportions in some species suggest sexual dimorphism. 2. A study of the superficial ornamentation reveals patterns of growth. From the primitive tessellated condition different evolutionary lines can be followed leading to the fusion of these small elements into large discrete plates. Among the latest group of heterostracans there was a secondary redevelopment of tesserae. 3. Impressions on the inner surface of the plates of the carapace enable certain aspects of the internal anatomy to be reconstructed. The nasal sacs were double, the acousticolateralis system was primitive. The brain was little more than the nerve cord swollen in three places, there was no cranial flexure. Impressions of two pre-otic somites were present, indicating that they had not migrated to form the extrinsic eye muscles. The branchial arches appear to have been of gnathostome type and in some genera spiracles were formed as an adaptation to a benthonic mode of life. 4. Microscopic sections of the armour demonstrate the existence of four tissues: aspidin, dentine, enameloid and calcified cartilage. Aspidin was originally acellular but later became cellular; the organic matrix was first organized like dentine, but subsequently like bone. Furthermore aspidin was capable of remodelling. Dentine appears to have acted as a skin-like tissue and was capable of regeneration. The significance of enameloid and calcified cartilage in the dermal armour is not well understood. 5. Until the end of the Silurian the heterostracans inhabited marine waters but from the beginning of the Devonian they colonized the fresh-water lakes and rivers of the Old Red Continent. One major group flourished in a large embayment on the edge of the Tungussian land mass. When the stratigraphical range and geographical distribution of the heterostracans is listed, evolutionary centres can be recognized and also migration routes: during the Upper Silurian the cyathaspids in the Canadian Arctic, the Lower Devonian pteraspids in Eastern Europe, the later Lower Devonian amphiaspids in north-west Siberia (Tungussian Realm), and the Middle and Upper Devonian psammosteids in the Baltic province. Periods of migration from the Baltic to Scotland, the Timan, Ellesmereland, the Urals and Donbas have been documented. For the illustrations the author is indebted to Miss Jennifer Middleton and Mr John Smith.

Zoogeography of some terrestrial micro‐arthropoda in antarctica

Abstract: Summary 1. A review is presented of the information relating to the distribution of free-living terrestrial Cryptostigmata, Prostigmata and Collembola in the south polar region. 2. The Maritime zone, comprising the Antarctic Peninsula and its neighbouring islands, forms a less-clearly definable faunal province than does the Continental zone where generic and specific endemism is high. The Maritime distribution of the micro-arthropods forms no consistent pattern, although in the case of the two groups of mites a distinction can be made between a southern and a northern Maritime fauna. The boundary between these two elements appears to lie between the South Orkneys and the South Sandwich Islands. 3. As far as distribution in the Sub-Antarctic is concerned, all three groups of micro-arthropods show certain general similarities, although in each case particular features can also be distinguished. All three show relatively high specific endemism in the Sub-Antartcic, which sugge:ts that this zone is a faunal province distinct from that of the cold temperate zone to the north and other parts of the Antarctic to the south. It seems equally reasonable to recognize that this province can be subdivided into western and eastern parts, each with its own distinct group of species, although the extent to which this separation is expressed varies from group to group. In all three groups there is an element which is circum-Sub-Antarctic in distribution, but this element is more in evidence in the Collembola than in the mites. From the study of the distribution of endemic species in the Sub-Antarctic it is concluded that the fauna of the eastern part has been isolated from the south temperate zone fauna for longer than it has in the west. 4. In all three groups of micro-arthropods a relic element can be identified, the present distribution of which is consistent with the idea of a former continuous distribution extending across a southern land mass incorporating both the eastern and western parts of the continent, the islands on the Scotia Ridge, and the older Sub-Antarctic islands of South Georgia, Macquarie and, possibly, Kerguelen. In the Cryptostigmata in particular, this relic element, which is represented by members of the Podacaridae, is joined in the Sub-Antarctic and Maritime zones by a penetrant element which probably has invaded the south polar region from the north during the post-Pleistocene period. 5. An alternative hypothesis is also discussed, namely that present distribution patterns of micro-arthropods can be explained solely in terms of post-Pleistocene colonization. It is accepted that limited overseas dispersal could have occurred in recent times, indeed may still be occurring between, for example, the various volcanic islands in the eastern Sub-Antarctic, between the south temperate zone and the Sub-Antarctic, and between the latter and the Maritime zone. However, there is no evidence to suggest that any appreciable amount of long-range dispersal is occurring, on the scale required to support this hypothesis. In addition, there is no evidence that rates of speciation in the south polar region are rapid enough to produce the kind of evolutionary divergence which is implicit in the theory of post-Pleistocene colonization.

Maternal processes in the cold-adaptation of mice.

Abstract: SUMMARY 1 Both laboratory and wild house mice, Mus musculus, given bedding, can breed in captivity in an environment kept at – 3°C. The nest temperature when a young litter is present then fluctuates widely. In a typical laboratory (at 21°C) the temperature of the nest is both higher and more constant. 2 The ovaries of pregnant mice breeding at – 3°C have more corpora lute a than controls at 21°C. This is not an index of a higher ovulation rate, but is evidently due to the presence of corpora lutea from a pievious ovulation. 3 In the absence of concurrent lactation, weights and numbers of foetuses at the sixteenth day of gestation are little affected by cold; but in both environments foetal weight diminishes with increasing size of litter. This is a systemic effect: foetal weight is hardly if at all influenced by the number of other foetuses in the same uterine horn. 4 Cold delays the onset of breeding and lengthens the interval between litters. Mean litter sizes are usually lower than in the warm environment, mainly through absence of large litters. 5 The body weights of laboratory mice are usually lower at – 3°C than 21°C at all ages from 3 weeks. This does not, however, apply to strain C57BL, which never stores much fat in adipose tissue. Wild mice bred at – 3°C are heavier than controls at 21°C, possibly because only the heavier individuals survive in early life. 6 F1 hybrids produced by crossing two inbred strains breed better and more consistently than the parent strains at both temperatures; but the effect of heterozygosis is much greater in the cold environment. 7 Food intake changes little during pregnancy, but rises greatly during the first 10 days of lactation at both temperatures. 8 At 21°C, body weight, excluding the weight of the litter, increases only slightly during pregnancy; but the weights of the heart and liver are greatly increased. The weight of the stomach also rises; the small intestine lengthens, but becomes lighter. During lactation the liver becomes still heavier, and the small intestine more than restores its loss of weight. The kidneys also become heavier. At – 3°C similar changes occur, but the heart is heavier at all stages of the reproductive cycle than it is at 21°C. The kidneys, too, are consistently heavier in the cold, and so is the small intestine. By contrast, the liver of pregnant or lactating females at – 3°C is no heavier than in the warm environment. 9 Pregnancy entails an increase in the absolute amount of nitrogen in the body, in both environments; but females at – 3°C have less nitrogen and collagen than controls. Pregnancy does not alter body fat at either temperature, but lactation is accompanied by some loss. At birth, mice born in the cold environment have more than twice as much body fat as controls. 10 When mice are bred for their full reproductive span, the effect of a cold environment depends markedly on genotype. Mice of strain A2G/Tb eventually produce as many young in the cold environment as in the warm, but take longer to do so; C57BL/Tb produce fewer young, Wild mice produce fewer litters at – 3°C, and have a much higher nestling mortality. Most of the mortality is due to loss of whole litters. 11 The preceding statements apply to mice of the first two or three generations in a cold environment, There are further effects of breeding for many generations in the cold. Wild mice bred for ten generations lose fewer litters in later than in earlier generations. After ten generations, some wild mice were moved from –3 to 21°C. Their reproductive performance was then much superior to that of controls which had been kept at 21°C throughout. The transferred mice were also quicker than the controls to make a nest of paper. 12 Genetically heterogeneous laboratory mice, after twelve generations in the cold, were similarly returned to the warm environment. Their offspring were heavier than controls; but there was no superiority in reproductive performance. 13 A2G/Tb mice kept at –3°C, though highly inbred, also improved in reproductive performance over a number of generations: in particular, their infant mortality declined. This was probably not due to a genetical change, but to a cumulative maternal effect. 14 Maternal performance was studied by cross-fostering young at birth between these ‘Eskimo’ mice, ‘immigrant’ mice of the first or second generation reared in the cold, and controls at 21°C. There was some evidence of an effect of true parentage, regardless of foster parentage, on body weight: the young of the Eskimo mice tended to be heavier than the others. There was also evidence that this influence persisted into a second generation. Mortality among the fostered young was influenced only by true parentage, not by foster parentage or environmental temperature. Some of the fostered mice were mated. Again, among their young, mortality in the nest was not affected by environmental temperature; but those whose true ancestry was Eskimo displayed a lower mortality than the others. 15 If a young mammal is given special treatment (such as exposure outside the nest), the treatment may influence, not only the individual treated, but also the behaviour of the parents; and the altered parental behaviour may in turn affect the development of the young. Enhanced parental attention in the nest has been directly observed after young have been exposed to cold or other treatment. It can probably accelerate maturation, and improve reproductive performance by lowering mortality among the young of the treated mice. Hence the direct effects of treatment in infancy can never be distinguished with certainty from indirect effects through changed parental behaviour, unless the experimental animals are reared artificially. 16 A comprehensive theory of ‘stress’, that is, of the response of a species to an environmental change for the worse, requires that attention should be paid to the following: (i) the effects of physiological (ontogenetic) adaptation to one ‘stressor’, such as cold, on response to another, such as infection; (ii) the ways in which conditions of rearing, especially early exposure to mildly adverse conditions such as lower temperature, influence later physiological, reproductive and behaviour al performance; (iii) the relationships of the above with the adaptive changes of pregnancy and lactation.

The significance of ionic concentrations in the internal media of animals

Abstract: Summary 1. The electrical and ionic gradients across a cell membrane depend on its permeability properties, on the concentration and net valency of the organic constituents of the cytoplasm and on the critical energy barrier to the extrusion of sodium. Such considerations do not, however, explain the small extent to which the concentration of potassium varies in myoplasm which may, instead, be related to the effects of potassium on particular enzymes. 2. The fact that the apparent optimum level of potassium cannot usually be maintained in animals in which the extracellular level of sodium is below about 140 mM may explain why so many non-marine animals have internal media of about that concentration, for more concentrated body fluids would require more work for their regulation. 3. In axoplasm, the concentration of potassium is more nearly proportional to the concentration of sodium in the internal medium and this may partly explain the general correlation between the extracellular levels of sodium and potassium. 4. The relation between pH and temperature in poikilothermic vertebrates is such as to suggest that the prime function of acid-base regulation is to control the ionization of imidazole groups. 5. High tensions of carbon dioxide cannot be maintained in water-breathing animals because of the high solubility of this gas in water as compared with oxygen. Bicarbonate levels are correspondingly low to give a suitable pH. Higher tensions are possible in air-breathing animals, and also necessary if water and heat are to be conserved, but an uncertain upper limit is set by the need for oxygen. The associated higher levels of bicarbonate confer the advantage of better buffering. 6. Calcium and bicarbonate levels are not obviously limited by the solubility of calcium carbonate and a more general limitation on the composition of body fluids seems to arise from the low solubilities of calcium phosphates. 7. The pattern of ionic balance in vertebrate plasma, reflected in a nearly constant value to the molar ratio ([Ca] + 5 × 10--4)/([K] +0.034 [Na]), may be explained in terms of the maintenance of a constant electrical gradient across certain areas of cell membrane, between the inner and outer double layers. 8. The patterns of cation balance in the haemolymphs of molluscs, crustacea and insects are also reviewed, with emphasis on the correlations existing between the concentrations of different cations. An attempt is made to relate the correlations in the mollusca to the concentrations of cations at the surfaces of cells.