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9 Major Groups of Invertebrate Animals

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Ray was 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.

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.

The 9 Main Phyla of Invertebrates

Groups of Invertebrate AnimalsExamplesEstimated 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

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.

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.

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.

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.

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.

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.

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.

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.

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.

References

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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

Question: What are some examples of Sponges?

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

Question: What is a cephalization?

Answer: 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.

Question: Are seahorses invertebrates?

Answer: Seahorses are actually vertebrate animals.

Question: What are the kinds of invertebrate animals?

Answer: Basically, when we speak of "kinds", it is a level lower than kingdom. So there will be more than 20 or 30 kinds of vertebrate animals. Some of these are spiders, crabs, sponges, insects, and worms. Refer to the Examples category in the first table presented in this article.

Question: Where do Arthropods usually live?

Answer: Arthropods basically live in a wide variety of ecosystem such as freshwater, ocean-based ecosystems, deep seas, frozen arctic regions and some even live in terrestrial ecosystem (land).

Question: Why are most invertebrate animals only seen in the water?

Answer: Fact is, most of the invertebrates are sea creatures.

Question: What type of invertebrate does the leech belong?

Answer: Leeches belong to Phylum Annelida.

Question: What are the phyla under vertebrates?

Answer: Vast majority of vertebrate animals fall under the Phylum Chordata which basically means "with spinal cord."

Question: How many groups of invertebrates are there? Are there only nine of them? Or there are more?

Answer: There are actually more than 30 groups of invertebrates. This list is the summary and the better grouping of invertebrates.

Question: Does an arachnid belong to a group of invertebrates?

Answer: Yes. Arachnid is a class of joint-legged invertebrate animals, in the subphylum Chelicerata.

Question: What is the name of the author of this article?

Answer: My name is John Ray Cuevas and I am the author of this article.

Question: What phylum do spiders belong to?

Answer: Spiders belong to Phylum Arthropoda.

Question: What animal kingdom lives in shallow and deep oceans?

Answer: Some of the animal kingdoms which live in shallow oceans are Phylum Arthropoda like crabs and scorpions, Phylum Porifera such as sponges and Phylum Echinodermata like starfish, sea cucumbers, and sea-urchins. On the other hand, the animal kingdoms living in deep oceans are Phylum Mollusca like squids and octopus, Phylum Nematoda like hookworms and pinworms, and Phylum Annelida like earthworms, leeches and lugworms.

Question: Why does cnidarians exist in two forms?

Answer: There are two major body forms among the Cnidaria - the polyp and the medusa. Sea anemones and corals have the polyp form, while jellyfish are typical medusae. I don't exactly know why there are two forms.

Question: What is the respiratory organ of mollusks?

Answer: Basically all molluscs breathe by gills that are called ctenidia (comb-gills) because of their comb-like shape. In terrestrial molluscs this respiration organ is reduced, but still respiration takes place in the pallial cavity. That is why it is also called the snail's respiratory cavity.

Question: What is the economic importance of these major phyla of invertebrates?

Answer: Invertebrates are brilliant aerators of soil as well as creating it. In other words, invertebrates not only help us to grow food crops through pollination, they help create and maintain soil quality. This is important for growing in agriculture, as well as in gardens and allotments.

Question: What are some easy examples of sponges are their respective groups?

Answer: There are three main kinds of Sponges namely Asconoid, Syconoid, and Leuconoid.

© 2018 Ray

Comments

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Calvin on April 29, 2020:

thanks a lot Ray, by any chance could you tell me what your favorite invertebrate is?

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Ray (author) from Philippines on March 08, 2020:

Please leave a comment or suggestions on what topics you would like me to cover about animals. Thanks!

Jamarion Gallaway on January 31, 2020:

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Ray (author) from Philippines on January 22, 2020:

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Steven on January 22, 2020:

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billy on January 22, 2020:

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Brayden on January 21, 2020:

Thank you

Ray (author) from Philippines on January 19, 2020:

You're all very welcome. Enjoy reading the article!

ŞÅFĔƏŘÏ ALI on January 19, 2020:

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IM YOU IN THE FUTURE on October 15, 2019:

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Viky on October 13, 2019:

Thank you very much for your help

Science on September 17, 2019:

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

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on May 18, 2019:

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.

lati on May 17, 2019:

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Someone on May 13, 2019:

This helped me with my science homework thank you so much

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on May 11, 2019:

All new comments were well noted. Thanks.

Abbas Ali on May 11, 2019:

Good Job Sir

Ray (author) from Philippines on March 29, 2019:

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!

Linus Divine on March 29, 2019:

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

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on March 25, 2019:

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anwaarnbatala@gmail.com on March 25, 2019:

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Lol on January 11, 2019:

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Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on July 18, 2018:

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

Ray (author) from Philippines on July 11, 2018:

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!

Eric Dierker from Spring Valley, CA. U.S.A. on July 11, 2018:

Mollusks because they are fun to eat!

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

Ray (author) from Philippines on July 11, 2018:

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!

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on July 11, 2018:

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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