An invertebrate is an animal without a backbone. Invertebrate animals include fruit flies and sea sponges.

Your backbone allows you to stand up straight, and it also allows you to be grouped with the other vertebrates: animals with backbones. Invertebrates are the opposite: they have no backbone. These are the two major groups of animals. Also, this word is used for people who are spineless in the sense of having no courage. You could say a coward acts in an invertebrate way. Whether it's a backbone-free animal or a courage-free person, all invertebrates lack spines.

Definitions of invertebrate
  1. noun
    any animal lacking a backbone or notochord; the term is not used as a scientific classification
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    invertebrate having jointed limbs and a segmented body with an exoskeleton made of chitin
    any of various invertebrate animals resembling a plant such as a sea anemone or coral or sponge
    parazoan, poriferan, sponge
    primitive multicellular marine animal whose porous body is supported by a fibrous skeletal framework; usually occurs in sessile colonies
    cnidarian, coelenterate
    radially symmetrical animals having saclike bodies with only one opening and tentacles with stinging structures; they occur in polyp and medusa forms
    comb jelly, ctenophore
    biradially symmetrical hermaphroditic solitary marine animals resembling jellyfishes having for locomotion eight rows of cilia arranged like teeth in a comb
    any of numerous relatively small elongated soft-bodied animals especially of the phyla Annelida and Chaetognatha and Nematoda and Nemertea and Platyhelminthes; also many insect larvae
    borer, woodborer
    any of various insects or larvae or mollusks that bore into wood
    minute aquatic multicellular organisms having a ciliated wheel-like organ for feeding and locomotion; constituents of freshwater plankton
    mollusc, mollusk, shellfish
    invertebrate having a soft unsegmented body usually enclosed in a shell
    hermaphrodite wormlike animal living in mud of the sea bottom
    bryozoan, moss animal, polyzoan, sea mat, sea moss
    sessile aquatic animal forming mossy colonies of small polyps each having a curved or circular ridge bearing tentacles; attach to stones or seaweed and reproduce by budding
    sessile mossy aquatic animal having the anus of the polyp outside the crown of tentacles
    any of various moss-like aquatic animals usually forming branching colonies; each polyp having a both mouth and anus within a closed ring of tentacles
    Symbion pandora
    only known species of Cycliophora; lives symbiotically attached to a lobster's lip by an adhesive disk and feeding by means of a hairy mouth ring; its complex life cycle includes asexual and sexual phases
    brachiopod, lamp shell, lampshell
    marine animal with bivalve shell having a pair of arms bearing tentacles for capturing food; found worldwide
    peanut worm, sipunculid
    small unsegmented marine worm that when disturbed retracts its anterior portion into the body giving the appearance of a peanut
    marine invertebrates with tube feet and five-part radially symmetrical bodies
    an extinct arthropod that was abundant in Paleozoic times; had an exoskeleton divided into three parts
    arachnid, arachnoid
    air-breathing arthropods characterized by simple eyes and four pairs of legs
    general term for any terrestrial arthropod having an elongated body composed of many similar segments: e.g. centipedes and millipedes
    Scutigerella immaculata, garden centipede, garden symphilid, symphilid
    minute arthropod often infesting the underground parts of truck-garden and greenhouse crops
    an arthropod of the division Tardigrada
    chiefly nocturnal predacious arthropod having a flattened body of 15 to 173 segments each with a pair of legs, the foremost pair being modified as prehensors
    millepede, milliped, millipede
    any of numerous herbivorous nonpoisonous arthropods having a cylindrical body of 20 to 100 or more segments most with two pairs of legs
    pycnogonid, sea spider
    any of various small spiderlike marine arthropods having small thin bodies and long slender legs
    Merostomata, class Merostomata
    used in some classifications; includes the orders Xiphosura and Eurypterida
    Limulus polyphemus, Xiphosurus polyphemus, horseshoe crab, king crab
    large marine arthropod of the Atlantic coast of North America having a domed carapace that is shaped like a horseshoe and a stiff pointed tail; a living fossil related to the wood louse
    Asian horseshoe crab
    horseshoe crab of the coast of eastern Asia
    large extinct scorpion-like arthropod considered related to horseshoe crabs
    pentastomid, tongue worm
    wormlike arthropod having two pairs of hooks at the sides of the mouth; parasitic in nasal sinuses of mammals
    glass sponge
    a siliceous sponge (with glassy spicules) of the class Hyalospongiae
    one of two forms that coelenterates take (e.g. a hydra or coral): usually sedentary with a hollow cylindrical body usually with a ring of tentacles around the mouth
    medusa, medusan, medusoid
    one of two forms that coelenterates take: it is the free-swimming sexual phase in the life cycle of a coelenterate; in this phase it has a gelatinous umbrella-shaped body and tentacles
    any of numerous usually marine and free-swimming coelenterates that constitute the sexually reproductive forms of hydrozoans and scyphozoans
    any of various usually free-swimming marine coelenterates having a gelatinous medusoid stage as the dominant phase of its life cycle
    hydroid, hydrozoan
    colonial coelenterates having the polyp phase dominant
    actinozoan, anthozoan
    sessile marine coelenterates including solitary and colonial polyps; the medusoid phase is entirely suppressed
    delicately iridescent thimble-shaped ctenophores
    ctenophore have long tentacles and flattened body
    sea gooseberry
    ctenophore having a rounded body with longitudinal rows of cilia
    Cestum veneris, Venus's girdle
    ctenophore having a ribbon-shaped iridescent gelatinous body
    helminth, parasitic worm
    worm that is parasitic on the intestines of vertebrates especially roundworms and tapeworms and flukes
    a larva of a woodborer
    acanthocephalan, spiny-headed worm
    any of various worms living parasitically in intestines of vertebrates having a retractile proboscis covered with many hooked spines
    arrowworm, chaetognath
    any worm of the Chaetognatha; transparent marine worm with horizontal lateral and caudal fins and a row of movable curved spines at each side of the mouth
    flatworm, platyhelminth
    parasitic or free-living worms having a flattened body
    nemertean, nemertine, proboscis worm, ribbon worm
    soft unsegmented marine worms that have a threadlike proboscis and the ability to stretch and contract
    beard worm, pogonophoran
    slender animal with tentacles and a tubelike outer covering; lives on the deep ocean bottom
    nematode, nematode worm, roundworm
    unsegmented worms with elongated rounded body pointed at both ends; mostly free-living but some are parasitic
    annelid, annelid worm, segmented worm
    worms with cylindrical bodies segmented both internally and externally
    burrowing marine mollusk
    gastropod, univalve
    a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes
    chiton, coat-of-mail shell, polyplacophore, sea cradle
    primitive elongated bilaterally symmetrical marine mollusk having a mantle covered with eight calcareous plates
    bivalve, lamellibranch, pelecypod
    marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together
    cephalopod, cephalopod mollusk
    marine mollusk characterized by well-developed head and eyes and sucker-bearing tentacles
    any mainly aquatic arthropod usually having a segmented body and chitinous exoskeleton
    onychophoran, peripatus, velvet worm
    any of numerous velvety-skinned wormlike carnivorous animals common in tropical forests having characteristics of both arthropods and annelid worms
    small air-breathing arthropod
    an insect or other arthropod between molts
    sea star, starfish
    echinoderms characterized by five arms extending from a central disk
    brittle star, brittle-star, serpent star
    an animal resembling a starfish with fragile whiplike arms radiating from a small central disc
    basket fish, basket star
    any starfish-like animal of the genera Euryale or Astrophyton or Gorgonocephalus having slender complexly branched interlacing arms radiating from a central disc
    sea urchin
    shallow-water echinoderms having soft bodies enclosed in thin spiny globular shells
    primitive echinoderms having five or more feathery arms radiating from a central disk
    holothurian, sea cucumber
    echinoderm having a flexible sausage-shaped body, tentacles surrounding the mouth and tube feet; free-living mud feeders
    type of:
    animal, animate being, beast, brute, creature, fauna
    a living organism characterized by voluntary movement
  2. adjective
    lacking a backbone or spinal column
    “worms are an example of invertebrate animals”
    synonyms: spineless
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    having a backbone or spinal column
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