Skeletal Systems of Organisms
Senior 1 · Biology
Skeletal Systems of Organisms
Lesson 2 of 5
25 min
On the bark of a tree you may find what looks like a whole insect, with legs, eyes and back, but it is light, hollow and split open along the back. It is not a dead insect. It is a coat of armour that a living insect has climbed out of. Insects, spiders and crabs wear their skeleton on the outside. You, a fish and a cow carry yours on the inside. Each design has its own strengths and its own problems.
Definition
Exoskeleton: a hard skeleton on the outside of the body, covering the soft tissues. It is made mainly of chitin, a tough, flexible material; in crabs it is hardened further with calcium carbonate. Found in arthropods: insects, spiders, millipedes and crustaceans such as crabs.
Moulting (also called ecdysis): shedding the old exoskeleton so that the animal can grow. The empty old exoskeleton is called an exuvia (plural exuviae).
Endoskeleton: a skeleton inside the body, surrounded by muscles and other soft tissues. In vertebrates (fish, amphibians, reptiles, birds and mammals) it is made of bone and cartilage.
Look closely at a dead grasshopper, or at a live one held gently in a clear, closed jar, with a hand lens. Press very gently on the body of a dead specimen with a blunt probe: it feels stiff, like thin plastic. Bend a leg slowly: it moves only at certain points. Count the body parts and the legs.
Figure 1: The exoskeleton of a grasshopper. Hard plates cover the whole body; the legs are hard tubes joined by soft, flexible joints.
The whole body is covered by a hard cuticle. It cannot bend except at the joints, where the cuticle is thin and soft. That is why arthropods are called "joint-legged" animals (arthro = joint, pod = foot). Tiny breathing holes, the spiracles, let air in through the armour.
Warning
Safety and care. Wash your hands after handling insects or soil. Some grasshoppers, including the brightly coloured variegated grasshopper, give off a bad-smelling, irritating froth when handled: do not rub your eyes. Release live insects where you found them.
Muscles are attached to the inside of the hard tubes and cross each joint. When a muscle contracts it pulls one tube against the next, and the leg bends. The exoskeleton also protects the soft organs and stops the animal drying out, which is why insects can live in dry places.
But an exoskeleton is not made of living cells, so it cannot grow. To get bigger, an insect or crab must moult:
Until it hardens, the animal is soft, weak and easily eaten, so it usually hides. A crab that has just moulted stays in a hole until its new shell is hard.
Figure 2: Growth of a young grasshopper (illustrative figures). Its length increases in sudden steps, one after each moult, while an animal with an endoskeleton grows gradually.
Worked example 1: reading the growth graph
Use Figure 2. (a) How many times did the grasshopper moult? (b) By how much did its length increase altogether, and what was the mean increase per moult? (c) Why does the line go up in vertical steps?
Answer: 5 moults; 35 mm in total, 7 mm per moult on average; growth happens only while the new exoskeleton is soft.
Vertebrates have their skeleton inside. It is made of two tissues:
Figure 3: Muscles are inside an exoskeleton but outside an endoskeleton. Both work the same way: a muscle pulls on one part across a joint.
| Feature | Hydrostatic skeleton | Exoskeleton | Endoskeleton |
|---|---|---|---|
| Made of | fluid under pressure, surrounded by muscle | chitin (plus calcium carbonate in crabs) | bone and cartilage |
| Position | body cavity (coelom) | outside the body | inside the body |
| Examples | earthworm, jellyfish, slug | grasshopper, crab, spider, millipede | fish, frog, lizard, bird, human |
| Muscles attached | around the fluid | to the inside of the skeleton | to the outside of the bones |
| Growth | grows with the body | cannot grow: the animal must moult | grows with the body (living tissue) |
| Protection | very little | very good: covers the whole body, reduces water loss | protects vital organs (brain, heart, lungs) only |
| Size of animal it can support | small, or large only in water | small: a large one would be too heavy | small to very large (elephant, whale) |
Worked example 2: classifying a skeleton step by step
Decide which type of skeleton each animal has: (a) a Lake Kivu sardine (isambaza), (b) a freshwater crab from a stream, (c) a jellyfish.
Answer: (a) endoskeleton, (b) exoskeleton, (c) hydrostatic skeleton.
Warning
Common mistakes. (1) "An exoskeleton grows as the insect grows." It does not; it must be shed and replaced. (2) "Bone is dead, like a stone." Bone is living tissue with cells and blood vessels, which is why a broken bone can heal. (3) "An insect has no muscles." It has many, attached to the inside of its exoskeleton.
Skeletal System (Amoeba Sisters). The first part compares the hydrostatic skeleton, exoskeleton and endoskeleton; watch for why exoskeletons must be moulted. Later parts about bone cells go beyond this lesson.
Tip
Memory hook: "EXO = EXit, outside; ENDO = ENter, inside." For comparison questions, compare the same feature on both sides, for example "An exoskeleton cannot grow, so the animal moults; an endoskeleton is living, so it grows with the body." Do not list facts about one side only.
Practice: exoskeletons and endoskeletons
Key points
Sources and credits
Written by Ganzaa: the observation, Figures 1 to 3 (the growth figures are illustrative), the comparison table, the worked examples and the practice questions.
