- macroplankton
- manca
- mandible
- mantle
- mantle cavity
- mantle gland
- mantle length
- mantle locking cartilage
- manubrium
- manus
- marginal bulb
- marginal spur
- marginal tentacle
- marginal tentacle bulb
- marginal vesicle
- marsupial
- marsupium
- masticatory pad
- masticatory process
- mastigopus
- maxilla 1
- maxilla 2
- maxillae
- maxillary carrier
- maxilliped
- median footlobe
- median furrow
- median lip depression
- median tubercle
- medusa
- medusoid
- Medusozoa
- megalopa
- meroplankton
- merus
- mesentery
- mesogloea
- mesohaline
- mesopelagic
- metamorphosis
- metanauplius
- metazoa
- metazoea
- micronekton
- migration
- molar process
- moult
- mouth
- mouth arm
- mouthplate
- mucus feeder
- mucus-producing glands
- muri
- muscle scar
- mysis