6.1 Support, Movement and Growth in Animals

Types of Support in Animals

• A skeleton is the support system for all types of animals.
• Vertebrates consist of fish, amphibians, reptiles, birds and mammals.

Type of support	Structure, functions and examples
Endoskeleton	Support for all vertebrates, including humans; made up of internal bones or cartilage; supports body weight, protects internal organs, maintains body shape and provides attachment sites for muscles to move body parts; examples: cat, frog, crocodile, fish and eagle
Exoskeleton	Support for most invertebrates; consists of an outer layer of hard, waxy chitin or a shell; supports body weight, maintains body shape, protects internal organs and provides attachment sites for muscles; examples: spider, prawn, crab and scorpion
Hydrostatic skeleton	Support for some soft-bodied invertebrates; consists of a muscular wall enclosing a fluid-filled body cavity; fluid pressure acts in all directions, making the soft body firm; maintains and controls body shape and assists movement; examples: earthworm, leech, starfish and jellyfish

Exoskeleton Size and Growth

• A growth curve is a graph of a growth unit against time.
• Growth units include height (cm), volume (cm³), wet mass (g) and dry mass (g).
• A growth curve shows an organism’s growth phases and growth rate.
• The basic growth curve of organisms is sigmoid-shaped, but its pattern varies among organisms.

• Animals with an exoskeleton, such as cockroaches and grasshoppers, show a step-shaped growth curve because:
  • the exoskeleton is made of hard chitin and cannot expand;
  • the animal repeatedly sheds its exoskeleton until adulthood;
  • a new, soft exoskeleton forms under the old exoskeleton.
• Ecdysis: the process of shedding the old exoskeleton.
• During ecdysis:
  1. The animal inhales air to expand its body.
  2. Expansion breaks the old, hard exoskeleton.
  3. Rapid growth increases body size before the new exoskeleton hardens.
• A hormone controls each stage of ecdysis.
• At the nymph stage, the animal eats a large amount of food to build new tissues and increase mass.

Interpreting the Step-shaped Growth Curve

• Vertical section: sudden growth occurs during ecdysis while the new exoskeleton is soft.
• Horizontal section: zero growth in body length occurs while the exoskeleton is hard; this stage is called an instar.
• Several instars and ecdyses occur before the animal becomes an adult.

Activity 6.2: Growth Curve of an Animal with an Exoskeleton

Aim: Study the growth curve of an animal with an exoskeleton.
Apparatus: Ruler

1. Observe pictures of five grasshopper nymphs, A–E, and an adult grasshopper over days 1–30.
2. Measure each grasshopper from the end of the head to the end of the abdomen.
3. Record each length in centimetres.
4. Plot grasshopper length (cm) against time (day).

Interpretation: The curve is step-shaped because growth in body length occurs rapidly during ecdysis and stops during each instar.

Hydrostatic Skeleton and Movement

• An earthworm has a fluid-filled body cavity and moves with the aid of chaetae, the bristles at the sides of its body.
• Its body wall contains circular muscles and longitudinal muscles that act antagonistically.

Muscle action	Effect on the body segment
Circular muscles contract; longitudinal muscles relax	Segment becomes thin and long
Longitudinal muscles contract; circular muscles relax	Segment becomes thick and short

Movement of an Earthworm

1. Alternating contraction and relaxation begins at the anterior and passes towards the posterior.
2. Circular muscles contract and longitudinal muscles relax in some segments.
3. These segments become long and thin; their chaetae release their grip so the body can extend forwards.
4. Longitudinal muscles contract and circular muscles relax in other segments.
5. These segments become short and thick; their chaetae grip the ground.
6. Hydrostatic pressure transfers body fluid into the shortening region, pulling the posterior forwards.
7. Repeated alternating waves move the earthworm forwards.

Functions of the Endoskeleton in Animals

Terrestrial Vertebrates

• Require a strong and rigid endoskeleton to support the body.
• Have a large skeleton proportional to body size.
• The pectoral girdle and pelvic girdle mainly support body weight and articulate with the limbs.
• In four-legged terrestrial vertebrates such as camels and horses, the curved vertebral column:
  • strengthens support for attached muscles;
  • enables the vertebral column to withstand gravitational force.
• Example: an elephant needs strong pectoral and pelvic girdles to support its body weight.

Aquatic Vertebrates

• Have a smaller endoskeleton relative to body size.
• Their pectoral and pelvic girdles are small and weak.
• Water buoyancy supports body weight; therefore, an aquatic vertebrate such as a whale can grow larger than the size of its skeleton.

Birds

• Have bone structures adapted for flight:
  • a flat, broad sternum provides a large attachment site for flight muscles;
  • hollow, light bones reduce body weight;
  • a relatively small skull makes flight easier.

Human Skeletal System

• The human skeleton is made up of 206 bones of various sizes and shapes.
• It is divided into the axial skeleton and appendicular skeleton.

Axial Skeleton

Structure	Composition and function
Skull	Consists of cranial bones and facial bones; cranial bones protect the brain; facial bones form the basic framework of the face and support the teeth
Vertebral column	Consists of 33 small bones called vertebrae; connected vertebrae form a strong, flexible column that protects the spinal cord
Ribs and sternum	12 pairs of ribs articulate with thoracic vertebrae; 7 pairs join the sternum directly, 3 pairs join indirectly through cartilage and the last 2 pairs hang freely; protect the heart and lungs

Appendicular Skeleton

• Pectoral girdle:
  • a pair connects the upper limbs to the axial skeleton;
  • consists of the clavicle and scapula.
• Upper limb:
  • consists of the humerus, radius, ulna, carpus, metacarpus and phalanx;
  • the rounded upper humerus articulates with the pectoral girdle;
  • the lower humerus articulates with the radius and ulna;
  • the radius and ulna articulate with the carpus to form the wrist;
  • the metacarpus forms the palm and articulates with the carpus;
  • the phalanges form the fingers and articulate with the metacarpus.
• Pelvic girdle:
  • consists of a pair of hip bones and articulates with the axial skeleton;
  • supports body weight;
  • protects the bladder and reproductive organs.
• Lower limb:
  • consists of the femur, patella, tibia, fibula, tarsus, metatarsus and phalanx;
  • the rounded upper femur articulates with the pelvic girdle;
  • the lower femur articulates with the tibia and fibula;
  • the tibia and fibula form the calf and articulate with the tarsus at the ankle;
  • the tarsus articulates with the metatarsus, which forms the foot;
  • the metatarsus articulates with the phalanges of the toes.

Experiment 6.1: Strength of Compact and Hollow Bones

• Large terrestrial vertebrates such as elephants have large, compact and strong bones because the endoskeleton supports their entire body weight.
• Birds have hollow bones that are:
  • light and strong;
  • able to allow faster movement;
  • able to reduce the need for calcium and phosphorus.

Aim: Compare the strength of a compact bone with a hollow bone.
Problem statement: Is a hollow bone stronger than a compact bone?
Hypothesis: A hollow bone is stronger than a compact bone.

Variables:

• Manipulated: Type of cylinder—hollow or compact
• Responding: Number of textbooks supported by the cylinders
• Constant: Length and diameter of each cylinder

Materials: A4 paper, cellophane tape, box covers
Apparatus: Textbooks, scissors

1. Roll A4 paper into a hollow cylinder 2.5 cm in diameter. Tape its upper and lower ends. Make three more identical cylinders.
2. Attach the four hollow cylinders to the corners of a box cover to make table model A.
3. Roll A4 paper into a compact cylinder 2.5 cm in diameter. Tape its upper and lower ends. Make three more identical cylinders.
4. Attach the four compact cylinders to another box cover to make table model B.
5. Place textbooks one at a time on each model until its paper cylinders bend.
6. Record the number of textbooks supported by each model.

Model	Observation
A: hollow cylinders	Supports more textbooks before bending
B: compact cylinders	Supports fewer textbooks before bending

Conclusion: Hollow cylinders are stronger than compact cylinders of the same length and diameter; the hypothesis is accepted.

Support System and Animal Stability

• An effective support system allows an animal to move smoothly and efficiently.
• Centre of gravity: the balance point of a support system or object; the point through which its total weight acts to balance its position.
• Stability: the ability of an object to maintain its original position.

Factor	Relationship with stability
Centre of gravity	A lower centre of gravity gives greater stability; a higher centre of gravity gives lower stability
Base area	A larger base area gives greater stability; a smaller base area gives lower stability

• A giraffe is less stable while standing because it has a high centre of gravity.
• A tortoise and crocodile are naturally more stable because they have low centres of gravity.

Solutions to Stability Problems

• Giraffe drinking water:
  • spreads its legs to increase the base area;
  • lowers its centre of gravity;
  • becomes less likely to topple.
• Stationary kangaroo:
  • stands on two hind legs and does not use its forelegs for standing;
  • uses its tail as a strut;
  • the tail increases its base area and prevents it from toppling.