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Home >>Biology Dictionary >> Exoskeleton - Extraembryonic - Eye Spot
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Evolutionary species. These are the lineages evolving on altogether different lines form one another. Lineages represent the ancestral decendant sequence of population.
Exarch. When xylem and phloem are arranged in two different radii in roots, then the arrangement is referred to as exarch condition.
Excretion. Getting rid of products of metabolism (either by storing them in insoluble form, or by removing them from body). In animals particularly applies to products of protein metabolism, organs mainly concerned being kidneys of vertebrates, Malpighian tubes of insects, and probably nephridia in any other invertebrates.
Excretory organs. It refers to those organs which are concerned with the excretion of waster products. In vertebrates they include skin, lungs, intestine and liver.
Exergonic.
(Of a chemical reaction) yielding energy, as in oxidation of glucouse to carbon dioxide and water during respiration.
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Exoskeleton. Skeleton covering the outside of the body, or situated in the skin. In Arthropoda the exoskeleton, e.g. cuticle (q.v.) of insects, or lime-impregnated cuticle of crabs, is superficial to the epidermis. Shell of many mollusks is a similar exoskeleton of lime – impregnated cuticle. In many vertebrates, e.g.scaly fish, tortoises, armadillo, the exoskeleton consists chiefly of bony plates beneath epidermis; in tortoise and armadillo these plates have a horny covering of keratin (q.v.) derived from epidermis.
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Exobiology. It is the study of living organisms in the universe.
Exocytosis. Passage of material from interior to surface of cell within a vacuole.
Exosmosis. If water can diffuse out of a cell into the surroundings then such an osmosis is termed exosmosis.
Expiration. It involves breathing out of air loaded with carbon dioxide.
Expired air. The composition of expired air is as follows:
Nitrogen - 79%
Oxygen - 16%
Carbon dioxide - 4%
Water vapour sufficient
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Expiratory reserve volume. It is the additional volume of air that can be breathed out with the maximum effort by a person after breathing out his tidal volume. A person can breath out an additional volume of about one litre of air.
Exteroceptor. Receptor which defects stimuli emanating from outside the animal, e.g.light, heat, sound.
Extra cellular. Wihin an organism, but not within its constituent cells.
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Extraembryonic. Derived from the zygote, but lying outside the epidermis of the embryo proper. E. membranes are those extraembryonic structures of vertebrates concerned in nutrition, respiration, and protection of embryo; yolk-sac, amnion, chorion, allantois.
Extrorse. (Of the dehiscence of an anther), away from centre of flower.
Exudate. The material that comes from a cut pore or break in the surface of an organism, such as sweat or cellular debris.
Eye. Receptor organ for light. Some form of eye occurs very widely among animals, varying from the simple ocellus to the very complex organs of insects (see Eye Compound), vertebrates (see Fig) and cephalopod mollusks. Eyes of the two latter groups are extraordinarily similar, offering an interesting example of analogous organs.
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Eye, Compound.
Eye of insects and Crustacea. Consists of a number of separate elements (ommatidia) each with lightsensitive cells and refractive system which can form an image; the ommatidia being separated from each other in varying degrees by pigment. According to the structure of the eye, and to the distribution of pigment between the ommatidia, the eye can from either apposition images, in which case each ommatidium will focus only rays almost parallel to its long axis, so that each forms an image of only a very small part of the visual field, an image of the whole resulting from combination of these part images; or superposition images in which the sense cells of an ommatidium may receive light from a large part of the visual field so that the image received may overlap those received by as many as thirty neighbouring ommatidia.
The superposition image thus gains in luminosity but loses in sharpness compared with the apposition image. Diurnal insects have apposition eyes, nocturnal ones superposition, but there are many intermediate grades:and in some cases one may change temporarily into other by movement of pigment between the ommatidia (dark adaptation).
Eye – muscles.(a) Extrinsic Eye –muscles. Those outside eye-ball. The set of sic muscles which move each eyeball, characteristic of all vertebrates with functional eyes. Consists of anterior (i.e. near to nose) pair of oblique muscles and four, more posterior (i.e. nearer to ears), rictus muscles. Supplied by third, fourth, and sixth cranial nerves.
Eye-Spot.
Light –sensitive organelle containing carotenoci pigment found in many Protozoa and motile unicellular and colonial green algae and in zoospores and gametes of non-motile forms, also known as stigma.
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