GLOSSARY. 805 



pfiora, or AcalepJm), so called because of the resemblance of their tentacles 



to the snaky hair of the Medusa. Many Medusas are now known to be 



merely the gonophores of Hydrozoa. 

 MEDUSIFORM. Resembling a Medusa in shape. 

 MEDUSOID. Like a Medusa; used substantively to designate the medusiform 



gonophores of the Hydrozoa. 



MEMBRANA NICTITANS (Lat. nicto, I wink). The third eyelid of Birds, &c. 

 MENTUM (Lat. the chin). The basal portion of the labium or lower lip in 



Insects. 

 MEUOSTOMATA (Gr. meron, thigh ; stoma, mouth). An order of Crustacea in 



which the appendages which are placed round the mouth, and which offi- 

 ciate as jaws, have their free extremities developed into walking or pre- 

 hensile organs. 

 MESENTERIES (Gr. mesos, intermediate ; enteron, intestine). In a restricted 



sense, the vertical plates which divide the somatic cavity of a Sea-anemone 



(Actinia) into chambers. 

 MESOPODIUM (Gr. mesos, middle ; pous, foot). The middle portion of the 



''foot of Molluscs." 

 MESOSTERNUM (Gr. mesos, intermediate; sternon, the breast -bone). The 



middle portion of the sternum, intervening between the attachment of the 



second pair of ribs and the xiphoid cartilage (xiphisternum). 

 MESOTHORAX (Gr. mesos; and thorax, the chest). The middle ring of the 



thorax in Insects. 



MESOZOIC (Gr. inesos ; and zoe, life). The Secondary period in Geology. 

 METACARPUS (Gr. meta, after ; karpos, the wrist). The bones which form the 



"root of the hand," and intervene between the wrist and the fingers. 

 METAMORPHOSIS (Gr. meta, implying change ; morphe, shape). The changes 



of form which certain animals undergo in passing from their younger to 



their fully-grown condition. 

 METAPODIUM (Gr. meta, after ; pous, the foot). The posterior lobe of the foot 



in Mollusca; often called the " operculigerous lobe," because it develops the 



operculum when this structure is present. 

 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 ki the hind-foot of the 



higher Vertebrates. 

 METATHORAX (Gr. meta, after ; thorax, the chest). 'She posterior ring of the 



thorax in Insects. 

 METAZOA (Gr. meta, implying change ; zob'n, 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 diphyo- 



dont Mammals which are not preceded by milk-teeth. 



MOLLUSCA (Lat. mollis, soft). The sub-kingdom which includes the shell- 

 fish 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. 

 'GNERA (Gr. moneres, single). An 



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 ; mum, muscle). Applied to those bivalves 



