Penis Worm

Image: University of Bristol

OK. Roll up your sleeves, put on your surgical gloves, prepare the censor bars. It's time for the Penis Worm.

No sniggering at the back!

Image: Shunkina Ksenia

The Penis Worm is a worm. It is. Just look. What else could it possibly be? There are some 20 or so species officially described and they make up a tiny phylum all of their own called Priapulida, a name which refers to Priapus, a Greek god. Of fertility. Very much male fertility. I guess he has a Penis Worm, too, in a way. It just isn't a worm. And you'd need a really big censor bar to cover it all.

Speaking of which, Priapulids can reach anywhere from a millimetre long to some 40 cm (16 in). 40 cm! More than a foot! Even Priapus himself would be happy with that one.

Image: Bruno C. Vellutini

Something he wouldn't be at all happy with is the fact that his namesakes live very much in the margins of marine society. They are not the mighty whales who breach the water's surface to spout out of their blowhole; imagine a 20 metre (65 foot) Penis Worm doing that and weep. They're not the fearsome shark who prowls the sea, psychopathy in their glassy eye; Jaws would have been a different movie. They're not the intelligent octopod who skulks in the shallows, surviving by wit and ninja skill; I dread to think what an eight-armed Penis Worm would look like.

Penis Worms are mud dwellers. They creep through the seabed, mostly in relatively cool, relatively shallow waters across the world. There they munch on other soft-bodied burrowers like polychaete worms and fellow Peripulids. Yup. Meat-eating Penis Worm. At least it's not the name of a parasite...

Video: BlackEagleNews

A Priapulid's body can be divided into two, sometimes three, parts.

First, the business end. The prosoma, or introvert, is basically the mouth. It's bulbous and, in most species, covered in rows of spines. It can also be withdrawn into the rest of the body and completely hidden away. Penis Worms burrow through the muddy seabed by extending their prosoma, gripping the mud with their spines, and then dragging their whole body forward by withdrawing the prosoma into the trunk.

Image: BOLD, Biodiversity Institute of Ontario

The actual mouth opening lies at the very tip of the prosoma and, while I've never seen a Penis Wom eat another worm, I'm sure we'd all be horrified by how wide that mouth can open when it needs to. I've no doubt it's all very nightmarish. We've all heard about how the world is "red in tooth and claw" but if you're a snail or a flatworm who doesn't have those kinds of teeth and claws, then other solutions need to be found. And they're usually horrible. Give me a good ol' tiger any day! A nice friendly one for preference.

The second part of the body is the trunk. It contains all the muscles for pulling in the prosoma as well as the few organs every Penis Worm is blessed with. It's also covered in a thin, chitinous cuticle which has to be moulted as the Penis Worm grows.

Image: Bernard Picton

Finally, larger species have a rather ghastly tail that looks like a whole mass of tiny, squirming worms. The extra surface area probably helps them get all the oxygen they need, something that smaller species have less trouble with.

As I say, were you mean enough to crack a Penis Worm open, you'd find that there's not much to see in there. Priapulids have precious few sense organs—no eyes or nose or ears, just a few sensory nerve endings in the body to reassure them that yes, they really are completely buried in total darkness. Which is great if you're into that sort of thing.

Image: Ralf Janssen, Sofia A Wennberg and Graham E Budd
Newly hatched Penis Worm

The nervous system itself is very simple, with a nerve ring just behind the mouth (eating is undoubtedly the most exciting thing in their entire lives) and a nerve cord running the length of their body with a few nerve bundles here and there. Gonads, either male or female, simply dump eggs or sperm into the sea. There's an organ called the protonephridium which helps to get rid of waste. And... that's about it! Once you include a digestive system which is essentially a tube going from one end to the other, you've pretty much got yourself a Penis Worm.

With this physical minimalism and a simple life spent in perpetual darkness, it's little wonder that poor old Penis Worms have been pushed to the outskirts of the marine community. But it wasn't always thus.

Image: Martin R. Smith
This fossil Ottoia Penis Worm is over 500 million years old

Priapulids are old. Really old. Modern species can trace their ancestors at least 300 million years. Before that, there was another group of very similar worms who are sometimes called Priapulids, just like modern Penis Worms but are sometimes called Archaeopriapulids. They date back to the Cambrian Period, some 500 million years ago. As trilobites explored the potential of their new-fangled exoskeletons and echinoderms were deciding what shape to be, these grand-daddyPenis Worms were spreading all over the world and eating pretty much everyone.

Penis Worms used to be fearsome hunters and ambush predators. They probably still burrowed through the mud or lay in wait in seabed burrows, but back then they were rulers of their domain. Not necessarily apex predators all the time, but at least competitors. At least they could stand up tall and take a bite out of just about anyone around them.

Image: Tom Harvey
Assortment of Ottoia teeth and scales

Today, Priapulids exist as a strange and rather isolated remnant of the past. Their closest relatives are tiny and microscopic creatures called Kinorhyncha and Loricifera. Penis Worms are usually placed within a larger group called Ecdysozoa, which also includes nematode worms and arthropods. But while nematodes took to parasitising all the latest animals, and while arthropods conquered the land, Penis Worms stayed exactly where they were. They watched the world change around them, like a thatched roof cottage surrounded by skyscrapers.

I can't help but admire them in a sad way.