Annelida: Skeleton & Segmentation

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

In this lesson, we'll discuss the hydroskeleton and segmentation of animals in the Phylum Annelida. Annelids include earthworms, leeches, and marine polychaete worms. Their body structure gives them support, even without a bony skeleton.

Annelid Structure

Slimy. Undulating. Squirming. Blood sucking, earth chomping, ocean swimming. The Phylum Annelida is the group that includes earthworms, leeches, and marine polychaete (POLY-keet) worms. These animals have unique internal and external structures that give them their shape and form.

When, if at all, do worms have skeletons? How are worms segmented and divided into their characteristic, ringed forms? Let's take a look at their adaptations.

When Is A Skeleton Not A Skeleton?

Does a worm have a skeleton? Well, not exactly. Not like you or I have a skeleton, which is made up of bones and cartilage.

Annelid worms have what's known as a hydrostatic skeleton, or hydroskeleton. This kind of skeleton is based on the Greek root hydro-, meaning water.

What does a water-based, or liquid-based, skeleton look and behave like? In annelid worms, there are two internal hollow body cavities, known as coelomata (singular coelom - pronounced SEE-lum). The coelomata are surrounded by a ring of muscles, which contract to move the supportive coelom fluid inside each cavity. Think of a small donut inside of another, bigger donut: that's the mental image of the coelom spaces, with fluid in between.

This interaction between the coelom fluid and the outer muscles produces hydrostatic pressure - pressure caused by fluid or water - which is why an earthworm or a leech is not completely flat. The body shape of the Phylum Annelida is therefore supported by this strange, liquid-based 'skeleton', which is not at all like our own skeleton. We humans do, however, have a coelom - though ours is full of our organs and organ cavities. Perhaps worms and humans aren't too different, after all!

Repeating Rings of Segmentation

An earthworm, showing typical annelid segmentation.

Think of an earthworm or a leech. These animals have lots of rings along their body, each cinching the body into many segments. These miniature compartments are a part of a type of biological organization known as segmentation. In fact, you can tell that annelid worms have segmented rings, just from their name - the Latin root anneli actually means 'rings'.

Each segment pinches off and divides a section of coelom, with a type of wall known as a septum (plural septa), from the Latin word for 'closing off'. From there, each individual segment also contains its own set of organs, often a repeating portion of a blood vessel or continuing part of a gut tube.

Annelid segmentation.
segmentation annelida

In the image above, the standard layout of annelid segmentation can be seen through a series of colors and partitions. The topmost, purple segment is known as the prostomium, and contains the brain and most sensory organs. Pro- means in front of, and stomium refers to the mouth. The yellow section is known as the peristomium, which surrounds the mouth - hence, peri- means 'around.'

The last segment is known as the pygidium - Greek for 'little tail' - which contains the anus. Immediately before the pygidium is the growth zone, labeled as green in the image above. Since annelids grow from the growth zone onward, with each new segment forming next to the pygidium, the oldest part of the animal is the peristomium.

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