31 Different Groups of Invertebrates

We all know that invertebrates lack backbones, but the differences among the various types of invertebrates go a lot deeper than that. On the following slides, you'll discover the 31 different groups, or phyla, of invertebrates, ranging from amoeba-like placozoans that stick to the sides of fish tanks to marine animals, like octopuses, that can achieve a near-vertebrate level of intelligence. 

Placozoans
Placozoans are considered to be the world's simplest animals. For over a century, this was the only species in placozoa, but a new species was named in 2018, another in 2019, and biologists are continuing to look for new species. One of them, Trichoplax adherens, is a small, flat, millimeter-wide blob of goo that can often be found sticking to the sides of fish tanks. This primitive invertebrate has only two tissue layers—an outer epithelium and an inner surface of stellate, or star-shaped, cells—and reproduces asexually by budding, much like an amoeba; as such, it represents an important intermediate stage between protists and true animals.

Sponges
Essentially, the sole purpose of sponges is to filter nutrients from seawater, which is why these animals lack organs and specialized tissues—and don't even possess the bilateral symmetry characteristic of most other invertebrates. Although they seem to grow like plants, sponges start their lives as free-swimming larvae that quickly take root in the seafloor (if they're not eaten by fish or other invertebrates, that is). There are about 10,000 sponge species, ranging in size from a few millimeters to more than 10 feet.

Jellyfish and Sea Anemones
Cnidarians, you may not be surprised to learn, are characterized by their cnidocytes—specialized cells that explode when irritated by prey and deliver painful, and often fatal, doses of venom. The jellyfish and sea anemones that make up this phylum are more or less dangerous to human swimmers (a jellyfish can sting even when it's beached and dying), but they are invariably a peril to the small fish and other invertebrates in the world's oceans. See 10 Facts About Jellyfish. 

Comb Jellies
Looking a bit like a cross between a sponge and a jellyfish, comb jellies are ocean-dwelling invertebrates that move by undulating the cilia lining their bodies—and, in fact, are the largest known animals to employ this means of locomotion. Because their bodies are extremely fragile and don't tend to preserve well, it's uncertain how many types of ctenophores swim the world's oceans. There are about 100 named species, which may represent less than half of the true total.

Flatworms
The simplest animals to display bilateral symmetry—that is, the left sides of their bodies are mirror images of their right sides—flatworms lack the body cavities characteristic of other vertebrates, have no specialized circulatory or respiratory systems, and ingest food and expel wastes using the same basic opening. Some flatworms live in water or damp terrestrial habitats, while others are parasites—yard-long tapeworms occasionally infesting human beings. The deadly disease schistosomiasis is caused by the flatworm Schistosoma.

Mesozoans
Just how obscure are mesozoans? Well, the 50 or so identified species of this phylum are all parasites of other marine invertebrates—which means that they're tiny, almost microscopic, in size and composed of very few cells. Not everyone agrees that mesozoans deserve to be classified as a separate invertebrate phylum. Some biologists go so far as to claim that these mysterious creatures are protists rather than true animals or flatworms (see the previous slide) that have "de-evolved" to a primitive state after millions of years of parasitism.

Ribbon Worms
Also known as proboscis worms, ribbon worms are long, exceptionally slender invertebrates that evert tongue-like structures from their heads to stun and capture food. These simple worms possess ganglia (clusters of nerve cells) rather than true brains, and respire through their skin via osmosis, either in water or damp terrestrial habitats. Nemerteans don't impinge much on human concerns unless you like to eat Dungeness crabs: One ribbon worm species feeds on this tasty crustacean's eggs, devastating crab fisheries along the West Coast of the United States.

Jaw Worms
Jaw worms look scarier than they are: Magnified a thousand times, these invertebrates evoke the monsters in an H.P. Lovecraft short story, but they're actually a few millimeters long and dangerous only to equally microscopic marine organisms. The 100 or so described gnathostomulid species lack internal body cavities and circulatory and respiratory systems. These worms are also hermaphrodites, meaning each individual bears a single ovary (the organ that produces eggs) and one or two testes (the organ that produces sperm).

Gastrotrichs
Greek for "hairy stomachs" (although some researchers call them hairy backs), gastrotrichs are near-microscopic invertebrates that live mostly in freshwater and ocean environments. A few species are partial to damp soil. You may never have heard of this phylum, but gastrotrichs are an essential link in the undersea food chain, feeding on the organic detritus that would otherwise accumulate on the seafloor. Like jaw worms (see the previous slide), most of the 400 or so gastrotrich species are hermaphrodites—individuals equipped with both ovaries and testes, and thus capable of self-fertilization.

Rotifers
Amazingly, considering how small they are—most species rarely exceed half a millimeter in length—rotifers have been known to science since around 1700 when they were described by the inventor of the microscope, Antonie von Leeuwenhoek. Rotifers have roughly cylindrical bodies and, atop their heads, cilia-fringed structures called coronas, which are used for feeding. As tiny as they are, rotifers are equipped with even tinier brains, a marked advance over the primitive ganglia characteristic of other microscopic invertebrates.

Roundworms
If you were to take a census of every single individual animal on the Earth, 80% of the total would consist of roundworms. There are more than 25,000 identified nematode species, accounting for over one million individual roundworms per square meter—on the seafloor, in lakes and rivers, and in deserts, grasslands, tundra, and just about all other terrestrial habitats. And that's not even counting the thousands of parasitic nematode species, one of which is responsible for the human disease trichinosis and others of which cause pinworm and hookworm.

Arrow Worms
There are only about 100 species of arrow worms, but these marine invertebrates are extremely populous, living in tropical, polar, and temperate seas worldwide. Chaetognaths are transparent and torpedo shaped, with clearly delineated heads, tails, and trunks, and their mouths are surrounded by dangerous-looking spines, with which they snatch plankton-sized prey out of the water. Like many other primitive invertebrates, arrow worms are hermaphroditic, each individual equipped with both testicles and ovaries.

Horsehair Worms
Also known as Gordian worms—after the Gordian knot of Greek myth, which was so dense and tangled that it could only be cleaved with a sword—horsehair worms can attain lengths of over three feet. The larvae of these invertebrates are parasitic, infesting various insects and crustaceans (but thankfully not humans), while the full-grown adults live in fresh water and can be found in streams, puddles, and swimming pools. There are about 350 species of horsehair worms, two of which infect the brains of beetles and prompt them to commit suicide in fresh water—thus propagating this invertebrate's life cycle.

Mud Dragons
Not the most widely known phylum of invertebrates, mud dragons are tiny, segmented, limbless animals, the trunks of which are made up of exactly 11 segments. Rather than propelling themselves with cilia (hairlike growths that grow out of specialized cells), kinorhynchs employ the circle of spines around their heads, with which they dig into the seafloor and inch themselves slowly forward. There are about 100 identified mud dragon species, all of which feed either on diatoms or organic matter lying on the seafloor.

Brush Heads
The invertebrates known as brush heads were only discovered in 1983, and for a good reason: These miniature (no more than one millimeter long) animals make their home in the tiny spaces in between marine gravel, and two species live in the deepest part of the Mediterranean Sea, about two miles beneath the surface. Loriciferans are characterized by their loricas, or thin external shells, as well as the brushlike structures surrounding their mouths. There are about 20 described brush head species, with another 100 or so awaiting more detailed analysis.

Spiny-Headed Worms
The thousand or so species of spiny-headed worms are all parasites and in an extremely complicated way. These invertebrates have been known to infect (among others) a small crustacean called Gammarus lacustris; the worms cause G. lacustris to seek out light rather than hiding from predators in the dark, as it normally does. When the exposed crustacean is eaten by a duck, the full-grown worms move to this new host, and the cycle begins again when the duck dies and the larvae infest the water. Moral of the story: If you see a spiny-headed worm (most measure only a few millimeters long, but some species are much bigger), stay far away.

Symbions
After 400 years of intense study, you may think human naturalists have accounted for every invertebrate phylum. Well, that wasn't the case for loriciferans (see Slide 15), and it certainly wasn't the case for Symbion pandora, the only existing species of phylum Cycliophora, discovered in 1995. The half-millimeter-long Symbion lives on the bodies of cold-water lobsters, and it has such a bizarre lifestyle and appearance that it doesn't fit well in any existing invertebrate phylum. (Just one example: Pregnant female Symbions give birth after dying, while they're still attached to their lobster hosts.)

Entoprocts
Greek for "interior anus," entoprocts are millimeters-long invertebrates that attach themselves by the thousands to undersea surfaces, forming colonies reminiscent of moss. Although they are superficially very similar to bryozoans (see next slide), entoprocts have slightly different lifestyles, feeding habits, and internal anatomies. For example, entoprocts lack internal body cavities, while bryozoans have internal cavities divided into three parts, making these latter invertebrates much more advanced, from the perspective of evolution.

Moss Animals
Individual bryozoans are extremely small (about half a millimeter long), but the colonies they form on shells, rocks, and seafloors are much bigger, extending anywhere from a few inches to a few feet—and looking uncannily like patches of moss. Bryozoans have complex social systems, consisting of autozooids (which are responsible for filtering organic matter from the surrounding water) and heterozooids (which perform other functions to maintain the colonial organism). There are about 5,000 species of bryozoans, of which exactly one (Monobryozoo limicola) does not aggregate in colonies.

Horseshoe Worms
Consisting of no more than a dozen identified species, horseshoe worms are marine invertebrates, the slender bodies of which are encased in tubes of chitin (the same protein that makes up the exoskeletons of crabs and lobsters). These animals are relatively advanced in other ways: For example, they have rudimentary circulatory systems. The hemoglobin in their blood (the protein responsible for carrying oxygen) is twice as efficient as that of humans, and they obtain oxygen from the water via their lophophores (the crowns of tentacles on top of their heads).

Lamp Shells
With their paired shells, brachiopods look a lot like clams—but these marine invertebrates are more closely related to flatworms than they are to oysters or mussels. Unlike clams, lamp shells usually spend their lives anchored to the seafloor (via a stalk projecting from one of their shells), and they feed via a lophophore or crown of tentacles. Lamp shells are divided into two broad categories: articulate brachiopods.

Snails, Slugs, Clams, and Squids
Considering the fine distinctions you've seen in this slideshow between, say, jaw worms and ribbon worms, it may seem strange that a single phylum should contain invertebrates as varied in structure and appearance as clams, squids, snails, and slugs. As a group, though, mollusks are characterized by three basic anatomical traits: the presence of a mantle (the rear covering of the body) that secretes calcareous (e.g., calcium-containing) structures; the genitals and anus both opening into the mantle cavity; and paired nerve cords.

Penis Worms
OK, you can stop laughing now: It's true that the 20 or so species of penis worms look like, well, penises, but that's a mere evolutionary coincidence. Like horseshoe worms (see Slide 20), penis worms are protected by chitinous cuticles, and these ocean-dwelling invertebrates protrude their pharynxes out of their mouths to snatch prey. Do penis worms have penises? No, they do not: The sex organs of males and females, such as they are, are merely tiny outgrowths of their protonephridia, the invertebrate equivalents of mammalian kidneys.

Peanut Worms 
Pretty much the only thing that keeps peanut worms from being classified as annelids—the phylum (see Slide 25) that embraces earthworms and ragworms—is that they lack segmented bodies. When threatened, these smallish marine invertebrates contract their bodies into the shape of a peanut; otherwise, they eat by protruding one or two dozen ciliated tentacles from their mouths, which filter organic matter from seawater. The 200 or so species of sipunculans have rudimentary ganglia instead of true brains and lack well-developed circulatory or respiratory systems.

Segmented Worms
The 20,000 or so species of annelids—including earthworms, ragworms, and leeches—all have the same basic anatomy. In between these invertebrates' heads (which contain the mouth, brain, and sense organs) and their tails (which contain the anus) are multiple segments, each composed of the same array of organs, and their bodies are covered by a soft exoskeleton of collagen. Annelids have an extremely wide distribution—including oceans, lakes, rivers, and dry land—and help maintain the fertility of the soil, without which most of the world's crops would eventually fail.

Water Bears
Either the cutest or the creepiest invertebrates on Earth, tardigrades are near-microscopic, multiple-legged animals that look uncannily like scaled-down bears. Perhaps even more eerily, tardigrades can thrive in extreme conditions that would kill most other animals—in thermal vents, in the coldest parts of Antarctica, even in the vacuum of outer space—and can withstand bursts of radiation that would instantly fry most other vertebrates or invertebrates. Suffice it to say that a tardigrade blown up to Godzilla size could conquer the Earth in no time flat.

Velvet Worms
Often described as "worms with legs," the 200 or so species of onychophorans live in tropical regions of the southern hemisphere. Aside from their numerous paired legs, these invertebrates are characterized by their small eyes, their prominent antennae, and their disconcerting habit of squirting mucus at their prey. Weirdly enough, a few velvet worm species give birth to live young: The larvae develop inside the female, nourished by a placenta-like structure, and have a gestation period as long as 15 months (about the same as that of a black rhinoceros).

Insects, Crustaceans, and Centipedes
By far the largest phylum of invertebrates, accounting for as many as five million species the world over, arthropods include insects, spiders, crustaceans (such as lobsters, crabs, and shrimp), millipedes and centipedes, and many other creepy, crawly creatures common to marine and terrestrial habitats. As a group, arthropods are characterized by their hard external skeletons (which need to be molted at some point during their life cycles), segmented body plans, and paired appendages (including tentacles, claws, and legs). See "10 Facts About Arthropods."

Starfish and Sea Cucumbers
Echinoderms—the phylum of invertebrates that includes starfish, sea cucumbers, sea urchins, sand dollars, and various other marine animals—are characterized by their radial symmetry and their ability to regenerate tissue (a starfish can often reconstitute its entire body from a single severed arm). Oddly enough, considering that most starfish have five arms, their free-swimming larvae are bilaterally symmetric, like those of other animals—it's only later in the growth process that the left and right sides develop differently, resulting in the unique appearance of these invertebrates.

Acorn Worms
You may be surprised to find a lowly worm at the end of a list of invertebrate phyla, ranked according to increasing complexity. But the fact is that acorn worms—which live in tubes on the deep seafloor, feeding on plankton and organic waste—are the closest living invertebrate relatives to chordates, the phylum that includes fish, birds, reptiles, and mammals. There are about 100 known species of acorn worms, with more being discovered as naturalists explore the deep sea—and they may shed valuable light on the development of the first animals with primitive spinal cords, way back during the Cambrian period.

Lancelets and Tunicates
Somewhat confusingly, the animal phylum chordata has three subphyla, once embracing all the vertebrates (fish, birds, mammals, etc.) and two others devoted to lancelets and tunicates. Lancelets, or cephalochordates, are fishlike animals equipped with hollow nerve cords (but no backbones) running the lengths of their bodies, while tunicates, also known as urochordates, are marine filter-feeders vaguely reminiscent of sponges but much more complicated anatomically. During their larval stage, tunicates possess primitive notochords, which is enough to cement their position in the chordate phylum.