Types of Skeleton: Hydrostatic, Exoskeleton and Endoskeleton

Hydrostatic skeleton

A hydrostatic skeleton is found in soft-bodied invertebrates such as earthworms and jellyfish. It consists of a fluid-filled body cavity enclosed by a muscular body wall; because the fluid cannot be compressed, contracting the surrounding muscles changes the shape of the body rather than its volume.

In an earthworm, alternating contraction of circular muscles (which make a segment long and thin) and longitudinal muscles (which make it short and fat) produces the waves of movement used to burrow and crawl.

Exoskeleton

An exoskeleton is a hard external covering found in arthropods, such as insects and crabs. Made mainly of a tough material called chitin, it protects the soft internal organs and provides a surface on the inside for muscles to attach to, allowing precise, rapid movement in small, lightweight animals.

Because an exoskeleton cannot grow once it has hardened, the animal must periodically shed it in a process called moulting (ecdysis) and grow a new, larger one, during which time it is soft and vulnerable.

Endoskeleton

An endoskeleton is an internal framework of bone and cartilage found in vertebrates, including humans. Unlike an exoskeleton, it grows together with the body, so the animal never needs to moult.

It can support a much greater body weight, allowing vertebrates to grow far larger than most invertebrates, and it protects delicate internal organs, such as the skull protecting the brain and the ribcage protecting the heart and lungs. Muscles attach to the outside of the bones, giving powerful and precise control over movement.

Comparing the three types

How it is examined

Exam questions on this standard often give an animal or a diagram and ask you to identify its type of skeleton, to compare two types in terms of support, growth or movement, or to explain why an exoskeleton limits growth while an endoskeleton does not.

Worked exam-style question

Question. Diagram X shows the elbow joint of a human arm, with muscle J attached above the joint that shortens when the arm bends, and muscle K attached below the joint that shortens when the arm straightens. Structure L connects each muscle to the bone.

(a) Name the type of skeleton found in a human arm. (b) State the term used for the way muscles J and K work as a pair.

(c) Name structure L, and state one way its structure suits its function. (d) Explain why muscle J alone cannot both bend and straighten the arm.

Model answer. (a) The human arm is supported by an endoskeleton. (b) J and K work as antagonistic muscles.

(c) L is a tendon; it is made of tough, non-elastic connective tissue, which lets it transmit the force of muscle contraction to the bone without stretching or snapping. (d) A muscle can only contract and pull, it cannot push, so J bending the arm must be reversed by a separate muscle, K, contracting on the opposite side to straighten it again.

Practice question

Try this. A crab's exoskeleton and a human's endoskeleton both provide support and allow movement, but an adult crab must moult several times before reaching full size while an adult human never needs to. Explain this difference.

Exam tip

Key terms

These Chapter 14 terms name the structures examined in this topic:

• Exoskeleton, a hard external skeleton, made mainly of chitin, found in arthropods such as insects and crabs.
• Antagonistic muscles, a pair of muscles that work against each other, one contracting as the other relaxes, to move a joint in opposite directions.
• Tendon, tough connective tissue that joins a muscle to a bone and transmits the force of contraction.
• Ligament, tough connective tissue that joins one bone to another at a joint, limiting its range of movement.

Source:SRC-DSKP-EN

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