All appearances to the contrary, Squirt the octopus doesn’t have any legs. She also has no tentacles. She doesn’t have any lungs, and the things that look like gills aren’t actually called gills. Ditto with the starfish. Starfish also don’t have legs; though despite that, they manage to have several hundred tube feet. Starfish have bony bits, which aren’t actually bones, and eyespots that don’t look anything like eyes. While we’re on the subject, they’re not actually called starfish, either.
So what are an octopus’ gills actually called? Ctenidium. My spell checker doesn’t even recognize that word. Neither would 99% of the aquarium guests I talk to, and the one guy who does know that the gills are actually called ctenidium probably has some sort of biology degree, and knows far more about octopus than I do, anyway. So, now you know what to properly call an octopus gill. But does that tell you anything more about the octopus itself?
I am of the opinion that some scientific terminology, a naturalist is better off ignoring. It is not worth correcting (or confusing) a visitor unless there is going to be some payoff in a visitor's improved understanding of the animal in question. Does knowing that an octopus gill is actually a ctendium improve anyone's understanding of an octopus? Does a gill by any other name still remove oxygen from the water column?
Certain inaccuracies I will correct, such as a completely wrong identification of an animal’s species. (Woody isn’t a mother walrus, for example; the common murres aren’t actually penguins, and the plumose anemones aren’t plants.) I’ll also (mostly) correct misinterpretation of an animal's behavior. A puffin who’s shaking his feathers is preening; not shivering with cold. A sea urchin in the touch tank moving his spines isn’t trying to eat you, he’s trying to discourage you from eating him. And that blue duck trying to mount the brown duck isn’t playing with her, though how I actually explain that behavior varies depending on how many children are within earshot. ('It's that time of the year.')
Other inaccuracies I will happily let stand, providing that they do not get in the way of the visitor understanding the basic gist (for lack of a better word) of the animal. This is especially true concerning animal features. For example, fins and flippers are both very good words to describe the appendages seals use to swim with, despite the fact that fins are only found on fish, whales, and scuba divers. Saying that Squirt the octopus has eight legs, while technically inaccurate (they’re called arms), doesn’t radically detract from understanding what Squirt’s arms do. She uses them to move around; she can crawl; her arms (or legs) are very dexterous, and strong enough to pry open clam shells.
One reason why octopus are such interesting critters is that they are very mammal-like animals, who happen to have a completely non-mammal-like biology. It’s sort of like someone gave God an oyster, and asked Him to turn it into a bunny. You might get long ears and a fluffy tail, but the ears and the tail are going to be coming from some very different places. So you end up with gills that are actually ctenidium, and tongues that are called radula, and weird mouth drilling equipment called salivary papilla, that don’t have any sort of biological counterpart, outside of maybe a DeWalt cordless drill. Partly for this reason, octopus seem to be particularly prone to confused nomenclature, such as having a non-gill-like structure that does very gill-like things, or a head that's actually called a mantle. Confusion is even implicit in the name. Quick – what’s the plural of octopus? Octopi? Octopuses? Octopus?
Technically, since octopus is originally a Greek word (not Latin), the plural, in Greek would be octopodes. Which sounds ridiculous in English, and thankfully no one, not even the biology textbooks, actually follows this usage.
Before we go any further, let me clarify that octopus do have eight arms, hundreds of sucker discs, and two mollusk lung-like organs called ctenidium, which is, basically, a marine-biology-degree word for a mollusk gill. Squirt the octopus also has an overabundance of a lot of other familiar organs – such as three hearts, and nine brains (or technically, one brain and eight nerve clusters, one in each arm). Seems a little excessive when you consider that animals like the plumose anemones can grow, move, reproduce, kill prey, and defend themselves without any sort of brain at all.
When dealing with invertebrates, a lot of the normal features of a mammal, the standard operating procedures, if you will, simply don’t apply. Invertebrates do the same sorts of things that mammals do – eat, reproduce, grow and defend themselves, but they tend to accomplish these things by very different methods. Which, for those with marine biology degrees, means a whole boatload of new terminology – such as a fancy new word for an organ that would be called a gill, if the owner of this organ happened to be a fish.
The fact that invertebrates have such different biology from us makes it even more startling when an animal happens to develop an organ that very closely parallels something owned by a vertebrate. Octopus have very well developed eyes, containing all sorts of mammal-like features, such as an iris, retina, pupil, and lens. Other features of its organization aren’t nearly so mammal-like, such as its approach to brain organization. The nerve clusters in octopus arms appear to be capable of responding to stimuli and carrying out commands (‘thinking’, in other words) independently from the rest of the octopus. How much oversight the main octopus brain has over the nerve clusters (or how much feedback the brain gets about what its arms are actually doing) is still being researched, and debated. For example, it may be possible for one individual arm to learn a behavior (opening a certain type of jar, for instance) that the other seven arms don’t know.
I’ve always thought that Squirt had a lot of personality. Some of this research is hinting that she might actually have nine of them…