9 Major Groups of Invertebrate Animals

Updated on October 17, 2019
JR Cuevas profile image

John Ray is a member of Science Olympiad, participates in science and health Writing competitions, and studied at a sci-tech school.

There are 30 phyla in the animal kingdom that include invertebrates. This article takes a closer look at the nine most important of these groups.
There are 30 phyla in the animal kingdom that include invertebrates. This article takes a closer look at the nine most important of these groups. | Source

What Are Invertebrates?

In contrast to bony vertebrates, invertebrates are animals with no vertebral column or backbone. Invertebrates comprise approximately 97% of the animal kingdom. Most of them are soft-bodied and do not develop rigid internal skeletons. Many invertebrates do, however, possess hard exoskeletons to protect their bodies from their environments. Some common examples of invertebrates are snails, sponges, earthworms, squids, sea-stars, centipedes, butterflies, spiders, and jellyfish.

Photos of Some Common Invertebrates

Click thumbnail to view full-size
Unidentified crab from PexelsUnidentified grasshopper from PixabayUnidentified butterfly from PexelsUnidentified wasp from PexelsUnidentified jellyfish from PexelsUnidentified inchworm from PexelsUnidentified starfish from Pexels Unidentified anemone from Pexels
Unidentified crab from Pexels
Unidentified crab from Pexels | Source
Unidentified grasshopper from Pixabay
Unidentified grasshopper from Pixabay | Source
Unidentified butterfly from Pexels
Unidentified butterfly from Pexels | Source
Unidentified wasp from Pexels
Unidentified wasp from Pexels | Source
Unidentified jellyfish from Pexels
Unidentified jellyfish from Pexels | Source
Unidentified inchworm from Pexels
Unidentified inchworm from Pexels | Source
Unidentified starfish from Pexels
Unidentified starfish from Pexels | Source
Unidentified anemone from Pexels
Unidentified anemone from Pexels | Source

The 9 Main Phyla of Invertebrates

Groups of Invertebrate Animals
Examples
Estimated Number of Species
Phylum Porifera
Sponges
3,000
Phylum Cnidarians
Jellyfish, corals, anemones, hydra
9,200
Phylum Platyhelminthes
Flatworms, flukes, tapeworms
15,000
Phylum Echinodermata
Sea stars, brittle stars, sea urchins, sea cucumbers
5,000
Phylum Mollusca
Snails, clams, squids, octopi, other mollusks
70,000
Phylum Nematoda
Ascaris, vinegar eels, hookworms, nematodes, pinworms
500,000
Phylum Arthropoda
Crabs, scorpions, insects, spiders, millipedes, centipedes, barnacles
750,000
Phylum Annelida
Earthworms, leeches, lugworms
22,000
Phylum Chordata
Sea squirts, lancelets
2,000
Click thumbnail to view full-size
Polymastia boletiformisUnidentified Porifera photo from WikipediaClathrina clathrusA variety of Porifera Unidentified Porifera photo from Wikipedia"Pipe organ" spongesUnidentified Porifera photo from Wikipedia
Polymastia boletiformis
Polymastia boletiformis | Source
Unidentified Porifera photo from Wikipedia
Unidentified Porifera photo from Wikipedia | Source
Clathrina clathrus
Clathrina clathrus | Source
A variety of Porifera
A variety of Porifera | Source
Unidentified Porifera photo from Wikipedia
Unidentified Porifera photo from Wikipedia | Source
"Pipe organ" sponges
"Pipe organ" sponges | Source
Unidentified Porifera photo from Wikipedia
Unidentified Porifera photo from Wikipedia | Source

1. Phylum Porifera (Sponges)

Sponges, or phylum Porifera, are one of the most common types of invertebrate animals. Currently, there are about 3,000 documented sponge species. The phylum name comes from the Latin words porus, which means "pore," and ferre which means "to bear." the phylum is so named because most sponges bear holes.

Porifera are multicellular animals that consist primarily of tissue and lack complex organs. Most reside in the ocean anchored to coral reefs, rocks, or shells. Sponges occasionally grow on oysters, cover their shells, and prevent them from feeding. This can result in the deaths of the affected oysters and may have an impact on the oyster industry.

Click thumbnail to view full-size
Pelagia noctilucaHydra vulgaris Mushroom coralsActinia fragaceaDendronephthya sp.Entacmaea quadricolor
Pelagia noctiluca
Pelagia noctiluca | Source
Hydra vulgaris
Hydra vulgaris | Source
Mushroom corals
Mushroom corals | Source
Actinia fragacea
Actinia fragacea | Source
Dendronephthya sp.
Dendronephthya sp. | Source
Entacmaea quadricolor
Entacmaea quadricolor | Source

2. Phylum Cnidaria (Coelenterates)

Cnidarians or Coelenterates are jelly-like aquatic invertebrate animals that are radially symmetrical and have tentacles that encircle a mouth at one end of the body. The life-cycle of a cnidarian includes multiple distinct stages. During the sessile stage, cnidarians resemble cylindrical polyps, while during the medusa stage, they are free-swimming and look like jellyfish.

The name Cnidaria is derived from the Greek word knide, which means "nettle" or "stinging." The word refers to specialized stinging cells called nematocysts that can be projected by certain cnidocytes as a defense. Nematocysts contain poison that can paralyze and sometimes kill other animals. Other examples of cnidarians include corals, hydra, and sea anemones.

Examples of Cnidarians (Coelenterates)

  • Jellyfish differ in shape, size, and color. Some have shorter tentacles, some have longer tentacles, and some have limbs with nematocysts.
  • Hydra are small, freshwater organisms that resemble jellyfish. Interestingly, hydra have regenerative properties and don't seem to "age" in a traditional sense.
  • Corals live in large colonies that serve as sanctuaries and nurseries for fish and other marine organisms.

Click thumbnail to view full-size
Photo of an unidentified marine flatwork from Wikipedia Photo of unidentified NZ flatworm from WikipediaCaenoplana coeruleaEgg of Fasciola hepaticaHelicometria sp.Taenia saginataProtoscolex of Echinococcus granulosus
Photo of an unidentified marine flatwork from Wikipedia
Photo of an unidentified marine flatwork from Wikipedia | Source
Photo of unidentified NZ flatworm from Wikipedia
Photo of unidentified NZ flatworm from Wikipedia | Source
Caenoplana coerulea
Caenoplana coerulea | Source
Egg of Fasciola hepatica
Egg of Fasciola hepatica | Source
Helicometria sp.
Helicometria sp. | Source
Taenia saginata
Taenia saginata | Source
Protoscolex of Echinococcus granulosus
Protoscolex of Echinococcus granulosus | Source

3. Phylum Platyhelminthes (Flatworms)

Platyhelminthes are structurally simple, wormlike invertebrate animals that lack anuses and circulatory systems. The name Platyhelminthes derives from the Greek terms platys, which means "flat," and helmin, which means "worm." There are about 15,000 known species of flatworm. Flatworms' bodies are flattened from the back, or dorsal side, to the belly, or ventral side.

Examples of Platyhelminthes (Flatworms)

  • Planaria maculata is a free-living invertebrate that occupies freshwater habitats. Adult Planaria are usually about one centimeter in length but can be longer.
  • Flukes are parasitic worms that live as ectoparasites or endoparasites in various parts of their hosts' bodies, including the intestines, blood, and liver. Flukes have a very simple digestive cavity.
  • Tapeworms are endoparasites that usually live in the organs of their hosts. They feed on their hosts' digested food but do not have digestive cavities of their own.

Click thumbnail to view full-size
Protoreaster linckii Acanthaster brevispinus Gilchrist's sea urchinUnidentified photo of a sea urchin and its eggs from WikipediaBrown-striped brittle starsAstroporpa annulataUnidentified photo of sea cucumbers from Wikipedia
Protoreaster linckii
Protoreaster linckii | Source
Acanthaster brevispinus
Acanthaster brevispinus | Source
Gilchrist's sea urchin
Gilchrist's sea urchin | Source
Unidentified photo of a sea urchin and its eggs from Wikipedia
Unidentified photo of a sea urchin and its eggs from Wikipedia | Source
Brown-striped brittle stars
Brown-striped brittle stars | Source
Astroporpa annulata
Astroporpa annulata | Source
Unidentified photo of sea cucumbers from Wikipedia
Unidentified photo of sea cucumbers from Wikipedia | Source

4. Phylum Echinodermata (Echinoderms)

Phylum Echinodermata is a group of invertebrate marine animals that have spiny skins. The word Echinodermata comes from the Greek terms echinos, which means "hedgehog," and derma, which means "skin." There are about 5000 species of echinoderms in the world. Members of this group display radial symmetry, and most individuals' bodies are divided into five equal parts that encircle a central axis.

Examples of Echinodermata (Echinoderms)

  • Starfish inhabit the shallow waters near ocean shores. They come in a variety of colors, shapes, and sizes. There are blue, bright red, reddish-brown, and flesh-colored starfish. They usually have five arms, but some have more than ten. Their arms encircle a small central disk. Some have no visible arms at all and are shaped like balls.
  • Sea-urchins are the spiniest of all echinoderms. There are several kinds of sea-urchins. The most common are black and have short spines. Some black and purplish sea-urchins with relatively small bodies and very long spines are dangerous. Their spines can irritate or injure human skin.
  • Brittle stars sport non-dangerous spines. However, their arms are fragile (which is how they got their name), so it is not advisable to hold or touch them. Through a process known as regeneration, brittle stars are able to regrow lost arms.
  • Sea cucumbers are not spiny-skinned. They have an ovoid body, similar to the shape of a real cucumber. Sea cucumbers have a habit of shooting out milky and sticky fluid as a defense tactic when disturbed.

Click thumbnail to view full-size
Tridacna maximaUnidentified mollusk shells from PixabayUnidentified land snail from PixabayUnidentified squid from PixabayUnidentified octopus from PixabayUnidentified mollusk shells from Pixabay
Tridacna maxima
Tridacna maxima | Source
Unidentified mollusk shells from Pixabay
Unidentified mollusk shells from Pixabay | Source
Unidentified land snail from Pixabay
Unidentified land snail from Pixabay | Source
Unidentified squid from Pixabay
Unidentified squid from Pixabay | Source
Unidentified octopus from Pixabay
Unidentified octopus from Pixabay | Source
Unidentified mollusk shells from Pixabay
Unidentified mollusk shells from Pixabay | Source

5. Phylum Mollusca (Mollusks)

Phylum Mollusca is a group of invertebrate animals whose members can be found both in water and on land. The word Mollusca comes from the Latin term mollis, which means "soft." Because mollusks are soft-bodied animals, most have hard shells made of calcium carbonate to protect their bodies from predators and the environment. Shelled mollusks are divided into two categories: univalves and bivalves. Most also have a muscular ventral foot. There are about 70,000 species of mollusks in the world.

Examples of Mollusca (Mollusks)

  • Snails are abundant on land and in water and vary in shape, size, color, and design. Snails have muscular ventral feet that they use to move themselves over surfaces.
  • Squids and octopi are examples of mollusks that don't have outer shells. The skins of these species are more durable than those of shelled mollusks. Squids have ten muscular arms that they use to capture prey.

Click thumbnail to view full-size
Ascaris suumAscaris lumbricoides
Ascaris suum
Ascaris suum | Source
Ascaris lumbricoides
Ascaris lumbricoides | Source

6. Phylum Nematoda (Nematodes)

Nematoda is a group of invertebrate animals commonly known as nematodes. Nematodes are unsegmented roundworms that are elongated and slender. The word Nematoda comes from the Greek term nematos which means "thread." Nematodes live in a wide variety of environments, including soil, freshwater, saltwater, and in the bodies of plants and animals as parasites.

Examples of Nematoda (Nematodes)

  • Ascaris lumbricoides is a nematode that often appears in pigs. Ascaris usually live in large numbers in pigs' intestines. They are also known as common roundworms. In humans, they affect children more often than adults. This is thought to be because children are generally less careful than adults in their hygiene habits.
  • Vinegar eels are not actually eels. They are nematodes whose scientific name is Turbatrix aceti. Generally, vinegar eels are smaller than Ascaris.
  • Filaria worms, hookworms, and pinworms are all common nematodes that act as parasites in human and animal bodies.

Click thumbnail to view full-size
Araneus diadematusGecarcinus ruricolaCicadella viridisAndroctonus crassicaudaPhoto of unidentified centipede from Wikipedia. Oxidus gracilis
Araneus diadematus
Araneus diadematus | Source
Gecarcinus ruricola
Gecarcinus ruricola | Source
Cicadella viridis
Cicadella viridis | Source
Androctonus crassicauda
Androctonus crassicauda | Source
Photo of unidentified centipede from Wikipedia.
Photo of unidentified centipede from Wikipedia. | Source
Oxidus gracilis
Oxidus gracilis | Source

7. Phylum Arthropoda (Arthropods)

Phylum Arthropoda is a group of invertebrate animals with jointed limbs and exoskeletons made of chitin. It is the largest phylum in the animal kingdom.

There are an estimated 750,000 species in this phylum. Of this number, 700,000 species belong to Class Insecta, 25,000 belong to Class Crustacea, 15,000 belong to Class Arachnida, 800 belong to Class Chilopoda, and 200 belong to Class Diplopoda.

The word Arthropoda comes from the Greek terms arthron, which means "joint," and pous, which means "foot." Butterflies, spiders, millipedes, centipedes, crabs, barnacles, scorpions, and ticks all belong to Phylum Arthropoda.

Click thumbnail to view full-size
EarthwormHaemopis sanguisugaScientific illustrations of leech anatomy
Earthworm
Earthworm | Source
Haemopis sanguisuga
Haemopis sanguisuga | Source
Scientific illustrations of leech anatomy
Scientific illustrations of leech anatomy | Source

8. Phylum Annelida (Annelids)

Phylum Annelida is a group of invertebrate animals with segmented, muscular bodies. The name Annelida comes from the Latin term annulus, which means "ring." This name refers to the ringlike segments of their bodies. The digestive systems of annelids stretch from the mouth to the anus, and different sections of the system have different roles.

Examples of Annelida (Annelids)

  • Earthworms belong to the phylum Annelida and are abundant in soil. They have cylindrical, segmented bodies. These annelids are beneficial to humans—they help enrich the soil by allowing the air to enter as they burrow through the earth.
  • Leeches, or Hirudo medicinalis, are another common annelid species. In the past, they have been used to remove blood from individuals for medical purposes.
  • Marine sandworms, Nereis virens, and Aelosoma are also annelids.

Click thumbnail to view full-size
Nembrotha lineolataPhoto of an unidentified sea squirt from WikipediaPolycarpa aurataDidemnum mollePolycarpa aurataPhoto of unidentified sea squirts from WikipediaBranchiostoma
Nembrotha lineolata
Nembrotha lineolata | Source
Photo of an unidentified sea squirt from Wikipedia
Photo of an unidentified sea squirt from Wikipedia | Source
Polycarpa aurata
Polycarpa aurata | Source
Didemnum molle
Didemnum molle | Source
Polycarpa aurata
Polycarpa aurata | Source
Photo of unidentified sea squirts from Wikipedia
Photo of unidentified sea squirts from Wikipedia | Source
Branchiostoma
Branchiostoma | Source

9. Phylum Chordata (Chordates)

While many species in Phylum Chordata have backbones, there are some that are invertebrates. The name Chordata comes from the Greek word chorde, which means string. While invertebrate chordates don't have backbones, they do have notochord structures that support their bodies.

Examples of Invertebrate Chordates

  • Sea squirts are animals that squirt water from openings in their body covers when they are touched suddenly. Adult sea squirts live underwater attached to harder objects near beaches. Sea squirts have muscular coats over their bodies that are referred to as "tunics". Members of their subphylum are called tunicates.
  • Lancelets, or amphioxus, are another type of invertebrate chordate. These slender, fish-like chordates live at the edge of the sea and burrow in the sand.

What is your favorite group of invertebrate animals?

See results

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

  • What are some examples of Sponges?

    Some examples of Sponges are Calcarea, Hexactinellida, Demospongiae, and Sclerospongiae.

  • What is a cephalization?

    Cephalization is the concentration of sense organs, nervous control, etc., at the anterior end of the body, forming a head and brain, both during evolution and in the course of an embryo's development.

  • Are seahorses invertebrates?

    Seahorses are actually vertebrate animals.

© 2018 Ray

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    • profile image

      Danish ali khoso 

      2 days ago

      thanks for given me knowledge

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      IM YOU IN THE FUTURE 

      4 weeks ago

      hi thanks for the help also pls answer my question thanks:]

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      Viky 

      4 weeks ago

      Thank you very much for your help

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      Science 

      8 weeks ago

      This help me with my homework thanks...... Tomorrow I will be active in school.....

    • Miebakagh57 profile image

      Miebakagh Fiberesima 

      6 months ago from Port Harcourt, Rivers State, NIGERIA.

      I can only note significant comments that relate to and is relevant to the article. All writers commenting would kindly take that into consideration. Many thanks, and enjoy the weekend.

    • profile image

      lati 

      6 months ago

      hi rayguy

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      Someone 

      6 months ago

      This helped me with my science homework thank you so much

    • Miebakagh57 profile image

      Miebakagh Fiberesima 

      6 months ago from Port Harcourt, Rivers State, NIGERIA.

      All new comments were well noted. Thanks.

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      Abbas Ali 

      6 months ago

      Good Job Sir

    • JR Cuevas profile imageAUTHOR

      Ray 

      7 months ago from Philippines

      Thank you very much Linus Divine. I am glad you enjoyed reading my article. You can share it with your friends and relative s who are interested with invertebrate animals as well. Have a great day!

    • profile image

      Linus Divine 

      7 months ago

      I enjoy this website a lot for its simplest explanation of terms, thank you so much.

    • Miebakagh57 profile image

      Miebakagh Fiberesima 

      7 months ago from Port Harcourt, Rivers State, NIGERIA.

      Hi. I agreed. Thanks.

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      anwaarnbatala@gmail.com 

      7 months ago

      thank you very much this website made me learn a lot thank you very much

    • profile image

      Lol 

      10 months ago

      Ew yak

    • Miebakagh57 profile image

      Miebakagh Fiberesima 

      16 months ago from Port Harcourt, Rivers State, NIGERIA.

      Hello JR, thanks for the feedback. I even like science subjects that related to the living organism. Will be watching for your articles.

    • JR Cuevas profile imageAUTHOR

      Ray 

      16 months ago from Philippines

      Me too, Sir Eric. I love eating mollusks. We have here in the Philippines a famous dish called "Adobong Pusit" (Squid Ink), and it tastes so good.

      Thanks, Sir Eric. Although I don't know if the pictures are children-friendly. Some pictures are cringy because of the slimy appearance. But, I hope you and your child enjoy reading and learning from my article. Have a great day!

    • Ericdierker profile image

      Eric Dierker 

      16 months ago from Spring Valley, CA. U.S.A.

      Mollusks because they are fun to eat!

      Well done. A much needed refresher as my son and I are way into knowing this stuff.

    • JR Cuevas profile imageAUTHOR

      Ray 

      16 months ago from Philippines

      Thanks, Miebakagh. I also love Biology eventhough it is not included in my path of study anymore. I was once a school representative in different Science competitions in our city so I was so inlove with this subject.

      Expect for more Biology-related articles Miebakagh. I'll be writing more science articles that I love. Thank you so much for the interest. Have a great day!

    • Miebakagh57 profile image

      Miebakagh Fiberesima 

      16 months ago from Port Harcourt, Rivers State, NIGERIA.

      Hi John, thanks for the update. I read Biology in secondary grammar school. It is one of my favorite and best science subject, and I made "A" in my external exams. I consistently it biology and related subjects and apply them in my life.

      AS an avid reader, I would read any subject that relate Biology. I will be watching for your fourth coming articles always.

      Many thanks again.

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