GLOSSARY. 589 



MAXILLA (Lat. jaws). The inferior pair or pairs of jaws in the Arthropoda 

 ([nsects, Crustacea, &c.) The upper jaw-bones of Vertebrates. 



MAXILLIPKDES (Lat. maxillce, jaws ; pes, the foot). The limbs in Crustacea 

 and Mynapoda which are converted into masticatory organs, and are com- 

 monly called "foot-jaws." 



MEDULLA (Lat. marrow). Applied to the marrow of bones, or to the spinal 

 cord, with or without the adjective <f spinalis." 



MEDUSA. An order of Hydrozoa, commonly known as Jelly-fishes (Disco- 

 phora, or Acalephce), so called because of the resemblance of their tentacles 

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

 merely the gonophores of Hydrozoa. 



MEDUSIFORM. Resembling a Medusa in shape. 



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

 gonophores of the Hydrozoa. 



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



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

 Insects. 



MEROSTOMATA (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 (xiphuternuni). 



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

 thorax in Insects. 



MESOZOIC (Gr. mesos; 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 deve- 

 lops 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 inter- 

 vene 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. 



MIMETIC (Gr. mimetikos, imitative). Applied to organs or animals which re- 

 semble 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 ijie Mollusca, 

 comprising the Polyzoa, Tunicata, and Brachiopoda. 



MONADS (Gr. monas, unity). Microscopical organisms of an extremely 

 simple character, developed in organic infusions. 



MONOCULOUS. Possessed of only one eye. 



MONODELPHIA (Gr. monos, single ; delphus, womb). The division of Mammalia 

 in which the uterus is single. 



