What are Invertebrate Species?

Jacob Queen

Invertebrates are animals without backbones. They have numerous possible body shapes and fall into many different animal families, including insects, jellyfish, and worms. Invertebrates are considered more primitive evolutionarily than vertebrate animals, and the variety between different kinds is much more extreme. Most experts agree that approximately 96% of all animals on the planet are invertebrates.

Squids are invertebrates.
Squids are invertebrates.

The invertebrate animals fall into eight different subtypes. Some of the more well-known types are the arthropods, nematodes, mollusks, and annelids. Arthropods are the insects, arachnids, and crustaceans, and they are the most numerous kind of invertebrate species. Mollusks are also very common, and they include snails and squids. Nematodes are round worms, and annelids are segmented worms like earthworms and leeches.

Earthworms are one invertebrate species in the Annelida phylum.
Earthworms are one invertebrate species in the Annelida phylum.

Invertebrates serve important purposes in terms of planetary ecology. For example, many crops are fertilized by invertebrates, and without them, much of humanity’s agricultural efforts could potentially be ruined. They are also an important food source for creatures all over the planet. Many mammals, including humans, consume large numbers of invertebrates, and if the invertebrate species population were to suffer, starvation would be a possibility for many species.

Inchworms are larvae that metamorphosize into the geometer moth.
Inchworms are larvae that metamorphosize into the geometer moth.

Some invertebrate species can also be harmful to people, and many of the world’s most dangerous parasites are members of the invertebrate family. Some of the more well-known invertebrate parasites include ticks, fleas, and leeches. There are also parasites like tapeworms that live within the body of their hosts, and these can potentially be even more dangerous than their external counterparts. Some parasites, like mosquitoes, can be especially damaging because they can potentially carry diseases.

Segmented animals like earthworms and leeches are called annalids.
Segmented animals like earthworms and leeches are called annalids.

Most kinds of invertebrate species go through a process called metamorphosis during their development. This is useful, because the animals can fill different ecological niches at different times in development, which allows for maximum exploitation of food opportunities. During each phase of its life, the animal undergoing metamorphosis will generally have a totally different lifestyle and survive in a different way. A common example of metamorphosis would be a caterpillar making a protective cocoon and turning into a butterfly.

Starfish are invertebrates.
Starfish are invertebrates.

Invertebrate species can vary greatly in size. Most of them are fairly small, like insects and mites. Others, like some squids and octopi, can be fairly large. The largest invertebrate in existence is the architeuthis, also known as the giant squid, which can potentially be 65 feet (20 meters) in length. Other invertebrates like the ciliated protozoan are too small to be seen with the human eye.

Snails are mollusks and therefore invertebrates.
Snails are mollusks and therefore invertebrates.

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Discussion Comments


@TreeMan - While the basic definition of an invertebrate usually revolves around an animal having a backbone, there are other requirements as well. Technically, the backbone itself doesn't make something a vertebrate, it's whether it has a spinal cord. The other things have to do with how the animals are formed after an egg is fertilized.

In the case of sharks and non-bony fish, they do have cartilage instead of bone. At the same time, they also have a spinal cord and meet all the other requirements to be a vertebrate. Off the top of my head, they are the only vertebrates I can think of that don't have true bones. I'm sure I could be missing something, though.

In addition to titans62's question, I'd be interested to know what the species distribution is of insects, too. What countries or continents have the most insects and the widest variety?


For some reason I always thought that fish were invertebrates, too, but they aren't mentioned here.

I know I have heard a lot of information about some types of fish, sharks especially, that don't have bones. Their structure is made up of cartilage. I know from experience that some fish do have bones, but I know the groups are split. I'm sure if sharks were invertebrates the article would have mentioned it. I'm just curious why they aren't when they don't have real bones.


@cardsfan27 - Most people don't know it, but almost all of the more "primitive" invertebrates go through various changes in form. For jellyfish, hydras, and similar animals, there are actually two stages call the medusa and polyp form. Jellyfish are born from eggs and start off as polyps that cling to rocks and float around in the water.

As they get older, they grow tenticles and take the form we normally think of. The opposite is true for hydras. If you've never seen a picture of a hyrda, they are very interesting.

Does anyone know what the insect species abundance is? I know as a group they make up a huge percentage of all the animals in the word, both in terms of species and numbers. I'm curious what the percentages are. I can't seem to find them anywhere.


Wow, I have seen pictures and read stories about giant squids, but I never realized how huge they could be. It's definitely impressive that something without bones can grow to be that large and still function.

The article says that a lot of invertebrates go through metamorphosis. Does this happen with anything besides insects? I have never heard of anything like a worm or snail doing through different life stages.

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