390 , PRINCIPLES OF ANIMAL BIOLOGY 



Mollusk {moV lusk). One of the MoUusca. 



Monaxon {tnon aks' on). Having one axis; rod-like; said of sponge spicules. 



Mondino da Luzzi (man dee' no da loot' see). Italian anatomist in University of 



Bolugna, in early part of fourteenth century. 

 Monocystis {mon' o sis' tis). A genus of gregarines, Protozoa of the class Sporozoa, 



order Gregarinida. M. agllis {aj' il is) is a common species. 

 MoncEcious {mon ee' shits). Having the organs of both sexes in the same individual 



which is thus a hermaphrodite ; said of species. 

 Monotreme {mon' o treem). One of the Monotremata (Prototheria) ; an egg-laying 



mammal having a cloaca. 

 Morphology (morfol' oji). The branch of biology which deals with the structure of 



living things. 



Motor {mo' ter). Pertaining to movement; applied to a neuron which conveys im- 

 pulses resulting in muscular movement, glandular action, and the like. 



Motor root {mo' ter 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. 



Mucin (mw' sin). A substance from which mucus is derived; it is secreted by certain 

 glands. 



Mucosa {muko'sa). The layer of cells lining the digestive tract of vertebrate 

 animals. 



Muellerian duct {mill le' ri an dukt'). A tube formed in the embryo of most verte- 

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

 exceptions) in the male. 



Miiller, Johannes {miiV ler). German physiologist and anatomist, 1801-1858. 



Muscle {mus' s'l). An aggregation of contractile cells. 



Mutation {mu ta' shun). A heritable modification arising from internal causes in an 



organism. 

 Mycelium {mi se' li urn) {pi., mycelia). The filamentous growths of fungi and 



related plants. 

 Myoneme {mi' o neem.). One of several contractile filaments in the stalks of ^'orti- 



cella and its alhes. 



Myotome {mi' o tome). One of the segments into which certain muscles are divided. 



Myriapoda {meer' i a-p' o da) . A class of Arthropoda having tracheae, one pair of 



antennae, and many unspecialized legs; centipedes and millipedes. 

 Myxomycetes {miks' o mi see' teez). The shme-molds. 



Nacre {na' ker). The pearly substance secreted by mollusks upon their shell or 

 other objects. 



Nais {na' is). A genus of freshwater worms, phlyum Annelida, subclass Oligochseta. 



Nasal pit {na' zal pit'). The ectodermal depression in an embryo which forms much 

 of the nostril. 



Natica {naf i ka). A genus of marine snails. 



Natural history {nat'ural his' tori). A descriptive account of things in nature, 



particularly animals and plants, though the term is sometimes used to include 



minerals, rocks, climate, etc. 

 Natural selection {naf u ral se lek' shun). The doctrine that through natural processes 



th(! fittest individuals are enabled to survive. 



Nautiloid {naw' ti loid). One of the extinct cephalopods resembling Nautilus. 



