The slow worm (Anguis fragilis) is considered a limbless lizard, not a snake.
From an evolutionary standpoint, there are a number of key differences. The slow worm has the ability to drop its tail and re-grow a new one, should it be caught by a predator. The slow worm also has small eyes with blinking eyelids, another ability snakes lack. They also have different scales, and a different manner of shedding them.
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