This kind of bug often appears in toilets, ranging from a dozen centimeters long to four or five centimeters small. What kind of bug is this? Where did it come from?

Bipalium (pinyin: jī zhì) is also called "snake of heaven", and it was called "indigenous method" in ancient times. Lentinus edodes, Hirudoidae. It is 4-30 cm long and its head is fan-shaped. It is yellow, with five black longitudinal stripes, and lives in the wet soil beside the roots or at the foot of the wall.