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Home >>Zoology Dictionary >> Nares - Nymph
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Nares - Nostrils the openings of the nasal or olfactory cavity. In most fishes the only openings are on the outside of the head external nares the olfactory organs being merely sunk into pits in the head. In longfishes and all higher vertebrates, however, there are also internal nares opening into the mouth.
Natural Selection - A naturally occurring mechanism which Darwin suggested as the basis of evolution and which is now widely held to be so.
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Darwin's theory appeared in print in 1859 as the famous 'Origin of Species' although he had previously lectured on his findings and those of Wallace who arrived independently at a similar theory at about the same time. Darwin witnessed the 'struggle for existence' among animals. Most of them produce many offspring but only a few survive. The others succumb to predators and disease; in other words there is 'survival of the fittest'. Darwin also noticed that individuals of a species all vary slightly. Such variations make some animals more suited to their surroundings than others. Those best sutited are more likely to survive and to reproduce and therefore the favourable variations will be passed on to the next generation. In this way an animal species gradually changes and becomes ideally suited to its surroundings. The latter are always changing however and so natural selection works continuously to produce new forms and, eventually, new species.
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Darwin's theory of Natural Selection can explain the evolution of the giraffe's neck quite easily. The early girafles competed with other animals for food. The giraffes with the longest necks were able to get better food and thus survived better and produced more offspring. These offspring too, had long necks but, more important, they varied among themselves. Selection again acted in favour of those with the longest necks. Over many generations the average neck length increased until the present day giraffes appeared. Lamarch (see Lamarckism) would have argued that by stretching to reach the higher branches, the giraffes gradually grew longer necks.
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Although Darwins theory showed clearly how natural variation could be the basis of evolutionary change, there was no explanation of how the variations occurred or how they were inherited. Later work on genetics, however, has shown how the natural. variation can come about and also how sudden charges may characteristics
Nekton - The free swimming life in the sea as opposed to the floating plankton and the bottom living benthos. Fishes, squids, and whales make up most of the nekton. There is no sharp division between nekton and plankton for some animals which can swim actively spend much of their time floating with the plankton.
Nematocyst - Stingin capsule' of Coelenterata.
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Nematoda - Roundworms or eel worms. Members of this phylum have cylindrical, unsegmented bodies, pointed at both ends. There is no coelom but a fluid filled space surrounds the gut and other organs of many species. Nematodes are found in every conceivable habitat and as individuals are extremely numerous. Many are parasitic and cause severe damage to crops (e.g. potato eelworm) or to animals, including man. Ascaris lumbricoides, found in the intestine of man and pigs, may reach a length of nine inches but the majority of nematodes are very much smaller.
Nematomorpha - Small phylum of very tine worm like creatures(hairworms) which in their young stages parasitise insects.
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Nemertea - Proboscis worms a small phylum of free living, unsegmented worms most of which are marine. In being flattened and possessing flame cells, they resemble the platybelminthes, but they show certain advances over the latter notably the possession of a blood system and two openings to the gut. The most characteristic feature is the proboscis which lies in a cavity above the gut. It is rather like the finger of a glove that has been pulled into the hand by a muscle attached to the tip. It may be sticky or armed with ,spines and it can be shot out by muscular action of catch food.
Neoteny - The persistence of larval features in an animal beyond the normal larval period. It may be temporary-caused by unsuitable conditions for further development or it may be a permanent retention of larval characteristics, in an otherwise adult animal. In permanent neoteny called paedogenesis the animal breeds in its larval form. Many salamanders show neoteny and paedogenesis. The phenomenon may have played a large part in evolution.
Nephridium - Excretory organ of many invertebrates consisting of a tube opening to the exterior. The inner end may be closed as in the flatworms and other acoelomates
Nerve - Group of nerve fibres, together with their surrounding membranes, running to or from particular region.
Nerve Cell - (=Neurone). Cell of the nervous system that actually conducts impulses. Typically the cell body is rounded and gives off one long axon and several short dendrites. The axon is not always, present however: brain cells and cells of primitive nerve nets give off a number of fairly equal processes and pass signals in all directions. The axon, together with the surrounding membranes and fatty sheath, forms the nerve fibre and it may be nearly as long as the animal itself. The impulses pass from the cellbody along the axon which ends in a muscle or gland or makes a synapse with the dendrites of another neurone. A synapse is a junction between neighbouring nerve cells. The cells do not join but impulses are able to 'jump' the gap and continue their way. Nerve fibres or axons can normally conduct impulses in only one direction towards the central nervous system in the case of sensory nerves, and away from it in the case of motor nerves. The majority of cell bodies lie within the central nervous system so that ther nerves running through the body arc composed almost entirely of axons and their sheaths. (
Nerve Cord - Any major nervous pathway, such as the spinal cord.
Nerve Fiber - (See Nerve Cell).
Nerve Net - Simple nervous system found in Coelenterata and Echimodermata. The cells branch out in all directions and connect up to form the net but there is nothing that can be called a brain.
Nerve Root - (See Spinal Nerve).
Nervous System - The whole system of nerves in an animal. A nervous system is found in all many-celled animals apart from the sponges. Every action involves the system to some degree.
Neuroptera - Order of containing the lacewing and related insects. The delicate are criss crossed by many tiny veins. Metamorphosis is complete and the larvae generally feed on other insects, notably aphids.
Nictitating Membrane - 3rd eyelid a transparent fold of skin found in many birds and reptiles and some amphibians. Only a few mammals have one.
Nidlecolus Birds - Those hatching at an early stage of development and remaining in the nest for a relatively long period.
Nidifugous Birds - Those that hatch at a relatively advanced stage and leave the nest almost immediately e.g. ducks.
Nitrogen Cycle - Nitrogen is one of the essential elements of life, being a major constituent of protein. In the form of nitrates, nitrogen is absorbed by plants and used to build up proteins. These are consumed by animals and converted to other proteins in the body. Upon the death of the organisms, the organic substance decay and bacteria convert the proteins back to nitrites and nitrates which can be used again by plants. This is the basis of the nitrogen cycle. A few bacteria, notably those forming nodules in the roots of leguminous plant, can convert free nitrogen into nitrates. Nitrates arc also formed during thunder-storms: the energy of lightning causes oxygen and nitrogen to combine. The compound so formed dissolves in the rain water and falls to earth as a very dilute solution of nitric acid. This acts on minerals in the soil to form nitrates. But the formation of nitrates in this way is offset by the activity of certain bacteria that break down protein and release freenitrogen to the air.
Notochord - A flexible skeletal rod found at some stage during the life of all chordates. In most vertebrates it is found only in the embryo and is later replace by the vertebral column.
Nucleic Acid - A complex compound made up of chains of pentose sugar molecules linked to molecules of phosphate and nitrogen containing molecules called bases. Nucleic acids occur in the chromosomes and in the cytoplasm of the cell. The genes are, in fact, chains of nucleic acid put together in various ways, and they control the features of the cells by controlling the types of protein that are made in the cells.
Nucleolus - Dense region of cell nucleus.
Nucleus - A feature of nearly all cells which contains the chromosomes
and appears to be the controlling int1uence of the cell.
Nymph - Young stage of an insect that differs from the adulf mainly in size and the absence of fully developed wings and sex organs. The wings develop gradually on the outside of the body (exopterygote condition).
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