6.1 Support, Movement and Growth in Animals
Animals require a support system to maintain body shape, support body weight, protect internal organs, and enable movement. There are three main types of animal skeletons: endoskeleton, exoskeleton, and hydrostatic skeleton.
Learning Objectives
- State and explain the three types of support systems in animals and provide examples.
- Explain the relationship between exoskeleton size and growth, including the step-shaped growth curve and ecdysis.
- Explain the movement mechanism in an animal with a hydrostatic skeleton involving antagonistic muscles.
- Compare the functions of endoskeletons in terrestrial vertebrates, aquatic vertebrates, and birds.
- Describe the components of the human skeletal system.
- Compare the strength of compact bones and hollow bones.
- Explain the factors that affect stability in animals and how animals overcome stability problems.
Human Skeletal System & Insect Growth Curve
The human skeleton is divided into axial and appendicular parts, while insects grow in a step-shaped pattern through ecdysis.
Short Explanation
What is a support system?
A skeleton is the support system for all types of animals. It maintains body shape, supports body weight, protects internal organs, and enables movement.
There are three main types of support systems in animals: endoskeleton, exoskeleton, and hydrostatic skeleton.
Endoskeleton
Support system for all vertebrates, including humans, fish, amphibians, reptiles, birds and mammals. It is made of internal bones and cartilage. Examples: cat, frog, fish, crocodile, eagle.
Exoskeleton
Support system for most invertebrates. It consists of an outer layer of waxy chitin or shell. Examples: spider, prawn, scorpion, crab, cockroach, grasshopper, beetle.
Hydrostatic skeleton
Found in soft-bodied invertebrates without bones, such as earthworm, leech, starfish and jellyfish. A muscular wall encloses a fluid-filled body cavity, and the fluid exerts hydrostatic pressure to make the body firm.
Functions of a skeleton
Supports body weight, maintains body shape, protects internal organs, and provides an attachment base for muscles to move body parts.
Exoskeleton Size with Growth
Insects grow in a step-shaped pattern because their hard chitin exoskeleton cannot expand.
Organism growth can be measured by plotting growth unit against time, forming a growth curve. Most organisms have a sigmoid-shaped growth curve, but animals with an exoskeleton have a step-shaped growth curve.
Ecdysis
Ecdysis is the repeated shedding of the old exoskeleton until adulthood. During ecdysis, a new and soft exoskeleton forms beneath the old one. The animal sucks in air to expand its body, breaking the old hard exoskeleton. Rapid growth occurs before the new exoskeleton hardens.
Growth curve stages of an insect. The horizontal parts are instar stages I–V, where no size increase occurs. The vertical parts are rapid growth immediately after ecdysis, before hardening. Growth is controlled by hormones; nymphs eat heavily during nymph stages to build tissues and increase weight.
Hydrostatic Skeleton and Movement in Earthworms
The earthworm moves using fluid pressure and two sets of antagonistic muscles.
The earthworm has a fluid-filled body cavity, chaetae on the body wall to grip the ground, and two sets of antagonistic muscles: circular muscles and longitudinal muscles.
Circular muscles contract
Longitudinal muscles relax. The body becomes thin and long. Fluid pressure shifts posteriorly, chaetae release their grip, and the body extends forward.
Longitudinal muscles contract
Circular muscles relax. The body becomes thick and short. Fluid is transferred to the shortened part, chaetae grip the ground, and the rear end is pulled forward.
Alternating peristaltic waves of contraction from anterior to posterior propel the earthworm forward.
Functions of Endoskeleton in Vertebrates
The endoskeleton is adapted differently in terrestrial vertebrates, aquatic vertebrates and birds.
Terrestrial vertebrates
Need a strong, firm, large skeletal frame compatible with body size. Body weight is supported mainly by the pectoral girdle and pelvic girdle, joined to limbs. Curved backbones in quadrupeds withstand gravitational forces and provide strong muscle attachment.
Aquatic vertebrates
The endoskeleton is smaller relative to body size; pectoral and pelvic girdles are small and weak. Body weight is supported by the buoyancy force of water, allowing animals like whales to grow much larger than their skeleton could otherwise support.
Birds
Adapted for flight: bones are hollow and light, and the skull is relatively small. Birds possess a wide and flat sternum for flight muscle attachment.
Compact vs hollow bones
Terrestrial animals have big, compact, strong bones to support full weight. Birds have hollow bones that are light and strong, allowing faster movement and requiring less calcium and phosphorus. Experiment 6.1 shows hollow cylinders can support more load than compact cylinders of the same length and diameter.
Human Skeletal System
The adult human skeleton has 206 bones divided into the axial skeleton and the appendicular skeleton.
Axial skeleton
Skull: cranial bones protect the brain; facial bones provide the framework for the face and support teeth.
Vertebral column: 33 small vertebrae forming a strong, flexible column that protects the spinal cord.
Sternum and ribs: 12 pairs of ribs — 7 pairs true ribs attached directly to the sternum, 3 pairs connected indirectly via cartilage, and 2 pairs floating ribs. They protect the heart and lungs.
Appendicular skeleton
Pectoral girdle: clavicle and scapula; connects upper limbs to the axial skeleton.
Upper limbs: humerus, radius, ulna, carpus, metacarpus, phalanx.
Pelvic girdle: pair of hip bones; supports weight and protects the bladder and reproductive organs.
Lower limbs: femur, patella, tibia, fibula, tarsus, metatarsus, phalanx.
Stability in Animals
Stability depends on the centre of gravity and the base area.
Centre of gravity
The centre of gravity is the point of equilibrium where the weight of the object or body acts to balance the system.
Factors affecting stability
A lower centre of gravity increases stability; a higher centre of gravity decreases stability. A larger base area increases stability; a smaller base area decreases stability.
Examples and adaptations
Tortoises and crocodiles have naturally low centres of gravity and broad base areas. Giraffes spread their legs wide when drinking water to increase base area and lower their centre of gravity. Kangaroos use their muscular tail as a third point of support to enlarge their base area when stationary.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Why is the growth curve of an insect such as a grasshopper step-shaped and not sigmoid?
Explain how the longitudinal muscles and circular muscles of an earthworm act when shortening and lengthening its body.
How does a giraffe maintain its stability while drinking water?
Common Mistakes
- Assuming all animals display a sigmoid growth curve. Animals with an exoskeleton have a step-shaped growth curve.
- Naming both earthworm muscles as contracting at the same time. Earthworm muscles are antagonistic — when one contracts, the other relaxes.
- Classifying the rib cage or vertebral column as appendicular skeleton. Both belong to the axial skeleton.
- Thinking compact bones are stronger than hollow bones at the same mass. Hollow bones are proven to be stronger and lighter.
Concept Misunderstandings
The exoskeleton grows together with the insect's body every day.
The chitin exoskeleton is rigid and prevents continuous growth. The animal must shed the exoskeleton through ecdysis to allow body size to increase.
Aquatic animals such as whales have a very large and heavy skeleton to support their large body.
A whale's body weight is supported mainly by the buoyancy force of water, allowing its pectoral and pelvic girdles to be small and weak.
Snakes have a hydrostatic skeleton because their body is flexible.
Snakes are vertebrates and have an endoskeleton consisting of internal bones and a long vertebral column.
Summary
- Animal support systems are divided into endoskeleton, exoskeleton, and hydrostatic skeleton.
- Animals with an exoskeleton undergo ecdysis, producing a step-shaped growth curve with instar phases.
- Earthworms use coelomic fluid pressure with antagonistic muscles and chaetae to move.
- The human skeleton contains 206 bones, divided into the axial skeleton and appendicular skeleton.
- Hollow bones give birds the advantage of being light and strong.
- Animal stability is influenced by a low centre of gravity and a large base area.
Short Activity
A. Objective Quiz
1 Which part of the human skeleton belongs to the axial skeleton?
2 Why does an insect suck in air during ecdysis?
B. Fill in the Blanks
3 The repeated process of shedding the exoskeleton in insects during growth is known as .
4 Birds have bones which make them light and strong for flight.
C. Drag and Drop: Match the Animal Feature to Its Function
Drag each animal feature to the correct function or description. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match the Animal Feature to Its Function
Write the correct animal feature next to each function.
| Function / Description | Animal Feature |
|---|---|
| 1. Bristles on an earthworm's body used to grip the ground during locomotion. | |
| 2. Broad, flat breastbone in birds that serves as an attachment point for flight muscles. | |
| 3. Acts as a strut to increase base area and improve stability while stationary. | |
| 4. Part of the axial skeleton that protects the brain. |
Answer choices: Chaetae • Sternum • Kangaroo tail • Cranial bones
