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Home >>Zoology Dictionary >> Histology - Hyracoidea
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Histology - The study of the structure and development of tissues.
Holometabola - (=Endopterygota).
Holostei - Order of ray finned fishes that first appeared in Permian times but now almost completely replaced by modem teleosts. Two genera survive Lepidosteus, the gar pike, and Amia, the bowfin both of which live in the Great Lakes of America. The swim bladder is used as a breathing organ to some extent.
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Holothuroidea - Sea cucumbers.
Holozoic - Feeding on solid organic material.
Homeostasis - The maintainance of constant conditions within the body, e.g. constant temperature and constant
composition of the blood.
Homodont - (See Heterodont).
Homoiothermy - Maintenance of a constant body temperature regardless of the surrounding temperature, a condition found only in birds and mammals. The so called 'cold blooded' animals whose temperatures vary with that of the surroundings are poikilothermic.
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Homologous Chromosomes - Each body cell has two sets of chromosomes, which can be arranged in pairs. The members of each pair are said to be homologous and each carries genes affecting the same features.
Homologous Organs - Organs of different animals which have the same fundamental structure and / or develop from corresponding parts of the embryos. The arm of a man is clearly homologous with the wing of a bird or the flipper of a whale, or even the fin of a ling fish. The bone structure is basically similar and so is their embryological development. But homologous organs are not always similar in structure or in function: the tiny bones that transmit sound in the mammalian middle ear are homologous with certain bones that are concerned with the articulation of the jaw in
lower vertebrates. They arise in the same way in the embryo but follow different paths of development. If organs are homologous it implies that the animals concerned have a common ancestor and that a feature of that ancestor has evolved along two or more different lines to produce the homologous organs.
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Homozygous - (S Gene).
Honeybee - (See Bee).
Hormone - A complicated substance produced by certain cells (normally collected into glands) that is released into the bloodstream continuously or periodically. Many hormones are known-e.g. adrenaline, insulin, thyroxine each with a specific composition and activity. Hormones are extremely important in the regulation of the internal activity of the body.
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Host - Organism that is being attacked by a parasite, A dog is host to the fleas in its hair and a man suffering from malaria is host to the malarial parasites in his blood.
Humerus - Bone of the upper forelimb.
Hybrid - An organism resulting from the mating of two different
sppecies. Only closely related species can mate and produce offspring and even then the offspring itself is normally unable to reproduce because its two sets of chromosomes do not match and cannot therefore produce sex cells, The mule, offspring of a horse and an ass, is one of the best known animal hybrids.
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Hydra - Small, freshwater coelenterate of class Hydrozoa 61.
Hydrozoa - Class of Coelenterata, Usually colonial and showing a
wellmarked alternation of generations ,although Hydra itself is an exception to both of these rules. Obelia is typical of the class. The Portuguese-man-ofwar (Physalia) also belongs to this class and each 'individual' is, in fact, a floating colony composed of numerous polyps.
Hymeaoptera - A large order of insects including the bees, wasps, ants, sawf1ies, and ichneumon flies.
There are two pairs of membranous wings with reduced venation.
The larvae are generally legless and metamorphosis is complete. The biting mandibles are retained in all members of the group but other parts of the head are often modified for sucking up nectar, etc. There is always an ovipositor, modified for boring or cutting into plants, and sometimes modified as a sting.
There are two sub orders: ;Symphyta, ;which contains more primitive forms such as sawflies and wood wasps which do not have the waspwaist' characteristic of the other suborder Apocrita which contains the parasitic ichneumon flies as well as the bees, wasps, and ants, many of which are social insects.
Hyoid Arch - Visceral arch, behind the jaws of fishes. It has an interesting evolutionary history.
Hyperparasite - Parasite that lives in or on another parasite.
Hypoglossal Nerve -Twellth cranial nerve, of vertebrates. Found only in amniotes, it serves the tongue. In fishes, the corresponding nerve arises from the spinal cord and serves the muscles under the pharynx.
Hypopharyax - A structurein the mouths of certain insects. It forms an important part of the sucking appartus of the mos quito.
Hyracoidea - An order of small mammals conies or hyraxes that show in their teeth and some other features a perhaps unexpected
relationship to elephants. They are herivorous and are found over
large parts of Africa and Middle East.
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