GENERAL ZOOLOGY 



MISSING LINK. See SYNTHETIC TYPE. 



MITOSIS. The process of indirect nuclear division during which chromosomes ap- 

 pear and duphcate lona;itudinally, the two members of each sister pair passing 

 into separate nuclei; cells arising after mitosis contain the diploid number of 

 chromosomes. CJ. AMITOSIS and MEIOSIS. 



MM. Abbreviation for millimeter. 



MOLT. To shed an exoskeleton or any integumentary structure. 



MONOBLASTIC. Having a closed cavity surrounded by a single layer of cells. CJ. 

 DIPLOBLASTIC and TRIPLOBLASTIC. 



MONOECIOUS. A condition, normal in manv animals, in which reproductive organs 

 characteristic of both sexes are present and functional in each individual; 

 hermaphroditic. CJ. DIOECIOUS. 



MORPHOLOGY. The science of structure. 



MOTORIUM. The neuromotor center responsible for integration and coordination 

 ol the ciliary system in ciliate protozoans. 



MOUTH. The opening of the digestive tract through which ingestion occurs. 



MOUTH CAVITY. The part of the digestive tract into which food is first taken. 



MOUTH PARTS. The appendages related to the mouth of an arthropod. 



MUCOSA. The cellular lining of the digestive tract, derived from endoderm. 



MUCUS {adj. MUCOUS). A sticky or slimy fluid secreted by certain glands, wide- 

 spread among animals and used in a variety of functions such as lubrication, 

 protection, etc. 



MULTI-. Combining form, meaning "many." CJ. UNI-. 



MUTATION. A modification of a character that is conditioned bv some change in 

 the gene complex; a heritable variation; specifically, an alteration of a gene. 

 (J. FLUCTUATION. 



MUTUALISM. A lorm of s\mbiosis consisting of an association between two or more 

 individuals, of dilTerent species, with benefil accruing to both members of the 

 as.sociation; the term symbiosis is .sometimes restricted to this situation. (./. 

 COMMENSALISM and PARASITISM. 



NASAL. Pertaining to the nose or nostril. 



NEPHRIDIUM {pi. NEPHRIDIA). An excretory tubule found in arthropods, mollusks, 

 and some other animals. 



NERVE. A bundle of nerve fibers lying outside the central nervous system. 



NERVE CELL. See NEURON. 



NERVE CORD. A compact cord composed of neurons and forming [jart of a central 

 ner\ous system; it may or may not be ganglionated. CJ. CONNECTIVE. 



NERVE FIBERS. Extensions of the cytosome of a neuron; dendrites and axons. 



NERVE IMPULSE. A physicochemical disturbance established and conducted in a 

 neuron as a result of a stimulus. 



NERVE NET. The association of neurons to form a diliuse network without ganglionic 

 aggregations; the only nervous system in some simple metazoans and a part of 

 the peripheral nervous system in many more complex forms. 



NERVE RING. A circular nerve cord. 



NERVOUS COORDINATION. Coordination brought about b\ (he n-action of parts 

 to the discharge of nerve impulses. Cf. COORDINATION and CHEMICAL CO- 

 ORDINATION. 



NEURAL. Pertaining to the nervous .system. 



NEURON. A nerve cell; a cell characterized b\- conspicuous extensions of the c\ to- 

 some, known as dendrites and axons, o\er which ner\e impulses pass. 



NICHE. The specific position or function of a particular type of organism in an 

 environment; comparable niches in different environments ma\- be filled bv a 

 variety of different organisms. 



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