- Macronucleus
- macroplankton
- Maddock's classification (1982)
- Main canal
- Main lateral spines
- Main spicule
- Main spines
- manca
- mandible
- Mantle
- Mantle cartilage
- mantle cavity
- Mantle gland
- mantle length
- mantle locking cartilage
- Manubrial pouch
- Manubrium
- Manus
- Marginal cirri
- Marginal cleft
- Marginal lappet
- Marginal ring canal
- Marginal sense organ
- Marginal tentacle
- Marginal vesicle
- Marginal wart
- marsupial
- marsupium
- masticatory process
- Maxilla
- maxilla 1
- maxilla 2
- Maxilla I
- Maxilla II
- maxillae
- maxillary carrier
- maxilliped
- Maxillipeds
- maxillula
- Maxillule
- Md
- Mdp
- Median bar
- Median footlobe
- Median lip depression
- Median tubercle
- Medullary shell
- Medusa
- Medusa budding
- Medusary nodule
- Medusozoa
- Membranelle
- merus
- Mesentery
- Meshwork
- Mesoglea
- mesogloea
- mesohaline
- mesopelagic
- mesoplankton
- Metagenesis
- metamorphosis
- Metastomium
- Micronucleus
- Micropores
- Microsphere
- midbrain
- migration
- Minute stage
- Mitral ring
- molar process
- Moniliform tentacle
- Monodisc scyphistoma
- Monokinetid
- Monolamella
- Monolocular shell
- Monothalamous shell
- moult
- Mouth
- Mouth arm expansions
- Mouth arms
- Mouth lip
- Mouth primary
- Mouth-arm disk
- Mouthlet
- mouthplate
- Mucocyst
- mucus feeder
- mucus-producing glands
- Multilocular cephalis
- Multilocular shell
- Multisegmented shell
- Muscle field
- Muscle fold
- muscle scar
- Muscle strand
- muscular fibers
- Mx1
- Mx2
- Mxp
- Myofriscs
- Myoneme
- Myonemes