Marsipobranchii

(Gr. marsipos, a pouch; bragchia, gill). The order of Fishes comprising the Hag-fishes and Lampreys with pouch-like gills.

Marsupialia

(Lat. marsupium, a pouch). An order of Mammals in which the females mostly have an abdominal pouch in which the young are carried.

Mastax

(Gr. mouth). The muscular pharynx or "buccal funnel" into which the mouth opens in most of the Rotifera.

Masticatory

(Lat. mastico, I chew). Applied to parts adapted for chewing.

Maxillae

(Lat. jaws). The inferior pair or pairs of jaws in the Arthropoda (Insects, Crustacea, &c). The upper jaw-bones of Vertebrates.

Maxillipedes

(Lat. maxillae, jaws; pes, the foot). The limbs in Crustacea and Myriapoda which are converted into masticatory organs, and are commonly called "foot-jaws."

Medulla

(Lat. marrow). Applied to the marrow of bones ; or to the spinal cord, with or without the adjective "spinalis."

Medusae

An order of Hydrozoa, commonly known as Jelly-fishes (Discophora, or Acalephae), 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, etc.

Mentum

(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 officiate as jaws, have their free extremities developed into walking or prehensile 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 (xiphistemum).

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 develops the operculum when this structure is present.