So when we think about invertebrates, we wana start to describe those organisms, or

those specimens, or those fossils that we find in our backyard, and

try to deduce how that organism lived.

And so, invertebrates can be wildly intricate, or

they can be fairly simple, if I'm not being too [LAUGH],

I don't want to insult those invertebrates, being too simple.

So, we want to start to deduce the type of environment that those organisms lived and

how they lived, actually.

And we can get a lot of information from just a simple little fossil and

considering the symmetry of it, and we can start to think about how it moved.

So let's discuss symmetry for just a moment here.

So the first thing you might notice about a specimen that you pick up, or

a dead organism that you find, or a fossil, is the general shape.

And so one of the descriptors of that shape is symmetry.

And so what symmetry is, is a recognition of some type of organized structure that

maybe can be reflected, where different sides of the organism are the same.

For example, a human, you can tell if you split it down the middle, the right and

the left half are the same.

And so that's a type of symmetry.

And so the first type of symmetry is the lack of symmetry.

For example, a shell, the general overall shape is asymmetrical.

So you want to look at it on all sides and consider it.

And I come up with the fact that, yes, it's very organized in its structure,

it has this spiral shape, but the overall shape is asymmetrical.

I can't find a mirror plane here where two or more sides are the same, so

that's asymmetry, the lack of symmetry.

Another type of symmetry is called radial symmetry and

that's where, the tight definition of radial

symmetry is where you almost have an infinite number of mirror planes.

You could cut that organism in any direction and

those pieces will be the same.

So if you think of a pizza, right, if you cut that pizza down the middle,

across the side, all the pieces look the same.

You can turn it around, you can fold it in half, and

all those folds are going to be represented equally on both sides.

So a more specific form of radial symmetry is called pentameral symmetry.

So pent or penta, that refers to the number five.

And so pentameral symmetry, for

example, is a type of radial symmetry where there are five equal pieces.

And so if I'm counting the pieces on this specimen, and actually,

the back kind of gives it away, [LAUGH] you can actually count five

areas that are the exact same as the one that's next to them, or mostly.

So if you can look at a crinoid, for example,

we'll talk about those eventually, or in a echinoderm here, you might say,

hey, I notice five structures on here.

And it's not only just the markings, but

you could start to split this organism into five equal pieces and

that's pentamorally radially symmetrical organism.

So pentamoral symmetry.

So the last type of symmetry we would say, as humans,

is the most sophisticated type of symmetry is bilaterally symmetrical organisms.

And so bi is two, and so bilaterally symmetrical organisms have a plane in some

direction that you can cut it, where those halves are mirror images of each other.

So again, as I said, humans, if you cut them down the center here, kind of

sagittally down the middle, the two, right and left sides should be mirror images.

A more ancient organism is this ammonoid here, and

maybe on the top here, on one plane that you would consider,

it doesn't look symmetrical, it looks, again, this spiral shape.

But if you turn it all around and you look at it kind of head on, you can see,

actually, the right and left sides, you could take those pieces apart and

they would look the same, they would be symmetrical.

So I can identify two sides and that's bilateral symmetry.