A cuticle that moults during growth.

A pseudocoelom.

A head.

Antennae.

It separates oxygenated and deoxygenated blood, improving efficiency of gas exchange.

It enables organisms to survive without a respiratory system.

It allows blood to bypass the lungs, increasing oxygen delivery to tissues.

It reduces the number of heart chambers, simplifying cardiac function.

It provided a way to reproduce on land without needing an external water source.

It led to the evolution of complex social behaviours.

It allowed for a greater number of eggs to be produced per clutch.

It provided an enhanced internal barrier against sperm from other species.

All Cnidarians

All Deuterostomes

All Ecydysozoans

All Eumetazoans

All Lophotrochozoans

All Protostomes

All Bilaterians

An animal with internal fertilisation.

An animal that shows parental care.

A broadcast spawner.

An animal that exhibits a K-strategy.

Presence of moist, thin body surfaces with a high surface-area-to-volume ratio.

Circulatory systems that transport oxygen directly to cells via haemoglobin.

Development of lungs with alveoli to increase surface area.

Use of diaphragm muscles to ventilate internal organs.

It allows for greater flexibility and movement in soft-bodied organisms.

It reduces the need for internal fluid pressure to maintain body shape.

It provides better protection against predators and environmental hazards.

It supports larger body sizes and complex jointed limbs.