They can be crunchy or squishy, 40 feet long or microscopic, and they often have very strange and wildly different body parts. One thing they all have in common, however, is what they are missing: their backbone.

Invertebrates, or animals that don't have a backbone, are an amazing group of animals. Perhaps the thing that is most amazing is the sheer number of them. In fact, if you combined all the birds, reptiles, amphibians and mammals that we know of into one group, they still wouldn't come close to the number of invertebrates known today.

Yet this only takes into account the invertebrates we currently know about. Scientists are constantly discovering new species of animals, many of which lack a backbone.

Since invertebrates do not have a backbone, or any other bones for that matter, some of them have developed hard outer shells to protect them and give their body shape and support. If you have ever accidentally stepped on an insect and heard a crunch, it is this hard outside skeleton (exoskeleton) that is making the noise.

The hard outer shell does pose a problem if you are trying to grow. Unlike turtles, whose shell grows with them, invertebrates must get rid of their old shell in order to get bigger. The process of crawling out of the old exoskeleton and having a new, bigger exoskeleton underneath is called "molting." The new exoskeleton is very soft at first but will harden over time, making the invertebrate more vulnerable than usual right after they molt.

However, not all invertebrates need an exoskeleton.

Many of the invertebrates that live in water use the water pressure around them to give shape to their body. Other animals like snails, oysters and clams will secrete a shell around their soft parts for protection.

The incredible diversity of the animals that lack backbones is sometimes hard for us to grasp. The invertebrates that people are most familiar with are, of course, the ones that they interact with on a daily basis and, therefore, the ones that live on land. However, the majority of invertebrates are found in the ocean.

Scientists once thought invertebrates such as sea sponges and anemones were plants. It wasn't until later that scientists figured out that they are actually animals. Sea anemones are closely related to sea jellies (jellyfish) and, like sea jellies, have stinging tentacles on their body that help them capture whatever small animal or other piece of food that comes floating their way. This is very different from sponges that filter small food particles out of the sea as the water flows through its many holes.

Perhaps the most interesting invertebrates of the ocean are the squids and octopi. Squids are masters of camouflage. They have special cells all over their body that change color to match their surroundings. Most squid are around a foot in length, but the Colossal squid can reach more than 40 feet long, making it the largest invertebrate in the world! A Colossal squid captured by a fishing vessel off the coast of Antarctica in 2007 weighed in at 1,091 pounds and was 33 feet long. Its eyes alone were 11 inches across, which make them the largest eyes of any known animal.

The octopus, while not quite as large, is no less amazing. Despite being able to grow up to 14 feet, they are able to squeeze through really tiny spaces. This is because the only hard part on their body is a parrot-shaped beak in their mouth. The rest of the body is very soft and can be easily compressed. For instance, a seven-foot, 30-pound octopus at the New England Aquarium was able to squeeze its entire body into a 14-inch square box. This flexibility makes their camouflage even more amazing, since not only can they change color like the squid, they can morph their body into different shapes. Octopi are also thought to be the most intelligent of all invertebrates. They can learn by observing, solve mazes, open jars, distinguish between different kinds of shapes and patterns, and possess both short- and long-term memory. However, the short life span of the octopus (around two years) limits the amount of things they can learn.

Then, of course, there are all the millions of invertebrates that live on land: spiders, snails, slugs, worms, insects, scorpions ... the list goes on and on. From millipedes that act like mini recycling machines that break down dead plants and animals, to dragonflies that live half their life in water and half on land, animals without a backbone are incredible.

Like all animals, each invertebrate is important to the environment in its own way, working hard to keep our planet healthy. So the next time you see a boneless animal outside, perhaps you might consider stepping over it rather than on it.

Our planet just wouldn't be the same without them.

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