As soon as the girl steps on the shell, the first thing that happens is that the sensory neuron, which in this case is located on the skin, receives the information and transfer it to the inter-neuron or the association neuron which is located between the motor and the sensory neuron. From there, the motor neuron relays the information to the effector e.g a muscle that responds to the stimulus by removing the foot immediately.
In humans, visible secondary sex characteristics include pubic hair, enlarged breasts and widened hips of females, and facial hair and Adam's apples on males.
