A plumbob is one of numerous species that evolved as a direct reaction to the convergent evolutionary being known as a Turd Burglar. Despite numerous origins on numerous worlds, and numerous biologies, turd buglary has evolved independaltly, and in almost identical ways, across known space. The resulting solution evolved across most of those worlds is the development of a plumbob.
Every normal Bob releases an excrement package or pellet in response to digestion. Every time he eats, he eventually produces such a product, his food's ultimate fruition being a waste concretion. Bob's produce, then, numerous excrescences a year.
However, the plumbob has the advantage of having produced only a single excrescence during an entire lifetime. Rather than deposit it in a specified or denoted area or chamber, the plumbob lowers his digested food matter egg or pod into the still liquid of a recepticle, allowing it to twist gently and rinse off, before raising it back into his canal to serve its ultimate purpose. That purpose is not yet known, but we fear the day we discover it.