Here Is Plato's Mollusk Trivia Quiz

Plato, the philosopher who believed in ideal forms, once defined a man as a "featherless biped" during a lecture. Diogenes the Cynic, whose principal occupation was making sloppy thinkers look ridiculous, decided to use a visual aid to show what he thought of this definition. He entered the Academy carrying a chicken he had plucked, and declared, "Here is Plato's man."

I tell you this story to express my befuddlement in generalizing what IS a mollusk. You can convince me there is a Platonic ideal of a bird, a lizard, a primate. But it seems like whenever you think you understand what a mollusk is - "a slimy creature with tentacles, a radula, and a shell!" - another one comes along to say, "Here is Etymon's mollusk."

So what *can* you say about all molluscs? First of all, they all have mantles. The mantle is a muscular wall that covers the single cavity containing all the mollusk's vital organs. Most mollusks have radulae, and many have shells made of calcium compounds.

Finally, there is the foot - just one, unlike most mammals. The "foot" is another large muscle that connects snails and slugs, clams and oysters, and chitons to the ground. What about cephalopods, like our nautilus? They are headfeet ("cephalo" - head + "pod" - feet): their crowns of arms and tentacles are actually evolved from the feet of snails and bivalves.

As in humans, the aorta of a limpet is the large, primary vessel of the circulatory system. Limpets have a three-chambered heart that circulates haemolymph, a usually clear fluid also found in crustaceans. Haemolymph binds oxygen to haemocyanin, a copper pigment, rather than the iron-based haemoglobin, so oxygen-rich mollusk blood has a pale blue color.

Most mollusks, besides the cephalopods, have an open circulatory system, meaning that the circulating fluid travels freely within the sinus cavities that contain their organs. (The advantage of keeping all your viscera in one mantle cavity is that it requires less complicated vessels to maintain everything.) Being less evolved than their cephalopod brethren, nautili are the only members of their clade which have open circulation.

A radula is a mollusk's spiral tongue which is completely covered in chitin. It is one of the few near-constants throughout the clade, though the two filter-feeding groups (the bivalves and the tusk shells) lack radulae, as do the obscure solenogasters. Basically, it's a tiny Rototiller for removing and pre-digesting the algae that many mollusks subsist on, common limpets among them. Limpets go through teeth faster than sharks - they can grow a new row on their radulae in just two days, and wear through a new set just as quickly. Quite impressive given that their teeth contain crystallized iron - in fact, these are the single strongest biological fibers known, stronger than spider silk.

Some mollusks have opted to eat things that don't sit there and let you eat them. Many have evolved the radula further, into sharp teeth that can shred or spear prey. In nautili, the radula has nine sharp teeth behind a sharp beak. (On our diagram, as the mouth is so well-hidden beneath all those tentacles, I opted not to mark those features.)

A mantle makes a mollusk mollusky. It is the muscular part of its body that cloaks its organs, and where it is situated determines the organism's overall body plan. Sounds vague? Now you're getting it! In a squid, the mantle is everything behind the tentacles and head up to the pointy tip. It includes fin structures and the core of its body. On a snail, the mantle is internal. It is the part of the snail which produces the compounds to form (and regenerate) its shell. The mantle is completely exposed in slugs: it's the thicker, saddle-shaped part where its breathing pore is, behind the head. The mantle also hides the shells of many slugs - yes, many slugs have internal shells!

The word "mantle" is originally from the Latin, meaning "cloak" (the Biblical expression "take up the mantle" uses the term in this sense). On many mollusks, including our snail, the mantle wall extends, cloak-like, beyond the wall of the body, creating the mantle edge. Nautiluses are actually an exception (if anything may be called an exception) among mollusks, in that their mantle is entirely internal.

Why was the robot mollusk sad? He was dismantled.

The shells of common limpets are extremely sturdy and hug the surface well, shaped roughly like a kneecap, hence the Latin name "Patella vulgata." Common limpets have a similar life plan to barnacles (a type of crustacean) and mussels (a fellow mollusk): cling to something hard, eat the algae on it, and have such a hard exoskeleton that your fleshy, surface-facing side stays impervious. Unlike barnacles and mussels, limpets can move - they just don't feel like it, for the most part.

The structure of the nautilus's shell is similar to that of other mollusk shells. About 99% of its weight is aragonite, a crystalized form of calcium carbonate. Other trade-secret mollusk polymers (including one with the beautiful name of "conchiolin") hold these crystals in place. In all shelled mollusks, the compounds that form the shell are secreted by the mantle. As the mantle glands are attached to nerves, it is believed that nautili can control the secretion of shell compounds with their brains.

It is true that both nautili and snails have parts of their bodies called "tentacles," although their functions and evolution are completely different. The tentacles of a cephalopod are what make them head-feet - they're modified from the "foot" muscle. The tentacles of a nautilus are used for hunting, feeding, locomotion, and reproduction. For a snail, they're really just sensory appendages.

Nautili have about ninety tentacles. The reason we don't call them nonagintapuses (ninety-foot) is that the number, oddly, isn't set in stone. Different individuals have different numbers of tentacles. A nautilus tentacle has a retractable sheathe, each containing a highly dexter "cirrus" with no suckers.

Snails, on the other hand, usually have two sets of tentacles: one for something resembling eyes, and one that contains "chemoreceptors." In fact, snails mainly orient using olfaction, not sight. Limpets are unusual in that they have only the one set: a pair of eyestalks. We'll talk more about limpet eyes in just a moment.

Another surprising similarity between these two mollusks is their rudimentary eyesight. Common limpets, like many snails, don't really have vision. They have light-sensitive eyespots at the ends of their eyestalks, which can only perceive the presence or absence of shadows, not images. (Other gastropods, such as conchs, do have more elaborate eyes.)

Nautili also have simple eyes - so simple, in fact, that I used to confuse them for siphons. Unlike some of their cuttlefish and squid cousins, nautili lack lenses. They focus via a pinhole, producing a blurrier image. Nautili are nocturnal animals that retreat to the ocean depths during the day, so you can understand why they've let their eyes stay simplified. It is believed that nautili depend on smell, rather than sight, to orient themselves.

Many mollusks have developed a siphon of some kind, formed from the excess flesh around the mantle. A nautilus is particularly dependent on its siphon. Helpfully for me, it demonstrates almost all of the things mollusk siphons do: it provides jet-propulsion, it floods the internal gills with water, and, unique for nautili, it fills the shell's chambers with water. About the only thing it doesn't do is squirt ink, which nautili lack.

Some snails also have siphons with chemoreceptors, functioning basically like an additional tentacle. Limpets, however, barely move, and live in the intertidal zone. Ergo, their bodies are optimized like nuclear bunkers - they don't allow a siphon to poke out when the tides have receded. (Note that the breathing pore on the side of a slug, called the "pneumostome," is not a siphon. Only land slugs have pneumostomes, while only aquatic mollusks have siphons.)

The part of a nautilus's shell that is broken into chambers is called the phragmocone. The chambers themselves are called camerae (simply the Latin word for "chambers"), and they grow and become sealed off as the nautilus gets older. The tube which runs through all of the camerae is called the siphuncle. When each camera is sealed, the water must be removed so the chamber will refill with air and maintain buoyancy. The siphuncle does exactly this, by interfacing with the animal's bloodstream.

The walls between the camerae are called septa ("septum" is Latin for partition, which is why the middle wall of your nose shares a name with nautiuls anatomy). The term "septal neck" is synonymous with "siphuncle." Of course, such a distinct structure is really only present in the nautiloids and some of their extinct ancestors.

If you've ever seen a bisection of a nautilus shell, or a picture of one, you know that you can clearly see each of the shell's camerae and the siphuncle that runs between them. Even fossils of nautilus shells (the only part of a cephalopod that can really survive fossilization) display these features prominently. Young nautili have 4 camerae, while fully grown adults can have more than 30.

When I hear "nautilus," I think of the Fibonacci spiral (a logarithmic spiral that follows the "golden ratio" phi, 1.618). But my math textbook may have told me a Fibonacci fib. The mathematician Clement Falbo found that the ratio of the spirals for nautilus shells ranged from 1.24 to 1.43, never 1.618. However, the blog GoldenNumber.net has argued that phi can be found hidden in the nautili in several other ways, such as by taking the ratio of the spiral's widths at different points.