GLOSSARY 



413 



vertebrates; although found elsewhere, it is the basis 

 for defining the Lophophorata, the Ectoprocta, 

 Phoronida, and Brachiopoda. 



"Low" a cyclone. 



macroscopic large enough to be seen with the naked eye. 



magma the molten and gaseous material that is formed 

 within the earth and cools to form igneous rock. 



mognetosphere the radiation belt about the earth. 



mandible a jaw; either jaw of an arthropod; the lower jaw 

 of a vertebrate. 



mantle (1) an animal's body fold, which encloses soft 

 structures; in a moUusk it secretes the shell; (2) the 

 middle layer of the earth between the surface crust 

 and the central core. 



marsh a semiaquatic habitat having emergent herbs. 



moss movement any movement of earth materials by grav- 

 ity, particularly a movement of the products of 

 weathering. 



massive rock or mineral without crystals; also a rock of 

 homogeneous or irregular structure, without layering 

 or capable of being separated into layers. 



matterhorn any needlelike peak resulting from glacial 

 activity. 



maxilla in certain arthropods, mouthparts behind and 

 lying' partly upon the mandibles; in vertebrates, the 

 large upper jaw bone. 



medial toward the center, midline, or middle of an or- 

 ganism's body. 



medusa the free-swimming, vaguely umbrellalike, adult 

 stage in the life cycle of coelenterates; a jellyfish. 



megaphyll a true leaf with many, often branching veins. 



meiosis a process, occurring in various possible stages in 

 the life cycles of different organisms, in which the 

 number of each kind of chromosome is halved; the 

 process compensating for chromosome doubling due 

 to fertilization and zygote formation. 



membrane any thin, pliable sheet or layer of cells, tissues, 

 or secretions of cells or tissues. 



meristem a group of cells or tissue that lack maturity and 

 contribute to essentially continuous growth of an or- 

 ganism, typically a plant. 



mesoderm the middle layer of cells, perhaps tissue, of the 

 gastrula or other embryo stages of animals other than 

 Porifera, Mesozoa, Coelenterata, and Ctenophora; of- 

 ten applied to any tissue formed from this layer. 



mesophyte any plant adapted to habitats lacking ex- 

 tremes of moisture and drought. 



metabolism the total chemical processes (mainly nu- 

 trition, formation of living substance, and energy 

 production) in protoplasm; the site is in cells or 

 cell-like organisms. 



metamerism segmental repetition of body units (somites) 

 of like basic structure and origin, as in annelids, 

 arthropods, and chordates; true segmentation. 



metamorphosis (1) in organisms, the marked change in 

 body form, during development, from one stage of the 

 life cycle to another; in insects, gradual or simple 

 metamorphosis involving egg, nymph, and adult and 

 complete or complex metamorphosis, egg, larva, pupa, 

 and adult; (2) in rocks, the alteration of any existing 

 rock to metamorphic rock. 



Metazoa a descriptive term or taxon synonymous with 

 animals as here defined. 



meteorology the study of the atmosphere and its 

 phenomena. 



microhabitat a small habitat within a larger habitat, gen- 

 erally confined to a single object or part of an object, 

 such as a fallen log or tree trunk. 



microphyll a true leaf having one, generally unbranched, 

 vein. 



micropyle the opening in the integuments of a spermato- 

 phyte's ovule, allowing pollen tube entrance to the 

 female gametophyte. 



microscopic invisible or not clearly discernable by the 

 unaided or naked eye. 



midvein the central, usually largest, vein of a leaf. 



migration not used here in the sense of any organism's 

 movement; the periodic, annual, or daily movement of 

 organisms between two areas. 



mimicry the imitation of the structure or behavior of a 

 comparatively protected species (the model) by a 

 comparatively defenseless species (the mimic). 



mineral refer to p. 68. 



"missing link" an organism possessing features, usually 

 structural, that cause it to appear intermediate be- 

 tween two other groups of organisms; usually applied 

 to one or a few species that are sole representatives of a 

 large taxon; an unfortunate connotation in that no 

 species is strictly intermediate in all structures, 

 functions, and behavior patterns between any two 

 other species or higher categories. 



mitosis nuclear division involving chromosomal duplica- 

 tion and the production of two daughter nuclei with 

 the same chromosomal composition as the parent 

 nucleus; usually ends in cell division and the produc- 

 tion of two daughter cells from a single parental cell. 



molecule the smallest part of a chemical compound or 

 element having the distinctive properties of any larger 

 unit of the same material. 



molt to shed a body covering. 



monophylletic pertaining to a group of species in a single 

 larger taxon that is believed to include only descend- 

 ants from a common ancestor. 



moraine an accumulation of glacial debris, mostly by 

 deposition from a glacier. 



mucous pertaining to mucus. 



mucus a sticky, slimy, and/or slippery substance secreted 

 by a cell, tissue, membrane, or organ. 



