Snake

"a mighty big river, that you could see on the map, resembling an immense snake uncoiled, with its head in the sea and, its body at rest curving afar over a vast country, and its tail lost in the depths of the land." (6)

The Congo River is being compared to a snake because snakes can be poisonous. Marlow is fascinated with the snake like a bird would be. It also symbolizes the snake in the Genesis, that convinces Eve to eat the forbidden fruit that causes original sin. The snake lures Marlow to go up the Congo, which is poisoned with deceiving imperialism.