GLOSSARY 389 



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Metatheria. A subclass of mammals including the marsupials or pouched mammals. 

 Metazoon. An animal composed of many cells. Although the term contrasts an 



animal with the protozoa, it is not a name of any taxonomic group of animals. 

 Miastor. A genus of fhes; the larvae are often paedogenetic. 

 Microgromia socialis. A species of protozoon which forms a gregaloid colony in a 



gelatinous supporting substance. 

 Micronucleus. The smaller nucleus in a cell or organism having two nuclei of unequal 



size. 

 Micropyle. A small hole in the shell of an egg through which the spermatozoon enters 



in fertilization. 

 Microstomum {mi Aros' to mum). A genus of rhabdocoele flatworms. 

 Microtus. A genus of field mice. 



Miocene. Belonging to middle Tertiary time; succeeding the Oligocene. 

 Miohippus {mi' o hip' pus). An extinct horselike animal from the Oligocene. 

 Mirbel, Charles Francois {mccr hcV). French botanist, 1776-1854. 

 Mississippian. The hfth period of the Paleozoic era, following the Devonian and 



preceding the Pennsylvanian. 

 Mitochondria {mi' to kon' dri a). Objects of unknown function and of various shapes 



(threadlike, rod-shaped, or granular) found in the cytosome of many cells. 

 Mitosis. Cell division involving the formation of chromosomes, spindle fibers, etc. 



Also called karyokinesis. 

 Moeritherium {me' ri the' ri um). An extinct animal from the Eocene of Egypt, 



probably an early ancestor of the elephants. 

 Molar. One of the grinding teeth of a mammal, back of the incisors and canines. 

 Mold. A cavity in a rock representing the form of an animal or plant or other 



object whose remains formerly occupied the cavity. 

 Molecule. Usually a group of atoms behaving as a imit of the substance which they 



compose. It is the smallest particle which possesses the chemical nature of the 



substance. 

 Mollusca. The phylum of animals including the clams, snails, cuttlefishes, etc. 



For definition see Chap. 19. 

 Mollusk. One of the ^Nlollusca. 

 Monoecious {mo nee' shus). Having the organs of both sexes in the same individual 



which is thus a hermaphrodite; said of species. 

 Monosaccharide {mon' o .sak' a ride). A simple sugar; one which cannot be broken 



down into simpler sugars. 

 Monotreme. One of the Monotremata (Prototheria) ; an egg-laying mammal having 



a cloaca. 

 Morgan, T. H. Leading American geneticist, 1866-1945. 



Morphology. The branch of biolog,y which deals with the structure of living things. 

 Motor. Pertaining to movement; applied to a neuron which conveys impulses result- 

 ing in muscular movement, glandular action, and the like. 

 Motor root. The ventral one of two roots by which a spinal nerve is connected with 



the spinal cord. So called because its fibers have a motor function. 

 Motor unit. The group of muscle cells innervated by a single nerve fiber. 

 Muellerian duct {mul le' ri an). A tube formed in the embryo of most vertebrate 



animals, becoming the oviduct in the female and degenerating (with few excep- 

 tions) in the male. 

 Muscle. An aggregation of contractile cells. 

 Mustelus mustelus {mus te' lus). A species of shark. 

 Mutation. A heritable modification arising in an organism. 

 Myelin. A fatty substance forming a sheath around many nerve fibers. 



