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Home >>Biology Dictionary >> Arachnida - Arteriole
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Aqueous humour
It is the fluid that is present between cornea and lens.
Arachnida
A class of terrestrial arthropods containing scorpions, spiders, ticks, mites, harves-men, king-crabs. Most living members are terrestrial, breathing air, but king-crabs are aquatic. No antenna-like or mandible-like appendages; the first pair of appendages, chelicerae, are used for grasping; the second, pedipalps, may be grasping, sensory or locomotory; the remaining four are locomotory.
Bases of second and following appendages are often modified for chewing. Head and thorax are combined into a prosoma; abdomen may be distinct. Contains, amongst others, the orders Acarina, Araneida, Eurypterida, Scorpionidea, Xiphosura.
Araneida
Spiders, order of Arachnida. Prosoma and abdomen distinct. Ends of chelicerae modified into piosonious fangs. Pedipalps of male are modified for copulation. Produce ‘silk’ from abdominal glands.
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Arable farming
The cultivation of land to produce crops.
Archegoniatae
Members of Cryptogamia in which female sex organ is an archegonium i.e. Bryophyta and Pteriodophyta.
Archegonium
Female sex organ of liverworts, mosses, ferns, and related plants, and most gymnosperms; multicellular, consisting of a neck, composed of one or more tiers of cells, and a swollen base (venter), containing the egg-cell.
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Archenteron (gastrocoel)
A cavity within an animal embryo at the gastrula stage of development. All or part of the archenteron eventually forms the cavity of the gut. It is connected to the outside by an opening (the blastospore), which becomes either the mouth, and anus, or the anal opening of the animal.
Archesporium
Cells or cell from which spores are ultimately derived, e.g. in developing pollen sac, fern sporangium.
Archichlamydeae
Sub-class of Angiospermae in which the individual member of the corolla are entirely separate from each other or the perianth is incomplete.
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Arsenic
A chemical element in the form of grey metal, more familiar in the extremely poisonous form of arsenious trioxide. It was at one time used in arsenical soap for the preservation of animal and bird skin in museums, so that particular care should be taken in handling old museum skins.
Arteriole
Small artery of vertebrates, less than 1/3 mm diameter. Smooth muscle of wall well developed and, under control of autonomic nervous system. Acts as a kind of stopcock regulating blood flow through capillaries (which are continuations of the arterioles).
Aril
A fleshy coloured, succulent investment of the seed in a few plants, e.g. yew, developed from stalk or base of ovule.
Arista
A large bristle, located on the dorsal edge of the apical antennal segement in the Deptera (house fly).
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Aristotle’s lantern
A five – sided, globular structure supporting the mouth and jaws of sea urchins.
Artery
A blood vessel that carries blood from the heart toward the other body tissues. In vertebrates arteries have thick walls, containing elastic and collagen fibres and smooth muscle, to withstand the high blood pressure near the heart. They re lined with smooth flat cells (endothelium), like other blood vessels. Arterial system starts with one or few arteries at heart, which by repeated branching as they proceed away from the heart, give rise to an increasing of ever smaller arteries, until the branches become arterioles.
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Arthropoda
A large phylum in the animal kingdom in number of species, including crabs, insects, spiders, centipes, etc., amounting to eighty per cent of known animals. Metamerically segmented; cuticle of chitin, usually hardened between joints; paired jointed appendages, segmentally arranged, used for locomotion, feeding and sensation; small coelom, body cavity a large haemocoel; heart a contractile tube in hoemocoel, with holes (ostia) in its walls; central nervous system ventral to gut, except for brain lying dorsal to oesophagus; no nephridia; no cilia (present in Onychophora).
Contains classes Onychophora, Trilobita, Crustacea, Myriapoda, Arachnida, Insecta. The living arthropods are sometimes treated as three distinct Phyla, the Arachnida, the Crustacea, and the Onychophora – Myriapoda – Insecta group.
Arthiritis
It is a disorder of joints due to any one or more of the following reasons.
(i) lack of synovial fluid
(ii) The ossification of articular cartilage
(iii) Deposition of uric acid crystals in synovial cavity.
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