In contains the anus. Immediately before the

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.

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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.
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.

Variations in Leeches

Leeches, those bloodsucking annelid worms that instill fear when we swim, are really fascinating creatures and relatively harmless.

Their segmentation differs from the arrangement seen in their kin, the earthworms and polychaete worms. The two coelomata are separated by a much greater distance in leeches, and they have no septa.

Lesson Summary

The weird and wonderful structure of annelid worms is properly highlighted by their strange, segmented bodies and skeleton-that-is-not-a-skeleton.

The hydrostatic nature of the annelid’s fluid-based skeleton gives their bodies shape and structure, while maintaining the squishy nature of these invertebrate animals. Annelid worms have two coelomata that overlap within their bodies, each filled with coelom fluid that allows liquid pressure to support the body.The segmentation of these annelid worms gives them their name – the annelids, or ringed, worms. Segmentation divides each annelid body into partitions, separated by a wall-like septum.

Segments of the annelid body are also given special names, with the prostomium containing the brain, the peristomium (the oldest segment) surrounding the mouth, and the pygidium or tail segment. Growth is regulated by a growth zone next to the pygidium. Annelid worms may look quite different from us, and their structures may seem strange – almost alien. But even worms have stuff inside them that keeps them supported, just like our own skeleton.


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