Mollusks are kind of the star of the show for the lophotrochozoans. And that's because they come in a wide variety of classes that have a bunch of different morphological features that are quite diverse. Now, mollusks, as a whole, are coelomates, and their bodies tend to have these three characteristic pieces, what we call the foot, which is basically a large muscle. Usually, it's going to be used for movement. We'll see a pretty interesting exception with creatures like squid. So anyway, this bottom piece right here in our example, that is going to be the foot.
We also have the visceral mass, which basically includes the internal organs of the organism and its external gills. And that's all this mess here. This is our visceral mass. I'm just going to write vm for visceral mass. And the cavity inside of this is called the hemocoel, and it's named for the fluid inside of it called hemolymph, which is similar to blood, basically. So, the hemocoel, that's the cavity, and it's filled with that fluid that is going to act similarly to blood, and that fluid is actually going to directly bathe the organs because these organisms mostly, there's going to be an exception we'll talk about later, have an open circulatory system. So, the fluids that are going to carry gases to the organs are not going to move through veins or arteries or anything like that. It's just kind of floating around in the hemocoel.
The feeding structure, briefly, I want to mention, as part of the visceral mass, is called the radula. It's at the anterior end, what you could, I guess, kind of think of, like the head. And it's a little hard to see in this image, but the radula is right there. And basically, this feeding structure, it functions kind of like a rasp. So, you know, think about an abrasive edge or abrasive surface kind of rubbing on the food to break it down, like, in a grinding type way. Now, not all mollusks have a radula, but many of them do, so that's why I mention it here as sort of a general feature of mollusks.
The last structure that's common to mollusks is the mantle, and this is basically just a covering for the visceral mass. So, you can see that it's actually this layer here. This layer just under that outer shell in the picture, that is a mantle. And it often forms a hardened calcium carbonate shell, which we can see right here, this is the shell. But this is not always the case, so again, we'll see some exceptions. So, it's worth noting that in some species there's something called a mantle cavity. And it's basically a water-filled chamber containing the gills and the anus. And you can see that little mantle cavity right there.
Now, I'm going to talk about a bunch of different classes of mollusks, and the first one, not a ton to say about. These guys are called chitons. They totally look like some type of prehistoric organism. And what defines them is that they have a shell, which you can see on the top right here, this is the shell. And the shell is made up of 8 dorsal plates. You can see there's like 8 segments to this shell. You can also kind of see the foot poking out from underneath this chiton.
Now, gastropods, probably an organism you're much more familiar with, these are slugs and snails. They're going to live both in marine and terrestrial environments. And while many of them have shells, not all do. Of course, our snail, pictured right here, has a shell, but, you know, there's plenty of these organisms, these gastropods, that don't have shells. Now, many of these species actually reproduce through sexual reproduction. If you ever want to see a wild YouTube video, look up something like slug reproduction. Pretty crazy acrobatic act. Some species, however, perform parthenogenesis, which is that type of asexual reproduction we discussed previously. With that, let's turn the page.