## Glossary of the Principal Scientific Terms Used in the Present Volume

(I am indebted to the kindness of Mr. W. S. Dallas for this Glossary, which has been given because several readers have complained to me that some of the terms used were unintelligible to them. Mr. Dallas has endeavoured to give the explanations of the terms in as popular a form as possible.)

Aberrant
  Forms or groups of animals or plants which deviate in important characters from their nearest allies, so as not to be easily included in the same group with them, are said to be aberrant.
Aberration (in Optics)
  In the refraction of light by a convex lens the rays passing through different parts of the lens are brought to a focus at slightly different distance: this is called Spherical Aberration; at the same time the coloured rays are separated by the prismatic action of the lens and likewise brought to a focus at different distance: this is Chromatic Aberration.
Abnormal
  Contrary to the general rule.
Aborted
  An organ is said to be aborted, when its development has been arrested at a very early stage.
Albinism
  Albinos are animals in which the usual colouring matters characteristic of the species have not been produced in the skin and its appendages. Albinism is the state of being an albino.
Algae
  A class of plants including the ordinary seaweeds and the filamentous freshwater weeds.
Alternation of Generations
  This term is applied to a peculiar mode of reproduction which prevails among many of the lower animals, in which the egg produces a living form quite different from its parent, but from which the parent-form is reproduced by a process of budding, or by the division of the substance of the first product of the egg.
Ammonites
  A group of fossil, spiral, chambered shells, allied to the existing pearly Nautilus, but having the partitions between the chambers waved in complicated patterns at their junction with the outer wall of the shell.
Analogy
  That resemblance of structures which depends upon similarity of function, as in the wings of insects and birds. Such structures are said to be Analogous, and to be Analogues of each other.
Animalcule
  A minute animal: generally applied to those visible only by the microscope.
Annelids
  A class of worms in which the surface of the body exhibits a more or less distinct division into rings or segments, generally provided with appendages for locomotion and with gills. It includes the ordinary marine worms, the earthworms, and the leeches.
Antennae
  Jointed organs appended to the head in Insects, Crustacea and Centipedes, and not belonging to the mouth.
Anthers
  The summits of the stamens of flowers, in which the pollen or fertilising dust is produced.
Aplacentalia
  , Aplacentata or Aplacental Mammals: See Mammalia.
Archetypal
  Of or belonging to the Archetype, or ideal primitive form upon which all the beings of a group seem to be organised.
Articulata
  A great division of the Animal Kingdom characterised generally by having the surface of the body divided into rings called segments, a greater or less number of which are furnished with jointed legs (such as Insects, Crustaceans and Centipedes).
Asymmetrical
  Having the two sides unlike.
Atrophied
  Arrested in development at a very early stage.
Balanus
  The genus including the common Acorn-shells which live in abundance on the rocks of the seacoast.
Batrachians
  A class of animals allied to the Reptiles, but undergoing a peculiar metamorphosis, in which the young animal is generally aquatic and breathes by gills. (Examples, Frogs, Toads, and Newts.)
Boulders
  Large transported blocks of stone generally embedded in clays or gravels.
Brachiopoda
  A class of marine Mollusca, or soft-bodied animals, furnished with a bivalve shell, attached to submarine objects by a stalk which passes through an aperture in one of the valves, and furnished with fringed arms, by the action of which food is carried to the mouth.
Branchiae
  Gills or organs for respiration in water.
Branchial
  Pertaining to gills or branchiae.
Cambrian System
  A series of very ancient Palaeozoic rocks, between the Laurentian and the Silurian. Until recently these were regarded as the oldest fossiliferous rocks.
Canidae
  The Dog-family, including the Dog, Wolf, Fox, Jackal, etc.
Carapace
  The shell enveloping the anterior part of the body in Crustaceans generally; applied also to the hard shelly pieces of the Cirripedes.
Carboniferous
  This term is applied to the great formation which includes, among other rocks, the coal-measures. It belongs to the oldest, or Palaeozoic, system of formations.
Caudal
  Of or belonging to the tail.
Cephalopods
  The highest class of the Mollusca, or soft-bodied animals, characterised by having the mouth surrounded by a greater or less number of fleshy arms or tentacles, which, in most living species, are furnished with sucking-cups. (Examples, Cuttlefish, Nautilus.)
Cetacea
  An order of Mammalia, including the Whales, Dolphins, etc., having the form of the body fish-like, the skin naked, and only the fore limbs developed.
Chelonia
  An order of Reptiles including the Turtles, Tortoises, etc.
Cirripedes
  An order of Crustaceans including the Barnacles and Acorn-shells. Their young resemble those of many other Crustaceans in form; but when mature they are always attached to other objects, either directly or by means of a stalk, and their bodies are enclosed by a calcareous shell composed of several pieces, two of which can open to give issue to a bunch of curled, jointed tentacles, which represent the limbs.
Coccus
  The genus of Insects including the Cochineal. In these the male is a minute, winged fly, and the female generally a motionless, berrylike mass.
Cocoon
  A case usually of silky material, in which insects are frequently enveloped during the second or resting-stage (pupa) of their existence. The term “cocoon-stage” is here used as equivalent to “pupa-stage.”
Coelospermous
  A term applied to those fruits of the Umbelliferae which have the seed hollowed on the inner face.
Coleoptera
  Beetles, an order of Insects, having a biting mouth and the first pair of wings more or less horny, forming sheaths for the second pair, and usually meeting in a straight line down the middle of the back.
Column
  A peculiar organ in the flowers of Orchids, in which the stamens, style and stigma (or the reproductive parts) are united.
Compositae
  or Compositous Plants: Plants in which the inflorescence consists of numerous small flowers (florets) brought together into a dense head, the base of which is enclosed by a common envelope. (Examples, the Daisy, Dandelion, etc.)
Confervae
  The filamentous weeds of fresh water.
Conglomerate
  A rock made up of fragments of rock or pebbles, cemented together by some other material.
Corolla
  The second envelope of a flower usually composed of coloured, leaf-like organs (petals), which may be united by their edges either in the basal part or throughout.
Correlation
  The normal coincidence of one phenomenon, character, etc., with another.
Corymb
  A bunch of flowers in which those springing from the lower part of the flower stalks are supported on long stalks so as to be nearly on a level with the upper ones.
Cotyledons
  The first or seed-leaves of plants.
Crustaceans
  A class of articulated animals, having the skin of the body generally more or less hardened by the deposition of calcareous matter, breathing by means of gills. (Examples, Crab, Lobster, Shrimp, etc.)
Curculio
  The old generic term for the Beetles known as Weevils, characterised by their four-jointed feet, and by the head being produced into a sort of beak, upon the sides of which the antennae are inserted.
Cutaneous
  Of or belonging to the skin.
Degradation
  The wearing down of land by the action of the sea or of meteoric agencies.
Denudation
  The wearing away of the surface of the land by water.
Devonian System
  or Formation: A series of Palaeozoic rocks, including the Old Red Sandstone.
Dicotyledons
  , or Dicotyledonous Plants: A class of plants characterised by having two seed-leaves, by the formation of new wood between the bark and the old wood (exogenous growth) and by the reticulation of the veins of the leaves. The parts of the flowers are generally in multiples of five.
Differentation
  The separation or discrimination of parts or organs which in simpler forms of life are more or less united.
Dimorphic
  Having two distinct forms: Dimorphism is the condition of the appearance of the same species under two dissimilar forms.
Dioecious
  Having the organs of the sexes upon distinct individuals.
Diorite
  A peculiar form of Greenstone.
Dorsal
  Of or belonging to the back.
Edentata
  A peculiar order of Quadrupeds, characterised by the absence of at least the middle incisor (front) teeth in both jaws. (Examples, the Sloths and Armadillos.)
Elytra
  The hardened forewings of Beetles, serving as sheaths for the membranous hind-wings, which constitute the true organs of flight.
Embryo
  The young animal undergoing development within the egg or womb.
Embryology
  The study of the development of the embryo.
Endemic
  Peculiar to a given locality.
Entomostraca
  A division of the class Crustacea, having all the segments of the body usually distinct, gills attached to the feet or organs of the mouth, and the feet fringed with fine hairs. They are generally of small size.
Eocene
  The earliest of the three divisions of the Tertiary epoch of geologists. Rocks of this age contain a small proportion of shells identical with species now living.
Ephemerous Insects
  Insects allied to the Mayfly.
Fauna
  The totality of the animals naturally inhabiting a certain country or region, or which have lived during a given geological period.
Felidae
  The Cat-family.
Feral
  Having become wild from a state of cultivation or domestication.
Flora
  The totality of the plants growing naturally in a country, or during a given geological period.
Florets
  Flowers imperfectly developed in some respects, and collected into a dense spike or head, as in the Grasses, the Dandelion, etc.
Foetal
  Of or belonging to the foetus, or embryo in course of development.
Foraminifera
  A class of animals of very low organisation and generally of small size, having a jellylike body, from the surface of which delicate filaments can be given off and retracted for the prehension of external objects, and having a calcareous or sandy shell, usually divided into chambers and perforated with small apertures.
Fossiliferous
  Containing fossils.
Fossorial
  Having a faculty of digging. The Fossorial Hymenoptera are a group of Wasp-like Insects, which burrow in sandy soil to make nests for their young.
Frenum
  (pl. Frena): A small band or fold of skin.
Fungi
  (sing. Fungus): A class of cellular plants, of which Mushrooms, Toadstools, and Moulds, are familiar examples.
Furcula
  The forked bone formed by the union of the collarbones in many birds, such as the common Fowl.
Gallinaceous Birds
  An order of birds of which the common Fowl, Turkey, and Pheasant, are well-known examples.
Gallus
  The genus of birds which includes the common Fowl.
Ganglion
  A swelling or knot from which nerves are given off as from a centre.
Ganoid Fishes
  Fishes covered with peculiar enamelled bony scales. Most of them are extinct.
Germinal Vesicle
  A minute vesicle in the eggs of animals, from which the development of the embryo proceeds.
Glacial Period
  A period of great cold and of enormous extension of ice upon the surface of the earth. It is believed that glacial periods have occurred repeatedly during the geological history of the earth, but the term is generally applied to the close of the Tertiary epoch, when nearly the whole of Europe was subjected to an arctic climate.
Gland
  An organ which secretes or separates some peculiar product from the blood or sap of animals or plants.
Glottis
  The opening of the windpipe into the oesophagus or gullet.
Gneiss
  A rock approaching granite in composition, but more or less laminated, and really produced by the alteration of a sedimentary deposit after its consolidation.
Grallatores
  The so-called wading-birds (storks, cranes, snipes, etc.), which are generally furnished with long legs, bare of feathers above the heel, and have no membranes between the toes.
Granite
  A rock consisting essentially of crystals of felspar and mica in a mass of quartz.
Habitat
  The locality in which a plant or animal naturally lives.
Hemiptera
  An order or suborder of insects, characterised by the possession of a jointed beak or rostrum, and by having the forewings horny in the basal portion and membranous at the extremity, where they cross each other. This group includes the various species of bugs.
Hermaphrodite
  Possessing the organs of both sexes.
Homology
  That relation between parts which results from their development from corresponding embryonic parts, either in different animals, as in the case of the arm of man, the foreleg of a quadruped, and the wing of a bird; or in the same individual, as in the case of the fore and hind legs in quadrupeds, and the segments or rings and their appendages of which the body of a worm, a centipede, etc., is composed. The latter is called serial homology. The parts which stand in such a relation to each other are said to be homologous, and one such part or organ is called the homologue of the other. In different plants the parts of the flower are homologous, and in general these parts are regarded as homologous with leaves.
Homoptera
  An order or suborder of insects having (like the Hemiptera) a jointed beak, but in which the forewings are either wholly membranous or wholly leathery, The Cicadae, froghoppers, and Aphides, are well-known examples.
Hybrid
  The offspring of the union of two distinct species.
Hymenoptera
  An order of insects possessing biting jaws and usually four membranous wings in which there are a few veins. Bees and wasps are familiar examples of this group.
Hypertrophied
  Excessively developed.
Ichneumonidae
  A family of hymenopterous insects, the members of which lay their eggs in the bodies or eggs of other insects.
Imago
  The perfect (generally winged) reproductive state of an insect.
Indigenes
  The aboriginal animal or vegetable inhabitants of a country or region.
Inflorescence
  The mode of arrangement of the flowers of plants.
Infusoria
  A class of microscopic animalcules, so called from their having originally been observed in infusions of vegetable matters. They consist of a gelatinous material enclosed in a delicate membrane, the whole or part of which is furnished with short vibrating hairs (called cilia), by means of which the animalcules swim through the water or convey the minute particles of their food to the orifice of the mouth.
Insectivorous
  Feeding on insects.
Invertebrata
  , or Invertebrate Animals: Those animals which do not possess a backbone or spinal column.
Lacunae
  Spaces left among the tissues in some of the lower animals and serving in place of vessels for the circulation of the fluids of the body.
Lamellated
  Furnished with lamellae or little plates.
Larva
  (pl. Larvae): The first condition of an insect at its issuing from the egg, when it is usually in the form of a grub, caterpillar, or maggot.
Larynx
  The upper part of the windpipe opening into the gullet.
Laurentian
  A group of greatly altered and very ancient rocks, which is greatly developed along the course of the St. Laurence, whence the name. It is in these that the earliest known traces of organic bodies have been found.
Leguminosae
  An order of plants represented by the common peas and beans, having an irregular flower in which one petal stands up like a wing, and the stamens and pistil are enclosed in a sheath formed by two other petals. The fruit is a pod (or legume).
Lemuridae
  A group of four-handed animals, distinct from the monkeys and approaching the insectivorous quadrupeds in some of their characters and habits. Its members have the nostrils curved or twisted, and a claw instead of a nail upon the first finger of the hind hands.
Lepidoptera
  An order of insects, characterised by the possession of a spiral proboscis, and of four large more or less scaly wings. It includes the well-known butterflies and moths.
Littoral
  Inhabiting the seashore.
Loess
  A marly deposit of recent (Post-Tertiary) date, which occupies a great part of the valley of the Rhine.
Malacostraca
  The higher division of the Crustacea, including the ordinary crabs, lobsters, shrimps, etc., together with the woodlice and sand-hoppers.
Mammalia
  The highest class of animals, including the ordinary hairy quadrupeds, the whales and man, and characterised by the production of living young which are nourished after birth by milk from the teats (Mammae, Mammary Glands) of the mother. A striking difference in embryonic development has led to the division of this class into two great groups; in one of these, when the embryo has attained a certain stage, a vascular connection, called the Placenta, is formed between the embryo and the mother; in the other this is wanting, and the young are produced in a very incomplete state. The former, including the greater part of the class, are called Placental Mammals; the latter, or Aplacental Mammals, include the Marsupials and Monotremes (Ornithorhynchus).
Mammiferous
  Having mammae or teats (see Mammalia).
Mandibles
  in insects, the first or uppermost pair of jaws, which are generally solid, horny, biting organs. In birds the term is applied to both jaws with their horny coverings. In quadrupeds the mandible is properly the lower jaw.
Marsupials
  An order of Mammalia in which the young are born in a very incomplete state of development, and carried by the mother, while sucking, in a ventral pouch (marsupium), such as the kangaroos, opossums, etc. (see Mammalia).
Maxillae
  in insects, the second or lower pair of jaws, which are composed of several joints and furnished with peculiar jointed appendages called palpi, or feelers.
Melanism
  The opposite of albinism; an undue development of colouring material in the skin and its appendages.
Metamorphic Rocks
  Sedimentary rocks which have undergone alteration, generally by the action of heat, subsequently to their deposition and consolidation.
Mollusca
  One of the great divisions of the animal kingdom, including those animals which have a soft body, usually furnished with a shell, and in which the nervous ganglia, or centres, present no definite general arrangement. They are generally known under the denomination of “shellfish;” the cuttlefish, and the common snails, whelks, oysters, mussels, and cockles, may serve as examples of them.
Monocotyledons
  , or Monocotyledonous Plants: Plants in which the seed sends up only a single seed-leaf (or cotyledon); characterised by the absence of consecutive layers of wood in the stem (endogenous growth), by the veins of the leaves being generally straight, and by the parts of the flowers being generally in multiples of three. (Examples, grasses, lilies, orchids, palms, etc.)
Moraines
  The accumulations of fragments of rock brought down by glaciers.
Morphology
  The law of form or structure independent of function.
Mysis
  -Stage: A stage in the development of certain crustaceans (prawns), in which they closely resemble the adults of a genus (Mysis) belonging to a slightly lower group.
Nascent
  Commencing development.
Natatory
  Adapted for the purpose of swimming.
Nauplius-Form
  The earliest stage in the development of many Crustacea, especially belonging to the lower groups. In this stage the animal has a short body, with indistinct indications of a division into segments, and three pairs of fringed limbs. This form of the common freshwater Cyclops was described as a distinct genus under the name of Nauplius.
Neuration
  The arrangement of the veins or nervures in the wings of insects.
Neuters
  Imperfectly developed females of certain social insects (such as ants and bees), which perform all the labours of the community. Hence, they are also called Workers.
Nictitating Membrane
  A semitransparent membrane, which can be drawn across the eye in birds and reptiles, either to moderate the effects of a strong light or to sweep particles of dust, etc., from the surface of the eye.
Ocelli
  The simple eyes or stemmata of insects, usually situated on the crown of the head between the great compound eyes.
Oesophagus
  The gullet.
Oolitic
  A great series of secondary rocks, so called from the texture of some of its members, which appear to be made up of a mass of small Egg-Like calcareous bodies.
Operculum
  A calcareous plate employed by many Molluscae to close the aperture of their shell. The Opercular Valves of Cirripedes are those which close the aperture of the shell.
Orbit
  The bony cavity for the reception of the eye.
Organism
  An organised being, whether plant or animal.
Orthospermous
  A term applied to those fruits of the Umbelliferae which have the seed straight.
Osculant
  Forms or groups apparently intermediate between and connecting other groups are said to be osculant.
Ova
  Eggs.
Ovarium
  or Ovary (in plants): The lower part of the pistil or female organ of the flower, containing the ovules or incipient seeds; by growth after the other organs of the flower have fallen, it usually becomes converted into the fruit.
Ovigerous
  Egg-bearing.
Ovules
  (of plants): The seeds in the earliest condition.
Pachyderms
  A group of Mammalia, so called from their thick skins, and including the elephant, rhinoceros, hippopotamus, etc.
Palaeozoic
  The oldest system of fossiliferous rocks.
Palpi
  Jointed appendages to some of the organs of the mouth in insects and Crustacea.
Papilionaceae
  An order of plants (see Leguminosae), The flowers of these plants are called Papilionaceous, or butterfly-like, from the fancied resemblance of the expanded superior petals to the wings of a butterfly.
Parasite
  An animal or plant living upon or in, and at the expense of, another organism.
Parthenogenesis
  The production of living organisms from unimpregnated eggs or seeds.
Pedunculated
  Supported upon a stem or stalk. The pedunculated oak has its acorns borne upon a footstool.
Peloria
  or Pelorism: The appearance of regularity of structure in the flowers of plants which normally bear irregular flowers.
Pelvis
  The bony arch to which the hind limbs of vertebrate animals are articulated.
Petals
  The leaves of the corolla, or second circle of organs in a flower. They are usually of delicate texture and brightly coloured.
Phyllodineous
  Having flattened, leaf-like twigs or leafstalks instead of true leaves.
Pigment
  The colouring material produced generally in the superficial parts of animals. The cells secreting it are called Pigment-Cells.
Pinnate
  Bearing leaflets on each side of a central stalk.
Pistils
  The female organs of a flower, which occupy a position in the centre of the other floral organs. The pistil is generally divisible into the ovary or germen, the style and the stigma.
Placentalia, Placentata
  Or Placental Mammals. See Mammalia.
Plantigrades
  Quadrupeds which walk upon the whole sole of the foot, like the bears.
Plastic
  Readily capable of change.
Pleistocene Period
  The latest portion of the Tertiary epoch.
Plumule
  (in plants): The minute bud between the seed-leaves of newly-germinated plants.
Plutonic Rocks
  Rocks supposed to have been produced by igneous action in the depths of the earth.
Pollen
  The male element in flowering plants; usually a fine dust produced by the anthers, which, by contact with the stigma effects the fecundation of the seeds. This impregnation is brought about by means of tubes (Pollen-Tubes) which issue from the pollen-grains adhering to the stigma, and penetrate through the tissues until they reach the ovary.
Polyandrous
  (flowers): Flowers having many stamens.
Polygamous Plants
  Plants in which some flowers are unisexual and others hermaphrodite. The unisexual (male and female) flowers, may be on the same or on different plants.
Polymorphic
  Presenting many forms.
Polyzoary
  The common structure formed by the cells of the Polyzoa, such as the well-known seamats.
Prehensile
  Capable of grasping.
Prepotent
  Having a superiority of power.
Primaries
  The feathers forming the tip of the wing of a bird, and inserted upon that part which represents the hand of man.
Processes
  Projecting portions of bones, usually for the attachment of muscles, ligaments, etc.
Propolis
  A resinous material collected by the hivebees from the opening buds of various trees.
Protean
  Exceedingly variable.
Protozoa
  The lowest great division of the animal kingdom. These animals are composed of a gelatinous material, and show scarcely any trace of distinct organs. The Infusoria, Foraminifera, and sponges, with some other forms, belong to this division.
Pupa
  (pl. Pupae): The second stage in the development of an insect, from which it emerges in the perfect (winged) reproductive form. In most insects the Pupal Stage is passed in perfect repose. The Chrysalis is the pupal state of butterflies.
Radicle
  The minute root of an embryo plant.
Ramus
  One half of the lower jaw in the Mammalia. The portion which rises to articulate with the skull is called the Ascending Ramus.
Range
  The extent of country over which a plant or animal is naturally spread. Range IN Time expresses the distribution of a species or group through the fossiliferous beds of the earth’s crust.
Retina
  The delicate inner coat of the eye, formed by nervous filaments spreading from the optic nerve, and serving for the perception of the impressions produced by light.
Retrogression
  Backward development. When an animal, as it approaches maturity, becomes less perfectly organised than might be expected from its early stages and known relationships, it is said to undergo a Retrograde Development or Metamorphosis.
Rhizopods
  A class of lowly organised animals (Protozoa), having a gelatinous body, the surface of which can be protruded in the form of root-like processes or filaments, which serve for locomotion and the prehension of food. The most important order is that of the Foraminifera.
Rodents
  The gnawing Mammalia, such as the rats, rabbits, and squirrels. They are especially characterised by the possession of a single pair of chisel-like cutting teeth in each jaw, between which and the grinding teeth there is a great gap.
Rubus
  The bramble genus.
Rudimentary
  Very imperfectly developed.
Ruminants
  The group of quadrupeds which ruminate or chew the cud, such as oxen, sheep, and deer. They have divided hoofs, and are destitute of front teeth in the upper jaw.
Sacral
  Belonging to the sacrum, or the bone composed usually of two or more united vertebrae to which the sides of the pelvis in vertebrate animals are attached.
Sarcode
  The gelatinous material of which the bodies of the lowest animals (Protozoa) are composed.
Scutellae
  The horny plates with which the feet of birds are generally more or less covered, especially in front.
Sedimentary Formations
  Rocks deposited as sediments from water.
Segments
  The transverse rings of which the body of an articulate animal or annelid is composed.
Sepals
  The leaves or segments of the calyx, or outermost envelope of an ordinary flower. They are usually green, but sometimes brightly coloured.
Serratures
  Teeth like those of a saw.
Sessile
  Not supported on a stem or footstalk.
Silurian System
  A very ancient system of fossiliferous rocks belonging to the earlier part of the Palaeozoic series.
Specialisation
  The setting apart of a particular organ for the performance of a particular function.
Spinal Cord
  The central portion of the nervous system in the Vertebrata, which descends from the brain through the arches of the vertebrae, and gives off nearly all the nerves to the various organs of the body.
Stamens
  The male organs of flowering plants, standing in a circle within the petals. They usually consist of a filament and an anther, the anther being the essential part in which the pollen, or fecundating dust, is formed.
Sternum
  The breastbone.
Stigma
  The apical portion of the pistil in flowering plants.
Stipules
  Small leafy organs placed at the base of the footstalks of the leaves in many plants.
Style
  The middle portion of the perfect pistil, which rises like a column from the ovary and supports the stigma at its summit.
Subcutaneous
  Situated beneath the skin.
Suctorial
  Adapted for sucking.
Sutures
  (in the skull): The lines of junction of the bones of which the skull is composed.
Tarsus
  (pl. Tarsi): The jointed feet of articulate animals, such as insects.
Teleostean Fishes
  Fishes of the kind familiar to us in the present day, having the skeleton usually completely ossified and the scales horny.
Tentacula
  or Tentacles: Delicate fleshy organs of prehension or touch possessed by many of the lower animals.
Tertiary
  The latest geological epoch, immediately preceding the establishment of the present order of things.
Trachea
  The windpipe or passage for the admission of air to the lungs.
Tridactyle
  Three-fingered, or composed of three movable parts attached to a common base.
Trilobites
  A peculiar group of extinct crustaceans, somewhat resembling the woodlice in external form, and, like some of them, capable of rolling themselves up into a ball. Their remains are found only in the Palaeozoic rocks, and most abundantly in those of Silurian age.
Trimorphic
  Presenting three distinct forms.
Umbelliferae
  An order of plants in which the flowers, which contain five stamens and a pistil with two styles, are supported upon footstalks which spring from the top of the flower stem and spread out like the wires of an umbrella, so as to bring all the flowers in the same head (Umbel) nearly to the same level. (Examples, parsley and carrot.)
Ungulata
  Hoofed quadrupeds.
Unicellular
  Consisting of a single cell.
Vascular
  Containing blood-vessels.
Vermiform
  Like a worm.
Vertebrata
  or Vertebrate Animals: The highest division of the animal kingdom, so called from the presence in most cases of a backbone composed of numerous joints or Vertebrae, which constitutes the centre of the skeleton and at the same time supports and protects the central parts of the nervous system.
Whorls
  The circles or spiral lines in which the parts of plants are arranged upon the axis of growth.
Workers
  See neuters.
Zoea-Stage
  The earliest stage in the development of many of the higher Crustacea, so called from the name of Zoea applied to these young animals when they were supposed to constitute a peculiar genus.
Zooids
  In many of the lower animals (such as the Corals, Medusae, etc.) reproduction takes place in two ways, namely, by means of eggs and by a process of budding with or without separation from the parent of the product of the latter, which is often very different from that of the egg. The individuality of the species is represented by the whole of the form produced between two sexual reproductions; and these forms, which are apparently individual animals, have been called Zooide.