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  Section: Zoological Terms used in General Zoology
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M - Zoological Terms

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(L. makros, long, large, + evolvere, to unfold). Evolutionary change on a grand scale, encompassing the origin of novel designs, evolutionary trends, adaptive radiation, and mass extinction.


(Gr. makros, long, large, + gamos, marriage). The larger of the two gamete types in a heterogametic organism, considered the female gamete.


(Gr. makros, long, large, + meros, part). The largest size class of blastomeres in a cleaving embryo when the blastomeres differ in size from one another.


A very large molecule, such as a protein, polysaccharide, or nucleic acid.


(Gr. makros, long, large, + nucleus, kernel). The larger of the two kinds of nuclei in ciliate protozoa; controls all cell functions except reproduction.


(Gr. makros, long, large, + phago, to eat). A phagocytic cell type in vertebrates that performs crucial functions in the immune response and inflammation, such as presenting antigenic epitopes to T cells and producing several cytokines.


(Fr. madr + pore, reef-building coral, + ite, suffix for some body parts). Sievelike structure that is the intake for the water-vascular system of echinoderms.

major histocompatibility complex (MHC)

Complex of genes coding for proteins inserted in the cell membrane; the proteins are the basis of self-nonself recognition by the immune system.


(Gr. malako, soft, + ostracon, shell). Any member of the crustacean subclass Malacostraca, which includes both aquatic and terrestrial forms of crabs, lobsters, shrimps, pillbugs, sand fleas, and others.


(L. hammer). The ossicle attached to the tympanum in middle ears of mammals.

malpighian tubules

(Marcello Malpighi, Italian anatomist, 1628 - 1694). Blind tubules opening into the hindgut of nearly all insects and some myriapods and arachnids, and functioning primarily as excretory organs.


Soft extension of the body wall in certain invertebrates, for example, brachiopods and molluscs, which usually secretes a shell; thin body wall of tunicates.


(L. handle). The portion projecting from the oral side of a jellyfish medusa, bearing the mouth; oral cone; presternum or anterior part of sternum; handle-like part of malleus of ear.


(Gr. marasmos, to waste away). Malnutrition, especially of infants, caused by a diet deficient in both calories and protein.


(Gr. marsypion, little pouch). One of the pouched mammals of the subclass Metatheria.

mast cells

Inflammatory cells in a variety of locations. Upon activation by an antigen they release pharmacologically active compounds leading to redness and swelling.


(Gr. jaws). Pharyngeal mill of rotifers.


(ma´triks) (L. mater, mother). The intercellular substance of a tissue, or that part of a tissue into which an organ or process is set.


(L. maturus, ripe). The process of ripening; the final stages in the preparation of gametes for fertilization.


(L. dim. of mala, jaw). One of the upper jawbones in vertebrates; one of the head appendages in arthropods.


(L. maxilla, jaw, + pes, foot). One of the pairs of head appendages located just posterior to the maxilla in crustaceans, a thoracic appendage that has become incorporated into the feeding mouthparts.


Situated, or occurring, in the middle.

mediated transport

Transport of a substance across a cell membrane mediated by a carrier molecule in the membrane.


(L. marrow). The inner portion of an organ in contrast to the cortex or outer portion. Also, hindbrain.


(Gr. mythology, female monster with snake-entwined hair). A jellyfish, or the free-swimming stage in the life cycle of cnidarians.

Mehlis' gland

Glands of uncertain function surrounding the ootype of trematodes and cestodes.


(Gr. meion, smaller, + L. faunus, god of the woods). Small invertebrates found in the interstices between sand grains.


(Gr. from mieoun, to make small). The nuclear changes by means of which the chromosomes are reduced from the diploid to the haploid number; in animals, usually occurs in the last two divisions in the formation of the mature egg or sperm.


(Gr. melas, black). Black or dark-brown pigment found in plant or animal structures.


(Gr. melania, blackness, + pherein, to bear). Black or brown chromatophore containing melanin.

memory cells

Population of long-lived B lymphocytes remaining after initial immune response that provides for the secondary response.


, sing. meninx (Gr. meninx, membrane). Any of three membranes (arachnoid, dura mater, pia mater) that envelop the vertebrate brain and spinal cord. Also, solid connective tissue sheath enclosing the central nervous system of some vertebrates.


(Gr. men, month, + pauein, to cease). In the human female, that time of life when ovulation ceases; cessation of the menstrual cycle.


(L. menstrua, the menses, from mensis, month). The discharge of blood and uterine tissue from the vagina at the end of a menstrual cycle.


(Gr. meros, part, + blastos, germ). Partial cleavage occurring in zygotes having a large amount of yolk at the vegetal pole; cleavage restricted to a small area on the surface of the egg.


(Gr. meros, part, + zoon, animal). A very small trophozoite at the stage just after cytokinesis has been completed in multiple fission of a protozoan.


(Gr. mesos, middle, + enchyma, infusion). Embryonic connective tissue; irregular or amebocytic cells often embedded in gelatinous matrix.


(L. mesenterium, mesentery). Peritoneal fold serving to hold the viscera in position.


(Gr. mesos, middle, + koilos, hollow). Middle body coelomic compartment in some deuterostomes, anterior in lophophorates, corresponds to hydrocoel in echinoderms.


(Gr. mesos, middle, + derma, skin). The third germ layer, formed in the gastrula between the ectoderm and endoderm; gives rise to connective tissues, muscle, urogenital and vascular systems, and the peritoneum.


(Gr. mesos, middle, + glia, glue). The layer of jellylike or cement material between the epidermis and gastrodermis in cnidarians and ctenophores; also may refer to jellylike matrix between epithelial layers in sponges.


(Gr. mesos, middle, + hyle, a wood). Gelatinous matrix surrounding sponge cells; mesoglea, mesenchyme.


(Gr. mesos, middle, + lekithos, yolk). Pertaining to a zygote (or ovum) having a moderate amount of yolk concentrated in the vegetal pole.


(Gr. mesos, middle, + nephros, kidney). The middle of three pairs of embryonic renal organs in vertebrates. Functional kidney of fishes and amphibians; its collecting duct is a Wolffian duct. Adj.,



(Gr. mesos, middle, + soma, body). The portion of the body in lophophorates and some deuterostomes that contains the mesocoel.

messenger RNA (mRNA)

A form of ribonucleic acid that carries genetic information from the gene to the ribosome, where it determines the order of amino acids as a polypeptide is formed.


(Gr. metabole, change). A group of processes that includes digestion, production of energy (respiration), and synthesis of molecules and structures by organisms; the sum of the constructive (anabolic) and destructive (catabolic) processes.


(Gr. meta, between, among, after, + kentron, center). Chromosome with centromere at or near the middle.


(Gr. meta, between, among, after, + kerkos, tail, + L. aria, connected with). Fluke juvenile (cercaria) that has lost its tail and has become encysted.


(Gr. meta, between, among, after, + koilos, hollow). Posterior coelomic compartment in some deuterostomes and lophophorates; corresponds to somatocoel in echinoderms.


(Gr. meta, after, + meros, part). A repeated body unit along the longitudinal axis of an animal; a somite, or segment.


(Gr. meta, between, among, after, + meros, part). Condition of being made up of serially repeated parts (metameres); serial segmentation.


(Gr. meta, between, among, after, + morphe, form, + osis, state of). Sharp change in form during postembryonic development, for example, tadpole to frog or larval insect to adult.


(Gr. meta, between, among, after, + nephros, kidney). A type of tubular nephridium with the inner open end draining the coelom and the outer open end discharging to the exterior.


(Gr. meta, between, among, after, + nephros, kidney). Embryonic renal organs of vertebrates arising behind the mesonephros; the functional kidney of reptiles, birds, and mammals. It is drained from a ureter.


(Gr. meta, after, behind, + soma, body). The portion of the body in lophophorates and some deuterostomes that contains the metacoel.


Gr. meta, after, + zoon, animal). Multicellular animals.


See major histocompatibility complex.


(L. mikros, small, + evolvere, to unfold). A change in the gene pool of a population across generations.


(Gr. mikros, small, + L. filum, a thread). A thin, linear structure in cells; of actin in muscle cells and others.


(Gr. mikros, small, + L. filum, a thread). Partially developed juveniles borne alive by filarial worms (phylum Nematoda).


(Gr. mikros, small, + gamos, marriage). The smaller of the two gamete types in a heterogametic organism, considered the male gamete.

microglial cells

Phagocytic cells in the central nervous system, part of the reticuloendothelial system.


(Gr. mikros, small, + meros, part). The smallest size class of blastomeres in a cleaving embryo when the blastomeres differ in size from one another.


(µ) (Gr. neuter of mikros, small). One one-thousandth of a millimeter; about 1/25,000 of an inch. Now largely replaced by micrometer (µm).


(Gr. mikros, small, + nema, thread). One of the types of structures composing the apical complex in the phylum Apicomplexa, slender and elongate, leading to the anterior and thought to function in host cell penetration.


A small nucleus found in ciliate protozoa; controls the reproductive functions of these organisms.


(Gr. mikros, small, + pileos, a cap). The small opening through which the cells emerge from a gemmule (phylum Porifera).


See microvillus


(Gr. mikros, small, + L. tubule, pipe). A long, tubular cytoskeletal element with an outside diameter of 20 to 27 µm. Microtubules influence cell shape and play important roles during cell division.


(Gr. mikros, small, + L. villus, shaggy hair). Narrow, cylindrical cytoplasmic projection from epithelial cells; microvilli form the brush border of several types of epithelial cells. Also, microvilli with unusual structure cover the surface of cestode tegument (also called microthrix [pl. microtriches]).


(Gr. miktos, mixed or blended). Pertaining to haploid egg of rotifers or the females that lay such eggs.


(M. E. minerale, ore, + L. cortex, bark, + oid, suffix denoting likeness of form). Hormones of the adrenal cortex, especially aldosterone, that regulate salt balance.


(Gr. meirakidion, youthful person). A minute ciliated larval stage in the life of flukes.


(Gr. mitos, a thread, + chondrion, dim. of chondros, corn, grain). An organelle in the cell in which aerobic metabolism takes place.


(Gr. mitos, thread, + osis, state of). Nuclear division in which there is an equal qualitative and quantitative division of the chromosomal material between the two resulting nuclei; ordinary cell division.


A configuration of atomic nuclei and electrons bound together by chemical bonds.


(Gr. monos, single, + kytos, hollow vessel). A type of leukocyte that becomes a phagocytic cell (macrophage) after moving into tissues.


(Gr. monos, single, + oikos, house). Having both male and female gonads in the same organism; hermaphroditic.


adj. monogamous (Gr. monos, single, + gamos, marriage). The condition of having a single mate at any one time.


(Gr. monos, single, + L. hybrida, mongrel). A hybrid offspring of parents different in one specified character.


(Gr. monos, single, + meros, part). A molecule of simple structure, but capable of linking with others to form polymers.


(Gr. monos, single, + phyle, tribe). The condition that a taxon or other group of organisms contains the most recent common ancestor of the group and all of its descendants; contrasts with polyphyly and paraphyly


(Gr. monos, one, + sakcharon, sugar, from Sanskrit sarkara, gravel, sugar). A simple sugar that cannot be decomposed into smaller sugar molecules; the most common are pentoses (such as ribose) and hexoses (such as glucose).


(Gr. monos, single, + zoon, animal). Tapeworms with a single proglottid, do not undergo strobilation to form chain of proglottids.


(Gr. morphe, form, + genesis, origin). Development of the architectural features of organisms; formation and differentiation of tissues and organs.


(Gr. morphe, form, + L. logia, study, from Gr. logos, work). The science of structure. Includes cytology, the study of cell structure; histology, the study of tissue structure; and anatomy, the study of gross structure.


(L. morum, mulberry, + ula, dim.). Solid ball of cells in early stage of embryonic development.

mosaic cleavage

Embryonic development characterized by independent differentiation of each part of the embryo; determinate cleavage.


(L. mucus, nasal mucus). Any of a group of glycoproteins secreted by certain cells, especially those of salivary glands.


(L. mucus, nasal mucus). Viscid, slippery secretion rich in mucins produced by secretory cells such as those in mucous membranes. Adj.,


Müller's larva

Free-swimming ciliated larva that resembles a modified ctenophore, characteristic of certain marine polyclad turbellarians.

multiple fission

A mode of asexual reproduction in some protistans in which the nuclei divide more than once before cytokinesis occurs.


(L. mutare, to change). A stable and abrupt change of a gene; the heritable modification of a characteristic.


(L. mutuus, lent, borrowed, reciprocal). A type of interaction in which two different species derive benefit from their association and in which the association is necessary to both; often symbiotic.


(Gr. myelos, marrow). A fatty material forming the medullary sheath of nerve fibers.


(Gr. mys, muscle,+ kytos, hollow vessel). Contractile cell (pinacocyte) in sponges.


(Gr. mys, muscle, + L. dim. of fibra, fiber). A contractile filament within muscle or muscle fiber.


(Gr. mys, muscle, + N.L., genic, giving rise to). Originating in muscle, such as heartbeat arising in vertebrate cardiac muscle because of inherent rhythmical properties of muscle rather than because of neural stimuli.


(Gr. mys, muscle, + meros, part). A muscle segment of successive segmental trunk musculature.


(Gr. mys, muscle, + in, suffix, belonging to). A large protein of contractile tissue that forms the thick myofilaments of striated muscle. During contraction it combines with actin to form actomyosin.


(Gr. mys, muscle, + tomos, cutting). That part of a somite destined to form muscles; the muscle group innervated by a single spinal nerve.

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