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.