If you’ve ever watched a bee buzz around your garden, spotted a spider spinning a web, or dug up an earthworm in moist soil, you’ve encountered an invertebrate. But what does invertebrates mean exactly? This seemingly simple question opens the door to understanding the vast majority of animal life on our planet.
In this guide, we’ll break down the definition, explore where the word comes from, and discover why these backbone-free creatures matter more than most people realize.
Quick answer: what does “invertebrate” mean?
An invertebrate is any animal that does not have a vertebral column, commonly known as a backbone or spinal column. This stands in direct contrast to vertebrate animals like mammals, birds, reptiles, amphibians, and fish, all of which possess a bony or cartilaginous spine running along their bodies.
It’s worth noting that “invertebrate” isn’t an official scientific classification on the same level as a phylum or class. Instead, it’s a descriptive, informal term that biologists and educators use to group together all animals lacking that internal skeletal structure. Think of it as a convenient label rather than a formal category.
Here’s a simple way to remember it: earthworms, butterflies, spiders, and squids are invertebrates, while humans, dogs, and salmon are vertebrates. The backbone is the dividing line.
When we talk about the animal kingdom, we’re usually talking about invertebrates. These creatures make up roughly 97 to 99 percent of all described animal species on Earth. So when someone says “animals,” the overwhelming majority of what that word covers are actually invertebrates.
Etymology: where the word “invertebrate” comes from
Understanding the history of a word often helps clarify its meaning, and “invertebrate” is no exception.
The term combines two Latin elements. First, there’s “vertebra,” a Latin noun meaning a joint in the spine. This word itself traces back to the verb “vertere,” meaning “to turn,” which refers to the turning or articulated nature of the backbone’s segments. Second, there’s the prefix “in-,” which means “not” or “without.”
Put them together, and you get “invertebrate”: without vertebrae, or lacking a backbone.
The term was popularized in the early 1800s by the French naturalist Jean-Baptiste Lamarck. In his groundbreaking work Histoire Naturelle des Animaux sans Vertèbres (1815-1822), Lamarck needed a practical word to group together all animals that lacked a backbone. The term stuck and has been used by scientists and educators ever since.
Taxonomic meaning (and limits) of the term “invertebrate”
While “invertebrates” is a useful everyday category, it’s important to understand that this grouping isn’t a formal taxonomic unit like “Phylum Arthropoda” or “Subphylum Vertebrata.” It’s more of a catch-all term defined by what these animals lack rather than what they share.
The group is what scientists call paraphyletic. This means it includes many unrelated lineages—insects, jellyfish, octopuses, worms—but excludes vertebrates, even though some invertebrates are actually more closely related to vertebrates than to other invertebrates. For example, tunicates (sea squirts) and lancelets are invertebrates within the phylum Chordata, making them closer relatives to fish and humans than to insects or snails.
Taxonomists today recognize more than 30 distinct animal phyla. Vertebrates make up only one small part: the subphylum Vertebrata within the larger phylum Chordata. Everything else falls under the informal umbrella of “invertebrate.”
Because of this incredible diversity, biologists usually don’t study “invertebrates” as a single scientific unit. Instead, they focus on specific groups—arthropods, molluscs, echinoderms, and so on. Each group has its own specialists, research questions, and dedicated journals. The field of invertebrate zoology serves as an umbrella discipline covering all these varied areas of study.
Main groups of invertebrates and what they include
The word “invertebrates” covers a staggering range of animal phyla with wildly different body plans, lifestyles, and habitats. From simple sponges to intelligent octopuses, this category contains more diversity than many people realize.
Porifera includes the sponges, ancient filter-feeding organisms that lack true tissues and spend their lives attached to rocks or seafloors. Despite their simplicity, sponges play crucial roles in marine ecosystems.
Cnidaria encompasses jellyfish, sea anemones, and corals. These animals possess specialized stinging cells called cnidocytes, which they use to capture prey. Coral reefs built by cnidarians house roughly 25 percent of all marine species.
Platyhelminthes, or flatworms, are soft-bodied creatures that include both free-living species like planarians and parasites like tapeworms. Nematoda, the roundworms, are even more numerous—over 25,000 known species, many of them living in soil or as parasites inside other organisms.
Annelida contains the segmented worms, including earthworms and leeches. The common earthworm, absent only from Antarctica, is one of the most familiar invertebrate animals to gardeners and farmers alike.
Mollusca is a remarkably diverse phylum of about 85,000 species. This group includes snails with coiled shells, clams that filter water, and cephalopods like octopuses and the giant squid. Cephalopods are known for their remarkable intelligence and complex behaviors.
Arthropoda is the largest group by far, accounting for over 80 percent of all described invertebrate species. This phylum includes insects, arachnids, crustaceans, and myriapods—all characterized by jointed legs and hard outer casing called an exoskeleton..
Echinodermata covers starfish, sea urchins, and sea cucumbers. These exclusively marine invertebrates display pentaradial symmetry as adults and possess a unique water vascular system for movement.
Some lesser-known groups like Bryozoa (moss animals), Brachiopoda (lamp shells), and Chaetognatha (arrow worms) may go unnoticed by most people, but they remain important players in marine ecosystems, much like the isopod species that thrive in vivarium ecosystems..
Familiar everyday invertebrate groups (insects, arachnids, crustaceans)
Most people first encounter invertebrates through three especially familiar groups: insects, arachnids, and crustaceans.
Insects are six-legged arthropods with three body parts: head, thorax, and abdomen. Many insects also possess one or two pairs of wings. With over one million described species, insects represent the largest group of animals on Earth. Common examples include bees, ants, beetles, butterflies, flies, and many insects you might find in your backyard on any given day. Some are beneficial pollinators, while pest insects can damage crops or spread disease. Social insects like ants and bees live in complex colonies with specialized roles.
Arachnids are eight-legged arthropods with two body parts (cephalothorax and abdomen) and no antennae. This group includes spiders, scorpions, ticks, and mites. Unlike other insects that have two antennae or crustaceans with four antennae, arachnids lack these sensory appendages entirely. Spiders serve as important predators that control insect populations naturally.
Crustaceans are mostly aquatic arthropods with multiple pairs of legs and typically two pairs of antennae. Concrete examples include crabs, lobsters, shrimps, krill, and barnacles. The terrestrial pillbug, or woodlouse, is a familiar exception that lives on land. Crustaceans support major fisheries worth billions of dollars annually..
All three groups qualify as invertebrates because they lack an internal backbone. Instead, they rely on an external exoskeleton—a hard outer casing—for structural support and protection.
Key characteristics of invertebrates
Beyond lacking a backbone, invertebrates display a huge variety of body designs. However, most invertebrates share certain broad features that distinguish them from plants, fungi, or single-celled organisms.
Most invertebrates are heterotrophic animals, meaning they must eat other organisms or organic matter to survive. They cannot make their own food like plants do. Nearly all possess specialized tissues organized into organs, and most have a digestive tract—sometimes with just one opening (like in jellyfish) and sometimes with two (like in earthworms).
One of the most interesting aspects of invertebrate biology is the diversity in body support systems. Some invertebrates have hydrostatic skeletons, using fluid-filled body cavities to maintain their shape. Earthworms and sea anemones fall into this category. Others, like insects and crabs, possess rigid exoskeletons made of chitin or calcium carbonate. Still other invertebrates have internal shells (like cuttlefish) or reduced skeletal elements (like some sponges with mineral spicules).
Invertebrates have colonized virtually every habitat on Earth. They thrive in deep-sea vents more than 3,000 meters below the ocean surface, in high-mountain soils, in arid deserts, in polar ice, in urban backyards, and even in carefully constructed bioactive vivariums that mimic natural ecosystems. Some even live as parasites inside vertebrate hosts, including humans..
Consider the contrast between a garden snail and a jellyfish. The snail carries a hard shell, moves slowly on a muscular foot, and scrapes algae from surfaces with a rasping tongue. The jellyfish drifts through open water, has soft bodies with no shell at all, and captures prey with trailing tentacles. Both are invertebrates, yet they could hardly be more different.
Body symmetry and morphology
One way to understand invertebrate diversity is to examine how their bodies are arranged—their symmetry.
Radial symmetry describes animals whose body parts radiate around a central axis, like spokes on a wheel. Jellyfish and sea anemones exhibit this arrangement, which suits their lifestyle of drifting in water or remaining attached to rocks. Prey can approach from any direction, and the animal can respond equally well.
Bilateral symmetry means the left and right sides of the body mirror each other. Insects, worms, and squids all display bilateral symmetry. This arrangement is common in animals that move in one main direction, with a distinct head end facing forward. Most species across many phyla share this body plan.
Some invertebrates show asymmetry, lacking any clear plane of symmetry. Adult sponges are a classic example, growing into irregular shapes dictated by their environment. Certain parasitic worms also display asymmetry. Even among otherwise symmetrical animals, individual asymmetries exist: fiddler crabs sport one greatly enlarged claw, and snail shells typically coil in just one direction.
Reproduction and life cycles
Invertebrates reproduce in remarkably varied ways, which is one reason scientists study them so intensively.
Most invertebrates use sexual reproduction. Males produce sperm, females produce eggs, and fertilization creates zygotes that develop into new individuals. However, many invertebrates don’t develop directly into miniature adults. Instead, they pass through larval stages that look nothing like the adult form. Sea urchin larvae float as plankton before settling to the seafloor. Caterpillars transform into butterflies through complete metamorphosis. Fly maggots become the flies we recognize. This complex life cycle allows different stages to exploit different ecological niches.
Some invertebrate groups can also reproduce asexually. Hydras and corals reproduce by budding, growing new individuals directly from the parent’s body. Certain worms and starfish can regenerate entire new organisms from fragments of themselves.
Famous model organisms have made invertebrates central to biological research. The fruit fly Drosophila melanogaster has been studied in genetics labs since the early 20th century. The nematode Caenorhabditis elegans helped scientists map an entire animal’s nervous system. These invertebrates continue to reveal fundamental truths about genes, development, and the life cycle of complex organisms.
How many invertebrates are there?
Invertebrates are by far the most numerous animals on Earth, whether you count species or individual organisms.
Biologists have formally described over 1.25 million invertebrate species as of the mid-2020s. Insects alone account for the majority of these records, with over one million catalogued species. According to Encyclopedia Britannica and other authoritative sources, invertebrates comprise more than 95 percent of all known animal species.
Compare this to vertebrates. The IUCN Red List and other global assessments document somewhere between 65,000 and 70,000 living vertebrate species—a small fraction of invertebrate numbers. For every species of mammal, bird, reptile, amphibian, or fish, there are dozens or hundreds of invertebrate species.
Many scientists estimate that the true number of invertebrate species could be several million more than currently described. Under-studied habitats like tropical forest canopies, deep-sea ecosystems, and soil communities almost certainly harbor countless undiscovered species. Some researchers suggest that as much as 80 percent of invertebrate diversity remains unknown to science.
Why invertebrates matter to ecosystems and people
Invertebrates are essential for healthy ecosystems and human wellbeing, even though they often go unnoticed and unappreciated.
In food webs, invertebrates occupy positions at the base and middle, serving as prey for fish, birds, amphibians, reptiles, and mammals. Caterpillars feed songbirds during breeding season. Krill support whale populations in polar seas. Earthworms sustain moles, shrews, and countless other predators. Remove invertebrates from the food chain, and the whole system collapses.
Ecosystem services provided by invertebrates are equally crucial. Bees, butterflies, moths, beetles, and flies pollinate roughly 75 percent of global crops, contributing an estimated $577 billion annually to agriculture. Dung beetles, earthworms, and wood-boring insects drive decomposition and nutrient recycling, breaking down dead animals and plant matter. Earthworms alone process up to 10 tons of soil per hectare annually, improving soil quality and water infiltration. Insects and other organisms provide nutrients back to the soil, enabling plants to grow.
Invertebrates also benefit humans directly. Honey bees produce honey and beeswax. Silkworms spin silk fibers. Horseshoe crab blood contains a compound essential for testing injectable medications, helping ensure the safety of 90 percent of US pharmaceuticals. Leeches are used in modern reconstructive surgery, and compounds from cone snails have yielded powerful pain medications, while pet keepers rely on specialized invertebrate supplies to maintain healthy captive populations..
Conserving invertebrates and their natural areas is critical for agriculture, water quality, and climate resilience. When insect populations decline, crops go unpollinated, pests go uncontrolled, and ecosystems unravel. A cleaner environment depends on the often-invisible work of invertebrates.
A brief history of invertebrates on Earth and in science
The evolutionary history of invertebrates stretches back hundreds of millions of years, far predating the first vertebrates.
The earliest known animal fossils are invertebrates. Sponge-like fossils discovered in South Australia date to approximately 665 million years ago, making them among the oldest evidence of animal life on Earth. These ancient organisms lived in seas that existed long before any fish, amphibian, or dinosaur walked or swam.
By the Cambrian Period, roughly 541 to 485 million years ago, many major invertebrate groups already existed. Fossil sites like the Burgess Shale in Canada preserve an extraordinary snapshot of early invertebrate diversity: trilobites with compound eyes, early molluscs, primitive echinoderms, and bizarre animals that defy easy classification. This “Cambrian explosion” represents one of the most dramatic events in the history of life.
Invertebrate fossils dominate the rock record throughout the Phanerozoic Eon, the past 540 million years. Geologists routinely use invertebrate fossils to date and correlate sedimentary rock layers. Certain species serve as index fossils, allowing scientists to determine the age of rocks across continents.
In recorded history, early biologists began systematically organizing invertebrate knowledge. Carl Linnaeus, in his landmark Systema Naturae (1758), established frameworks for classifying animals. Jean-Baptiste Lamarck, as noted by Cambridge University Press and other scholarly sources, advanced invertebrate taxonomy in the early 19th century, describing thousands of species and coining new terms.
Since the 20th century, modern genetics, advanced microscopy, and molecular phylogenetics have transformed our understanding of how invertebrate groups are related. Scientists have discovered unexpected relationships among worms, molluscs, and arthropods, reshaping the tree of life. Research continues to reveal new species at a rapid pace—over 100 new jellyfish species were described in 2023 alone.
Invertebrates in modern research and education
Invertebrates remain central to biological research and classroom learning today.
Model invertebrates have driven major scientific discoveries. The fruit fly Drosophila melanogaster helped establish the chromosomal theory of inheritance and remains a workhorse for genetics research. The nematode Caenorhabditis elegans was the first multicellular organism to have its entire genome sequenced and its complete neural wiring mapped. Mosquitoes like Aedes aegypti are studied intensively because they transmit diseases affecting hundreds of millions of people annually.
Applied research also relies heavily on invertebrates. Aquatic macroinvertebrates—mayfly nymphs, caddisfly larvae, freshwater snails—and isopods used as cleanup crews in bioactive enclosures serve as bioindicators for water quality monitoring.. Their presence or absence tells environmental scientists whether a river or stream is healthy or polluted. This approach is used worldwide by government agencies and conservation organizations.
Zoos, aquariums, and insectariums use live invertebrates to teach children and adults about biodiversity and conservation, and enthusiasts also gather at invert and reptile shows that showcase diverse species and supplies. Interactive exhibits let visitors observe leaf-cutter ants building fungus gardens, watch butterflies emerge from chrysalises, or touch sea stars in tide pool tanks. School programs bring live insects and other invertebrates into classrooms, connecting students with the incredible diversity of the animal kingdom..
Whether you’re a professional biologist, an environmental monitor, or simply a curious learner, invertebrates offer endless opportunities for discovery. They outnumber all other animals combined, underpin ecosystems worldwide, and continue to surprise scientists with new species and remarkable adaptations.
Next time you encounter a bee visiting flowers, a spider building its web, or an earthworm in your garden, take a moment to appreciate what you’re seeing: members of the vast invertebrate world that makes life on Earth possible—and remember that even lesser-known species like Trachelipus trilobatus isopods kept in terrariums are part of this incredible diversity..
