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Home >>Zoology Dictionary >> Miocene Epoch - Myriapoda
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Miocene Epoch - Divistion of geological time.
Miracidium - A stage in the life cycle ofliver fluke.
Mite - (See Acarina).
Mitochondria - Tiny particles in cells, that appear to be concerned with
respiration.
Motor - Motor nerve fibres are those that carry impules to muscles and glands, stimulating them to action. Sensory fibres are those that carry messages to the brain from sense organs.
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Mitosis - Nuclear division chief feature of which is that the new nuclei receive identical chromosomes and genes to those of the parent cell. This is the normal process of division that takes place every time new cells are formed in the body. Before the nucleus begins to divide the chromosomes duplicate themselves exactly, forming pairs of chromatids, and then during the dividing process one of each pair of chromatids goes to each new nucleus. After the nucleus has divided, the cell itself divides.
Molar - Crushing and grinding tooth in the cheek region of mouth.
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Mollusca - A large phylum of softbodied, unsegmented animals, normally covered with a calcareous shell. The head is usually well-developed and there is a muscular 'foot'. All or part of the body is covered by a thick sheet of skin the mantle-which secretes the shell outside it. The animals have a coelom but it is reduced and largely replaced by a haemocoel. Living forms include the slugs and snails, bivalves such as cockles and mussels and oysters, and the squids and octopuses. Among the many oxtinct forms the ammonites are the most important. The three main classes: classes: Gasteropoda, Lamellibranchiata; and Cephalpoda have diverged considerably from the ancestral form which must have resembled the present-day chitons somewhat. These animals (Class Amphineura) creep about the sea bed on their muscular feet, their bodies protected by a shell consisting of eight plates. The remaining class (Scaphopoda) contains a few creatures that are in some respects part. way between gasteropods and lamellibranchs. They have a tubular, open-ended shell and burrow in the sea-bed. Like the gasteropods they have a radula.
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Monotremata - Order of mammals showing many primitive features that indicate the reptilian ancestry of mammals, although the monotremes the echidnas and the duck billed platypus are only distantly related to the other living mammals. The affinities of the monotremes with the reptiles are especially obvious through similarities in the bones of the skull, the back bone. and the limbs and limb girdles. The gut, reproductive and excretory systems open into a common chamber the cloaca and perthaps the most striking characteristic of all, they lay large, yolky eggs that have shells. The young are suckled by the female, however, and feed on milk produced by specialised sweat glands on the abdomen. These open as slits; the ducts do not form nipples as in other mammals.
The female echidna grows a pouch during the breeding season. She lays only one egg and, with her beak like muzzle places it in the pouch to incubate it. The youngster hatches in the pouch and feeds there on milk. After a period of weaning, the young are left to fend for themselves.
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Morphology - The study of the external form of animals and organs.
Mosquito - An insect of the order Diptera family Culicidae. Mosquitoes are extremtly important because the females are blood-feeders and transmit a number of serious diseases including malaria and yellow fever after feeding on infected persons. The mouthparts are modified into sharp needles which puncture the skin and which enclose two channels one up which blood is sucked, the other down which saliva and infectious parasites enter the wound. The eggs of mosquitoes are laid in water and the larvae develop there. Unlike the pupae of most insects, the mosquito pupa can move about, although it does not feed in this stage.
Moth - (See Lepidoptera).
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Muscle - A highly contractile tissue that is responsible for the majority of animal movements. Muscle is composed of fibres of which there are three main types. Striped,or skeletal muscle fibres, which make up the bulk of the body's muscles, are about 1/l0th millimeter in diameter but they may be several centimeters long. Each contains hundreds of nuclei and is made of long, thin strands (fibrils) which are striped with alternate light and dark bands. The width of these bands alters when the muscle contracts. Striped muscle is under conscious control and can contract very rapidly. Unstriped muscle is found in the lining of the gut and the blood vessels and certain other places in vertebrates. It is controlled by the nervous system but not consciously (See Autol1omic Nervous System).Each fibre is about 1/5th millimetre long and 1/15th millimetre wide and contains only one nucleus. Unlike striped muscle, it can maintain a state of contraction for long periods without fatigue. Also called smooth or involuntary muscle. Cardiac muscle is found only in the heart. Its fibres branch and form an elaborate network well-suited to the continual contraction and relaxation of the heart. Its fibrils are similar to those of ordinary striped muscle.
Musculo-epithellal Cell - Characteristic cell of Coelenterata.
Mutation - A sudden change in a gene or chromosome that leads to the appearance of new features in the organism possessing it. Because they are controlled by genes, these new features can be inherited: Most mutations are harmful and. the animals do not normally survive, but some are useful and make the animal more successful. These useful mutations will be passed on to succeeding generations.
Myelin - Fatty substance sheathing nerve fibres of vertebrates and some other animals.
Myriapoda - Class of Arthropoda, containing the centipedes (Chilopoda) and millipedes (Diplopoda). They have one pair of antennae, many pairs of legs and breathe by means of tracheae. Centipedes have one pair of legs per segment while millipedes have two pairs per segment. Among many other differences, centipedes are carnivorous while millipedes feed on vegetable matter. Although included in one class, the two groups are not closely related.
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