Tubes, garden hoses, highways with water, sewage pipes, water pipes, canals, rivers of cells, Erie Canal, the Panama Canal, the Suez Canal, this canal, a stream, a brook, a storm drain. In short they are kinda like any kind of circular liquid transport system.
Feel free to be the first
Please review the linked page for context.
If you can think of something better than this,
please add it to the database