Skeletal Systems of Organisms
Senior 1 · Biology
Skeletal Systems of Organisms
Lesson 1 of 5
25 min
After a heavy shower in Musanze, earthworms come up onto the paths. An earthworm has no bones and no shell, yet it holds its shape, pushes its way through firm soil and crawls forward. Try this: fill a balloon with water and tie it. Squeeze it round its middle and it becomes long and thin; squeeze it from both ends and it becomes short and fat. The water cannot be squashed into less space, so pressing in one direction makes the balloon bulge in another. An earthworm is built on exactly this idea.
Definition
Skeleton: the structure that supports an animal's body, protects its soft parts and gives its muscles something firm to pull against, so that the animal can move.
Hydrostatic skeleton: a skeleton made of fluid held under pressure inside a body cavity and surrounded by muscles. Examples: earthworms, jellyfish, sea anemones, slugs and caterpillars.
Coelom: the fluid-filled body cavity of an animal such as an earthworm. In an earthworm it is divided into separate compartments, one in each segment.
You need a live earthworm from damp garden soil, a sheet of damp paper towel, a sheet of rough paper, a hand lens and a small container of moist soil to return it to.
What you will see. A wave passes along the body from front to back. As each part stretches forward it becomes longer and thinner; as it pulls in, it becomes shorter and fatter. The scratching is the sound of the bristles gripping the paper.
Warning
Safety and care. Earthworms breathe through their moist skin and die if it dries out, so keep them damp, out of the sun, and out of the classroom for as short a time as possible. Handle them gently with wet hands; never pull a worm that is gripping. Return every worm to moist soil and wash your hands with soap afterwards.
Definition
Circular muscles: muscles that run round the body. When they contract, the body becomes longer and thinner.
Longitudinal muscles: muscles that run along the length of the body. When they contract, the body becomes shorter and fatter.
Antagonistic muscles: a pair of muscles that pull in opposite directions, so that when one contracts the other is stretched.
Chaetae (singular chaeta): small bristles on the segments of an earthworm that grip the soil.
Figure 1: A lengthwise section through part of an earthworm. The fluid in each segment cannot be squashed, so the two muscle layers can change the segment's shape.
The fluid in each segment of the coelom has a fixed volume. Muscles cannot push, only pull, but because the fluid cannot be squashed, a pull in one direction changes the shape in the other:
The two layers are antagonistic: each one stretches the other. The septa keep the fluid in each segment separate, so different parts of the body can have different shapes at the same moment. Figure 2 shows how waves of contraction, called peristalsis, move the worm forward.
Figure 2: An earthworm crawling. Long, thin segments push forward; short, fat segments grip the ground with their chaetae. Both ends end up further to the right.
Worked example 1: reading the movement diagram
Use Figure 2. (a) Which muscles are contracting in the front half of the worm in stage 2? (b) Why must the chaetae at the back grip the ground in stage 2?
Answer: (a) circular muscles; (b) to anchor the back so the stretching front moves forward, not the back backwards.
Worked example 2: how much does a segment stretch?
A student measures an earthworm at rest as 12 cm long. When it stretches its front forward, it is 15 cm long. Calculate the percentage increase in length.
Answer: 25%. (The lengths are illustrative; real worms vary.)
| Advantages | Limitations |
|---|---|
| flexible: the animal can squeeze through narrow gaps and burrow | gives almost no protection against being crushed or bitten |
| light, and cheap to build: it is mostly water | cannot hold up a large animal on land; big soft-bodied animals live in water, which supports them |
| the whole body can change shape | fails if the body is punctured and fluid leaks out, or if the animal dries out |
Plants use a similar trick. In Unit 3 you saw how soft stems and leaves are held up; in Unit 5 you will meet turgid cells, each one pressing water against its cell wall. When a plant is short of water, the cells lose their pressure and the leaves wilt, just as an earthworm on a hot, dry path loses its shape.
Because their skeleton lets them change shape, earthworms can burrow through soil, swallowing it as they go. Their tunnels let air and rainwater into the soil, and their droppings (casts) are rich in plant nutrients. Farmers who add compost and manure to their fields feed the earthworms, which in turn keep the soil loose and fertile.
Hydrostatic skeleton (MooMooMath and Science). Watch how squeezing a fluid-filled body in one direction makes it lengthen in another, and the examples of animals that use this skeleton.
Tip
Memory hook: "Circle squeezes, long and lean; Longitudinal pulls, short and keen." Circular muscles make the worm long and thin; longitudinal muscles make it short and fat. In an exam, always name which muscles contract and what shape the segment becomes.
Practice: the hydrostatic skeleton
Key points
Sources and credits
Written by Ganzaa: the observation, the balloon model, Figures 1 and 2, the table, the worked examples and the practice questions.
