Metastoma

(Gr. meta, after; stoma, mouth). The plate which closes the mouth posteriorly in the Crustacea.

Metatarsus

(Gr. meta,, after; tarsos, the instep). The bones which intervene between the bones of the ankle (tarsus) and the digits in the hind-foot of the higher Vertebrates.

Metathorax

(Gr. meta, after; thorax, the chest). The posterior ring of the thorax in Insects.

Metazoa

(Gr. meta, implying change ; zoon, animal). Applied to animals in which the primitive indifferent tissue of the embryo becomes converted into cells, which in turn may or may not be developed into more complex tissues. Under this head are included all animals except the Protozoa.

Mimetic

(Gr. mimetikos, imitative). Applied to organs or animals which resemble each other in external appearance, but not in essential structure.

Molars

(Lat. mola, a mill). The "grinders" in man, or the teeth in diphyodont Mammals which are not preceded by milk-teeth.

Mollusca

(Lat. mollis, soft). The sub-kingdom which includes the shellfish proper, the Polyzoa, the Tunicata, and the Lamp-shells ; so called from the generally soft nature of their bodies.

Molluscoida

(Mollusca; Gr. eidos, form). The lower division of the Mollusca, comprising the Polyzoa, Tunicata, and Brachiopoda.

Monads

(Gr. monas, unity). Microscopical organisms of an extremely simple character, developed in organic infusions.

Monera

(Gr. moneres, single). An order of Protozoa, comprising animals composed of simple undifferentiated sarcode.

Monoculous

Possessed of only one eye.

Monodelphia

(Gr. monos, single; delphus, womb). The division of Mammalia in which the uterus is single.

Monoecious

(Gr. monos, single; oikos, house). Applied to individuals in which the sexes are united.

Monomyary

(Gr. monos, single; muon, muscle). Applied to those bivalves (Lamellibranchiata) in which the shell is closed by a single adductor muscle.

Monophyodont

(Gr. monos; phuo, I generate; odous, tooth). Applied to those Mammals in which only a single set of teeth is ever developed.

Monothalamous

(Gr. monos; and thalamos, chamber). Possessing only a single chamber. Applied to the shells of Foraminifera and Mollusca.

Monotremata

(Gr. monos; trema, aperture). The order of Mammals comprising the Duck-mole and Echidna, in which the intestinal canal opens into a "cloaca" common to the ducts of the urinary and generative organs.

Multilocular

(Lat. multus, many; loculus, a little purse). Divided into many chambers.

Multivalve

Applied to shells which are composed of many pieces.

Multungula

(Lab. multus, many; ungula, hoof). The division of Perissodactyle Ungulates, in which each foot has more than a single hoof.