All sentient beings?

This is a question about Pure Land Buddhism. What is a sentient being ? What is a living being ? Examples, e.g. slug, tree, bacteria, cat, aliens ? How do you teach enlightenment to a cat ? Are the snow lions in a pure land enlightened ? How do slugs get to be enlightened ? Do they have to be reborn as human first ? How can every living being be enlightened ? Do they all have to be human first ? Thanks.