When upset, worried or shy, you can retreat into your shell. You can even develop a hard shell of self-protection, when underneath you're soft and quite vulnerable. Sometimes, you can be snappy and rather 'crabby'. When hurt or addressing a touchy issue, you can sometimes take an indirect approach and move sideways, like the crab.
Cancer is a water sign.
This represents an emotional, nurturing, intuitive, creative nature. You have strong feelings and live life from the heart. The Moon symbolises changeable moods, a sensitive, responsive nature and reflective perceptive qualities. It represents the Eternal Mother i.e. caring, protective and sheltering. This denotes a deeply emotional nature.