Skeleton, Muscle and Movement

Lessons

Senior 6 · Biology

Skeleton, Muscle and Movement

4 lessons

Lesson 1 of 4

Hydrostatic skeletons, exoskeletons and endoskeletons

25 min

Pushing against something

A muscle can only pull: it shortens and then waits to be stretched again. To turn that pull into movement, an animal needs something firm for its muscles to pull against, and something that will carry the force to where it is needed. That something is a skeleton. In Senior 1 you met the three kinds of skeleton by name. This lesson asks how each one actually works as a machine: why an earthworm's fluid-filled body can burrow, why no insect is the size of a goat, and why your thigh bone is a hollow tube rather than a solid rod.

Definition

Skeleton: a structure that supports the body, keeps its shape, protects organs and gives muscles something to pull against.

Hydrostatic skeleton: a volume of fluid held under pressure in a closed body compartment and surrounded by muscle; because the fluid cannot be compressed, muscle contraction changes the body's shape.

Exoskeleton: a hard external covering secreted by the epidermis; in arthropods it is the cuticle, made of chitin and protein.

Endoskeleton: an internal framework of living tissue, bone and cartilage in vertebrates, with the muscles attached to its outside.

Ecdysis (moulting): shedding the old exoskeleton so that the animal can grow; it is triggered by the steroid hormone ecdysone.

Antagonistic muscles: a pair of muscles that act in opposite directions, so that when one contracts it stretches the other.

Hydrostatic skeletons: fluid under pressure

Liquids are almost incompressible. If fluid is trapped in a closed bag, squeezing the bag in one direction must make it bulge in another, because its volume cannot change. Soft-bodied animals use this. In Hydra and sea anemones the fluid is the water in the gut cavity; in roundworms (nematodes) and annelids it is the body-cavity fluid.

The earthworm is the classic example. Its body cavity, the coelom, is divided by cross-walls (septa) into a separate fluid-filled compartment in each segment, so each segment can change shape on its own. Two layers of muscle surround the fluid and act as an antagonistic pair:

When the circular muscles contract, the segment becomes thinner and longer. This pushes the front of the worm forward.
When the longitudinal muscles contract, the segment becomes shorter and fatter. Fat segments press against the soil, and tiny bristles called chaetae grip it so that these segments act as anchors.

Waves of contraction (peristalsis) pass from the front of the worm to the back. At any moment some segments are long and thin and others are short, fat and anchored, so the body is pushed and pulled forward through the soil.

Worked example 1: same volume, new shape

Treat one earthworm segment as a cylinder of fluid, 5.0 mm long with a radius of 2.0 mm. The circular muscles contract and reduce the radius to 1.6 mm. Assuming the volume stays constant, calculate the new length.

Volume of a cylinder: $V = \pi r^{2} L$ . The fluid cannot be compressed, so $\pi r_{1}^{2} L_{1} = \pi r_{2}^{2} L_{2}$ .
Rearrange: $L_{2} = L_{1} \times \left( \frac{r_{1}}{r_{2}} \right)^{2} = 5.0 \times \left( \frac{2.0}{1.6} \right)^{2}$ .
$\left( \frac{2.0}{1.6} \right)^{2} = 1.5625$ , so $L_{2} = 5.0 \times 1.5625 = 7.8$ mm (2 s.f.).

Interpretation. A 20% cut in radius gives about a 56% increase in length, because length depends on the square of the radius. A small squeeze produces a large forward push, which is why burrowing works so well.

Exoskeletons: armour that must be shed

Arthropods (insects, spiders, crustaceans, millipedes) are covered by a cuticle secreted by the single layer of epidermal cells beneath it. Its main material is chitin, a polysaccharide made of β-linked N-acetylglucosamine units, which forms long fibres embedded in protein. The cuticle has layers:

a thin, waxy epicuticle on the outside, which cuts water loss and helped arthropods to colonise dry land;
a hard exocuticle, stiffened by cross-linking of its proteins (sclerotisation) and, in crabs and crayfish, by calcium carbonate;
a thicker, more flexible endocuticle next to the epidermis.

At each joint the hard layers are missing and the cuticle is thin and flexible, so the limb can bend. Muscles attach to the inside of the cuticle tube, often to inward projections, and each joint is moved by an antagonistic pair: a flexor that bends it and an extensor that straightens it.

The cuticle cannot grow. To get bigger, the animal must moult: the epidermis secretes a new soft cuticle under the old one, digests and reabsorbs much of the old endocuticle, and then splits and climbs out of what is left. Until the new cuticle hardens, the animal is soft, slow and easily eaten.

Endoskeletons: living, growing scaffolding

The vertebrate skeleton is made of two living tissues:

Bone has a matrix of collagen fibres, which resist stretching, hardened by crystals of calcium phosphate (hydroxyapatite), which resist compression. In compact bone the matrix is laid down in rings (lamellae) around central (Haversian) canals that carry blood vessels and nerves; bone cells (osteocytes) sit in small spaces (lacunae) and connect through fine channels (canaliculi).
Cartilage has cells (chondrocytes) in a firm, rubbery matrix of collagen and proteoglycans, with no blood vessels. Hyaline cartilage covers the ends of bones at joints, and forms most of the skeleton of an embryo.

Because bone is alive, it grows with the body, repairs fractures and remodels itself where loads change. Muscles attach to its outside through tendons. The skeleton also protects organs (skull, rib cage), makes blood cells in the red marrow, and stores calcium and phosphate.

Notice that both skeletons are built as hollow tubes. For the same mass of material, a hollow tube is much stiffer against bending than a solid rod, because the material far from the centre does most of the work of resisting a bend. The exoskeleton puts this tube on the outside of the whole limb; the endoskeleton puts a narrower tube in the middle.

Worked example 2: why giant insects are impossible

A beetle and an imaginary beetle with exactly the same shape but 3 times the length are compared. By what factor does (a) its mass, (b) the cross-sectional area of its legs, and (c) the stress (force per unit area) in its legs change?

(a) Mass depends on volume, which scales with length cubed: $3^{3} = 27$ times heavier.
(b) Area scales with length squared: $3^{2} = 9$ times the cross-section.
(c) Stress $= \frac{\text{weight}}{\text{area}}$ , which changes by $\frac{27}{9} = 3$ times.

Interpretation. Every time an animal's length is multiplied by $k$ , the stress in its skeleton rises by about $k$ , so a large animal needs disproportionately thicker supports. For an exoskeleton that means a heavy, thick cuticle, and a moulting animal of great size would collapse under its own weight while the new cuticle was soft. An endoskeleton grows continuously and never has to be shed, which is one reason why all the largest land animals are vertebrates.

Comparing the three skeletons

Feature	Hydrostatic	Exoskeleton	Endoskeleton
Examples	Earthworm, Hydra, roundworms	Insects, crabs, spiders	Fish, birds, mammals
Material	Fluid under pressure	Chitin and protein (with calcium carbonate in crustaceans)	Bone and cartilage, living tissue
Where muscles attach	Surround the fluid in the body wall	Inside the skeleton	Outside the skeleton, by tendons
Growth	Grows with the body	Must be moulted	Grows and repairs itself
Protection	Little	Excellent, covers the whole body, cuts water loss	Protects key organs only (brain, heart, lungs)
Main limitation	Little support on land; no rigid levers for fast movement	Heavy at large size; animal is vulnerable after each moult	Surface left unprotected; bone is costly to build

Nuffield Foundation, Practical Biology: "Worm movement". Watch which segments are long and thin and which are short and fat as each wave passes along the body.

Warning

Common mistakes

"Muscles can push." No muscle can push. Every skeleton works by pairing muscles so that each one is stretched again by its antagonist.
"The exoskeleton is dead, so it does nothing after it hardens." It is non-living, but it carries the muscle attachments, the joints and many sense organs, and it is constantly maintained by the epidermis.
"Bone is just a mineral." About a third of its mass is organic, mainly collagen. Without the collagen bone would shatter; without the mineral it would bend like rubber.
"An earthworm moves by wriggling sideways." It moves by peristalsis: waves of lengthening and shortening, with chaetae anchoring the short, fat segments.

✦

Practice: types of skeleton

State three functions shared by all three types of skeleton.
Name the fluid that forms the hydrostatic skeleton of (a) an earthworm, (b) Hydra.
Describe what happens to the shape of an earthworm segment when (a) the circular muscles contract, (b) the longitudinal muscles contract.
Explain why it is important that the earthworm's coelom is divided by septa.
An earthworm segment is 6.0 mm long with a radius of 1.5 mm. The longitudinal muscles shorten it to 4.0 mm. Assuming constant volume, calculate the new radius.
Give the function of (a) the waxy epicuticle, (b) the thin cuticle at a joint, (c) collagen in bone.
Explain why an arthropod is especially likely to be eaten soon after it moults.
A student claims that "an exoskeleton is always better than an endoskeleton because it protects the whole body". Evaluate this claim.

Show answersHide answers

Any three: support, keeping the body's shape, giving muscles something to pull against (movement), protection.
(a) Coelomic fluid. (b) Water in the gut (gastrovascular) cavity.
(a) It becomes thinner and longer. (b) It becomes shorter and fatter.
Each segment's fluid is separate, so different segments can be long and thin or short and fat at the same time; this makes peristaltic waves possible.
$r_{2} = r_{1}\sqrt{\frac{L_{1}}{L_{2}}} = 1.5 \times \sqrt{\frac{6.0}{4.0}} = 1.5 \times 1.225 = 1.8$ mm (2 s.f.).
(a) Reduces water loss by evaporation. (b) Allows the limb to bend. (c) Resists tension (stretching) and makes bone less brittle.
The new cuticle is still soft, so the muscles have nothing rigid to pull against: the animal moves slowly and has little armour until the cuticle hardens.
Partly true: it covers the whole body and cuts water loss. But it must be moulted, leaving the animal vulnerable, and its mass grows faster than its strength as size increases, which limits animal size. An endoskeleton grows continuously, repairs itself and supports large bodies. Each suits a different size and way of life, so "always better" is wrong.

Key points

Muscles only pull; every skeleton gives them something to pull against and pairs them antagonistically.
Hydrostatic: incompressible fluid in a closed space. In the earthworm, circular muscles make a segment long and thin, longitudinal muscles make it short and fat, and chaetae anchor it; volume stays constant.
Exoskeleton: a chitin cuticle with muscles inside; protects and waterproofs, but must be moulted, and it limits size.
Endoskeleton: living bone (collagen plus hydroxyapatite) and cartilage, with muscles outside; it grows, repairs and supports large bodies.
Mass grows as length cubed but strength as length squared, so large animals need disproportionately stronger skeletons.

Sources and credits

38.1 Types of Skeletal Systems, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax (Rice University), licensed under CC BY 4.0. Used for the three types of skeleton and the structure of bone. Adapted by Ganzaa.
Photo: Cicadidae with exuvia, immediately after moulting, in Laos, by Basile Morin, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: "Worm movement", Practical Biology, Nuffield Foundation, embedded from YouTube.

Figures 1 and 2, the comparison table, the worked examples and the practice questions were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Pushing against something

2Hydrostatic skeletons: fluid under pressure

3Exoskeletons: armour that must be shed

4Endoskeletons: living, growing scaffolding

5Comparing the three skeletons

Pushing against something

Hydrostatic skeletons: fluid under pressure

Exoskeletons: armour that must be shed

Endoskeletons: living, growing scaffolding

Comparing the three skeletons