Ecdysozoans: Videos & Practice Problems

Ecdysozoans, including arthropods, myriapods, crustaceans, and hexapods, are characterized by a cuticle that they periodically shed to grow. Arthropods possess segmented bodies, exoskeletons made of chitin, and jointed appendages. Myriapods, like millipedes and centipedes, have many legs, while crustaceans, such as crabs and lobsters, have a cephalothorax and abdomen. Hexapods, or insects, are the most diverse group, featuring three pairs of legs and compound eyes. They undergo complete or incomplete metamorphosis, showcasing their adaptability and evolutionary success.

Arthropods and Chelicerata

Hi. In this video, we'll be talking about ecdysozoans and mainly focusing on arthropods. Now, ecdysozoans are protostomes that grow incrementally by shedding their outer layer. This outer layer is known as the cuticle, and it's a tough external coating. In some cases, it's actually hardened into what we call an exoskeleton. Every time the animal sheds its cuticle, its body swells, allowing it to grow. If you've ever had a soft shell crab, for example, you're actually eating a crab that has recently molted and is in the process of regrowing its cuticle. That's why it has a "soft shell." By and large, the biggest group of ecdysozoans is the phylum of animals known as arthropods. These organisms have segmented bodies—more on that in a second—and they have exoskeletons. They also have jointed appendages. Now, their exoskeletons are mostly made of chitin. And in the case of crustaceans, which are organisms like shrimp or lobsters, those exoskeletons are actually reinforced with calcium carbonate. Now, their bodies are grouped into what are called tagmata: segments like the head, thorax, and abdomen of insects. We can actually see an example of tagmata right here with this organism. Technically, this is a trilobite. These guys, well, they've been gone for a little while. But it nicely illustrates the three tagmata of the trilobite body in these different colors. So, you can see we have the cephalon, thorax, and pygidium. Don't worry about these names. Just know that these are all tagmata.

It should be noted that changes in Hox genes, which are those genes that control the development of the various segments in the animal body, allow for a huge variety of form and function, which has led to the massive success of arthropods. Arthropods are just everywhere. There's tons of them on the planet, many different types. And part of what allowed them to spread so effectively is that great diversity in form and function. Think about it. Some of these Hox genes have allowed for things like pincers, wings, and antennae, many different sensory organs. There's a lot of variety in the body plans of arthropods, thanks to these variations in the Hox genes. Now, the main body cavity of arthropods is called the hemocoel and is due to the fluid it's filled with called hemolymph, which is basically just like their version of blood. And, arthropods have an open circulatory system, so that fluid is basically going to directly bathe the organs in order to do gas exchange. And you can see a simple body plan of an arthropod right here. And you might be surprised to see that they actually have brains as well as hearts. But yes, things like insects and organisms like that absolutely have brains and hearts. And here you can see an insect in the process of molting.

Now, let's talk about one of our first groups of arthropods that are known as the Chelicerata. These are organisms that have claw-like feeding appendages. They actually lack antennae and have simple eyes, as opposed to something we're going to talk about momentarily called compound eyes. Basically, its eyes, like you can see right here in this kind of cute little spider. I mean, not that spiders are cuddly or cute. But, notice how its eye just has this one single lens. It's like a single dome. Later we'll look at compound eyes, which have many different lenses, or rather, many lenses on one eye. So, Chelicerata includes organisms like horseshoe crabs, sea spiders, scorpions, ticks, mites, all these lovely things, as well as spiders. And we're going to talk about arachnids right now. So, organisms like spiders. And they're also the largest group of Chelicerata. They actually have two tagmata, and we'll look at that right here on this nice little diagram of the inside of an arachnid. There's the cephalothorax, which is this region right here. And then this back region called the abdomen. Now, arachnids have these special appendages called pedipalps, which you can see right here. They're actually going to use these to reproduce. And believe it or not, these guys reproduce through internal fertilization. We'll see that a lot of other arthropods reproduce through external fertilization. These guys happen to use internal fertilization. They also have the characteristic chelicerae, those claw-like feeding appendages, you can see right here, and also here on our little spider friend. So that is the chelicera. Arachnids, one of the defining features of arachnids, I'm sure you're familiar with, is their four pairs of walking legs, or 8 legs, basically. Here you can see four legs, well, there are four on this side and, you know, ostensibly four behind it on the other side for a total of 8 legs. So, with that, let's turn the page.

Crustaceans and Hexapods

Hello everyone. In this lesson, we are going to continue our discussion of ecdysozoans, specifically talking about myriapods, crustaceans, and hexapods. So the first group of animals here that we're going to talk about are the myriapods, and they are invertebrate organisms, representing what you think of when you think of millipedes and centipedes. Those little bug-like structures that seem to have a million legs, those are myriapods. They have segmented bodies with legs emerging from each segment. 'Myriapod' is an interesting name for them, making a lot of sense when you break it down. 'Myriad' in English implies a huge quantity of something, hence 'many'. And if you recall, 'pod' means legs or foot, translating to 'many legs'. Indeed, myriapods do have many legs; the fewest number of legs they can have is 10, while the highest number of legs recorded is 750. There are approximately 16,000 species of millipedes and centipedes within this category.

Next, let's discuss crustaceans. These animals tend to be more familiar to most. They inhabit both aquatic and terrestrial environments, existing in fresh and saltwater. Common edible examples include lobsters, crabs, crayfish, shrimp, and barnacles, all primarily aquatic. However, one terrestrial crustacean to note is the wood louse, also known as a roly-poly, which can curl into a ball for protection. Fascinatingly, all crustaceans are equipped with 19 appendages and two pairs of antennae. They possess two tagmata: a cephalothorax (a fusion of the head and thorax) and an abdomen. Their head houses sensory structures and the brain, while the abdomen contains the digestive organs. The thorax supports respiratory and circulatory organs. Most crustaceans need to molt their exoskeleton, including the carapace, to grow, which makes them vulnerable but is necessary for their development. There are around 67,000 species of crustaceans ranging greatly in size from as small as 0.1 millimeters to the Japanese spider crab, which can span up to 3.8 meters. Small species breathe through their skin, while larger ones, like the Japanese spider crab, use gills.

Moving on to hexapods, often referred to as insects, they are arguably the most successful group of animals on Earth. With over a million known species inhabiting diverse environments, their adaptability is clear. They are mostly terrestrial but some are aquatic. Hexapods are distinguished by their three pairs of walking legs attached to their thorax, and many have wings. They are segmented into three tagmata: a separate head, thorax, and abdomen (no cephalothorax). Famous for their compound eyes, which are excellent for detecting movement, hexapods also have an open circulatory system with spiracles that facilitate gas exchange. Most reproduce sexually with internal fertilization.

Finally, a key characteristic of hexapods is their ability to undergo metamorphosis. Complete metamorphosis, seen in butterflies and bees, involves a striking transformation from larva to adult, each stage differing significantly and serving unique life functions: larvae prioritize growth, while adults focus on reproduction. Conversely, in incomplete metamorphosis, exemplified by dragonflies and grasshoppers, juveniles (nymphs) resemble adults but are non-reproductive, undergoing less dramatic changes.

Let’s now proceed to our next topic.

Nematodes

The last dicotomous organisms we're going to talk about are nematodes, and these are not arthropods, but they are dicotomous. So, they still have a body covered in a cuticle that they shed periodically to grow. It's kind of hard to see here in this image, but they are such small creatures that it’s hard to visualize. We looked at annelids before, and those were segmented worms. These are unsegmented worms. They are sometimes casually referred to as roundworms. They have a pseudocoelom and they actually exhibit sexual dimorphism, believe it or not. This means that the males and females of the species have morphological differences; they appear different. Now, you'd have to be an expert to tell by looking at them, though. Obviously, these are subtle differences that our untrained eyes are not going to readily pick up on. However, I do want to briefly mention the picture that we have here: this guy is C. elegans, and you probably, if you haven't already seen that name in biology, you'll definitely see it come up later. This is one of the most studied organisms in laboratory experiments. It's actually so well studied that scientists basically know how every single cell in the C. elegans body develops. It's a really well-studied organism. So, if you see that name come up in the future, now you know. It's this little nematode. Alright, with that, let's end the video here. I'll see you guys next time.

Here’s what students ask on this topic:

What are the main characteristics of arthropods?

Arthropods are characterized by their segmented bodies, exoskeletons made of chitin, and jointed appendages. Their bodies are divided into segments called tagmata, such as the head, thorax, and abdomen in insects. The exoskeleton provides protection and support but must be shed periodically for growth, a process known as molting. Arthropods also have an open circulatory system, where hemolymph (a fluid similar to blood) bathes the organs directly. Additionally, they possess a variety of specialized appendages, including antennae, pincers, and wings, which contribute to their adaptability and success in diverse environments.

How do hexapods differ from other arthropods?

Hexapods, commonly known as insects, differ from other arthropods in several ways. They have three pairs of walking legs, always located on the thorax, and many species possess wings. Hexapods have three distinct body segments: the head, thorax, and abdomen, unlike crustaceans, which have a fused cephalothorax. They also feature compound eyes, which consist of multiple lenses, providing excellent motion detection. Hexapods utilize spiracles and tracheal tubes for gas exchange, as they lack lungs. Additionally, they undergo metamorphosis, either complete or incomplete, which significantly changes their morphology and life stages.

What is the significance of Hox genes in arthropods?

Hox genes play a crucial role in the development and diversification of arthropods. These genes control the formation and differentiation of body segments, leading to the vast variety of forms and functions seen in arthropods. Variations in Hox gene expression can result in different appendages, such as antennae, pincers, and wings, allowing arthropods to adapt to various ecological niches. This genetic flexibility has contributed to the evolutionary success and widespread distribution of arthropods, making them one of the most diverse and abundant groups of animals on the planet.

What are the differences between complete and incomplete metamorphosis in insects?

Complete metamorphosis involves four distinct life stages: egg, larva, pupa, and adult. The larval stage looks entirely different from the adult and focuses on growth and feeding. After the larva becomes a pupa, it undergoes significant transformation to emerge as an adult, whose primary role is reproduction. Examples include butterflies and beetles. Incomplete metamorphosis, on the other hand, consists of three stages: egg, nymph, and adult. Nymphs resemble smaller versions of the adult but cannot reproduce. They gradually develop into adults through a series of molts without a pupal stage. Examples include grasshoppers and dragonflies.

What are nematodes and how do they differ from arthropods?

Nematodes, also known as roundworms, are unsegmented worms covered by a cuticle that they shed periodically for growth. Unlike arthropods, nematodes lack segmented bodies and jointed appendages. They possess a pseudocoelom, a fluid-filled body cavity, and exhibit sexual dimorphism, meaning males and females have different morphological features. Nematodes are ecologically diverse, inhabiting various environments, and some species, like Caenorhabditis elegans, are extensively studied in scientific research due to their simple body plan and well-mapped cellular development.