Answer
Environmental pressures
Basically every animal adapts to pressures from their environment in different ways, causing them to have different shapes and sizes
B. The Threshold describes the minimum level of stimulus required to ctive a neuron. Hope that helps
The answer is a simple radial nervous system.
Echinoderms do not have brains. Around their mouth, they have central rings from which nerves radially run into arms and along the body. Movement is controlled by the branches of those radial nerves.
