Updated: October 3, 2022
© I Wei Huang/
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The definition of an invertebrate is any animal that does not have a backbone or vertebral column. The most prolific and easily recognizable members of the invertebrate family are insects. It’s estimated that upwards of 30 million individual species of invertebrates may exist accounting for between 90-95 percent of all organisms on the planet.

All phyla with the exception of Chordata are made up solely of invertebrates, and even the phylum Chordata is primarily comprised of invertebrates. Only the subphylum of Chordata called Vertebrata contains any vertebrate animals.

Some invertebrates have a hard exoskeleton, or shell, like snails, crabs, or mussels. The majority have compound eyes that form as an extension of their skin, and all invertebrates are cold-blooded. This means that they are totally unable to regulate their own body temperature through internal methods, and they rely on external temperature to maintain their own.

Methods of Breathing

Invertebrates breathe through many different means. Some arthropods use spiracles which are small holes in their exoskeleton, and other invertebrates exchange gases across their entire body surface like several species of marine worms.

A large number of invertebrates breathe through gills, but these gills are just as wildly different as the invertebrates themselves. Most people think of gills as they appear on fish, but the definition of a gill is any thinly walled structure that permits the exchange of gases. That leaves a lot of room for variation within just one organ.

Some of the most interesting means of breathing are amongst the various aquatic invertebrates. Freshwater clams use microscopic cilia to propel water and its contents through their body. This allows the clam to both eat and push water through its modified gills. Starfish take in oxygen through the same suction cups on their appendages that allow them to move, and sea cucumbers actually breathe through lungs that are able to process water.

Aquatic Invertebrates

While insects may be the most common invertebrate that humans experience daily, it is the aquatic invertebrates that permeate virtually all of the most common classes of invertebrates. That is in comparison to just one class of insects. Marine invertebrates are found in freshwater, saltwater, and brackish water.

Here are some of the most common groups of invertebrates and some of their species.

  1. Protozoans: Amoeba and other single-celled organisms
  2. Echinoderms: Sea urchin, sea cucumber, starfish
  3. Annelids: Earthworms and leeches
  4. Arthropods: Insects, spiders, and crustaceans
  5. Mollusks: Snails, octopi, and squid

Outstanding Invertebrates

The largest invertebrates on record are members of the squid family. The giant squid is the longest with the largest specimen recorded clocking in at an impressive 59 feet long. That is approximately the length of a school bus! While colossal squid are slightly shorter, they are much heavier weighing in at around half a ton.

Giant squid are extremely elusive, and scientists have found it virtually impossible to record many details about the species due to the inhospitable climate of the deep ocean that it resides in. Colossal squid are slightly more well-known but still not common. These animals also hold the record for the largest eyes on any invertebrate with a range of 10 to 16 inches in diameter.

The smallest invertebrates are single-cell organisms such as amoebas. Sponges are among the simplest invertebrates with most not having developed many of the organs or systems present in other related species.

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Invertebrates FAQs (Frequently Asked Questions) 

What are invertebrates?

By definition, invertebrates are animals that do not develop a backbone or vertebral column, and they do not possess a notochord during development. The exception to the lack of a notochord are the members of the phylum Chordata that are not vertebrates contained in the subphylum Vertebrata. In excess of 90 percent of all known animals are invertebrates, and scientists estimate that there may actually be as many as 30 million total invertebrate species.

What are the characteristics of invertebrates?

Invertebrates have many differences across all of the classes and species that exist. Insects are significantly different from starfish, but there are common characteristics that all invertebrates share.

  1. Complete lack of a vertebral column or developmental notochord
  2. Multiple, varied methods of breathing
  3. Open circulatory system
  4. No red blood cells
  5. Vast majority are hermaphroditic
  6. One-layer skin
  7. Compound eyes that are a growth out of the skin

What are some examples of marine invertebrates?

There are a large number of marine invertebrates in every possible shape or size that you could imagine. From coral polyps to sea urchins to colossal squid, invertebrates saturate both salt and freshwater environments. Here are some other common aquatic invertebrates:

  1. Mussels
  2. Crabs
  3. Shrimp
  4. Jellyfish
  5. Clams
  6. Lobster

What are the differences between invertebrates and vertebrates?

The major difference between vertebrates and invertebrates is the lack of a backbone in invertebrates. Typically, invertebrates are slow-moving and smaller organisms when compared with the larger and generally faster vertebrates. Invertebrates are also cold-blooded compared to the warm-blooded vertebrates.

Five classes comprise all vertebrates, whereas invertebrates are made up of over thirty different classes. While there are invertebrates that have bony body structures, only vertebrates possess an endoskeleton made of living tissue.

Are snakes invertebrates?

No. Snakes are vertebrates. While at first glance it may be difficult to believe that such a flexible animal has a bony skeleton, snakes have a spinal column that is made up of between 200-400 vertebrae depending on their size.

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