Re: How do invertebrates solve the problem of support without a backbone?

Dear Anonymous,

You have asked two very interesting questions, and I hope that I can answer them for you. For people who are used to thinking about vertebrates, it may not be obvious how the animals that lack bones (unceremoniously and unfairly lumped into a hodgepodge group called "the invertebrates"; I prefer to think of them as the "spineless wonders") manage without them. However, the invertebrates do quite well without any bones at all, backbone or otherwise.

First of all, although the invertebrates do indeed lack a backbone, many of them still have a skeleton. It's just not the kind of skeleton that you're used to thinking about. You probably know that one of the functions of our own skeleton is to provide mechanical support to hold our bodies upright. This is very important to us because we live on land, and must support our weight against the pull of gravity. However, most of the non-insect invertebrates are aquatic, so they can use the support of the water in which they live to support their bodies' weight. In fact, the largest invertebrates live in the sea, and can grow to be longer than all but the largest whales. They can get this big in the sea partly because they can use the buoyancy of seawater to support their bodies, which may be quite flimsy (think of jellies and such beasts). You will notice that there aren't any monstrously huge terrestrial invertebrates lumbering around like elephants. That's because the structures that are best suited to support a large body on land aren't available to the invertebrates.

The second function of our own skeleton is to provide a place for muscles to attach. Our muscles are attached to bones (a different bone at each end of the muscle), and the contraction of muscle fibers causes the bones to move relative to each other. This is what happens every time we move a body part. Even though they don't have bones, the invertebrates have devised other kinds of skeletons to which they attach their muscles.

Many invertebrates have what is called a hydrostatic skeleton -- a fluid-filled internal body cavity that provides mechanical support. Because fluid (water) is incompressible, the contraction of muscles around any fluid-filled sac will deform the sac. The best example of this kind of skeleton, and how it can be used for locomotion, is for an earthworm. An earthworm's body is divided into many segments, and each segment contains a fluid-filled space called a coelom. Each coelom is surrounded by circular muscles that go around the segment like a series of hoops and longitudinal muscles that run down the length of the worm's body. Here's a diagram of an earthworm in cross-section. When the circular muscles around a segment contract against the hydrostatic pressure of the coelom, that segment becomes long and skinny (try this with a water balloon to prove it to yourself). When the longitudinal muscles contract, the segment becomes short and fat. The worm can control the actions of muscles associated with each segment independently, so that part of the body can be short and fat while an adjacent section is long and skinny.

This ability to deform its body by the selective contraction of circular or longitudinal muscles against its hydrostatic skeleton is what enables an earthworm to burrow into the ground. The worm starts by contracting the circular muscles at its anterior end, which makes its head long and skinny. Then it drives its head into the ground and contracts the longitudinal muscles at the head, which makes the head fat and anchors that end of the worm into the ground while it pulls the rest of the body along behind. Once the body has been pulled forward, the worm contracts the longitudinal muscles at the posterior end to anchor it in place and contracts the circular muscles at the head again to burrow farther into the ground.

Another kind of skeleton used by many invertebrates is an exoskeleton that covers the outside of the body. The arthropods (crabs, insects, etc.) and molluscs (snails, clams, etc.) make use of this kind of skeleton. The advantage of having an exoskeleton is that it provides protection as well as a site for muscle attachment. In fact, the arthropod exoskeleton has numerous invaginations called apodemes which serve solely as attachment sites for muscles. The downside of an exoskeleton is that unless it grows with the animal's body, as it does for molluscs, the animal can't get any larger once it has filled all the space inside. That's why arthropods have to molt periodically.

In terms of how lacking a backbone limits the invertebrates, I'm not sure they have been limited at all. Much of the biodiversity in the seas is made up of invertebrates, and the most successful group of animals, in terms of both species diversity and abundance, is the arthropods, primarily the insects. So while the invertebrates may not have achieved large size on land, they have had excellent success both in the seas and in the air.

Whew! This is a long response, probably more information than you wanted. But I hope it answers your questions. I had a lot of fun putting it together.

Allison J. Gong
Mad Scientist