Complex animals: Annelids & arthropods발음듣기
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Complex animals: Annelids & arthropods
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I got to say I got a lot of respect for these guys because they are some of the most successful organisms on the earth.발음듣기
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On one acre of cultivated land, annelids like this earthworm process about 16 thousand pounds of soil a year, which makes plant life and our lives therefore possible.발음듣기
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Not only are there about a billion billion arthropods like this in the world, that's 10 to the 18th power, by the way, but also 80 percent of known animal species are arthropods.발음듣기
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Don't get me wrong, even though bugs and spiders and worms and shrimp totally outnumber us humans, we are far more complex than them when it comes to arm wrestling, and guitar playing, and long division, we totally school them.발음듣기
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Can you see the resemblance? (lively music) Last time we talked about how, in the very simplest animals, there are two traits that indicate an animal's relative complexity.발음듣기
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There's how many germ layers they develop when they're embryos, and whether or not they have a [selum] or a body cavity that holds their organs.발음듣기
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It's in the next two phyla, annelida, and arthropoda where we find the new big thing in animal complexity, segmentation.발음듣기
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Segmentation is the repetition of anatomically identical units that can be added to and modified to serve different purposes as animals evolve.발음듣기
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In fact, the three biggest and most diverse groups of animals in the world are the ones that display segmentation, annelida, arthropoda, and chordata, which includes the vertebrates.발음듣기
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All segmented animals have a common ancestor that probably lived about 600 million years ago.발음듣기
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That's how long ago it was when one of your grandparents, and one of the earthworm's grandparents, and one of the beetle's grandparents all played on the same softball team; pretty crazy.발음듣기
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In humans, you see anatomically identical pieces repeated along an axis, from our butts to our heads.발음듣기
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Among today's animal phyla, the earliest to display segmentation is annelida, which includes leeches, and earthworms, and lugworms.발음듣기
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Synapomorphies are traits that set one group of animals apart from its ancestors and from other groups that came from the same ancestors.발음듣기
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They've also got little bristles on their bodies called chaetae that provide traction and help them move through the dirt.발음듣기
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These are both little extra somethings that annelids have that less complex relatives don't have and other common ancestors didn't have.발음듣기
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Synapomorphies, literally shared derived traits, are usually the defining traits of a phylum.발음듣기
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You can also learn something about an animal's lineage by comparing plesiomorphies, very basic traits that are shared by animals with a common ancestry.발음듣기
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Between the platyhelminthes, and the nematodes, and the annelids, one pleisomorphy is that they're all worm-shaped, which tells you that they have a common distant ancestor that was wormy-looking.발음듣기
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As we talk about these phyla and the classes within them, notice how they are similar and different from each other.발음듣기
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They're everyone's favorite because they eat soil and crap it out the other end allowing air and water to circulate in soil.발음듣기
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Plus, their poo is rich in things that plants need to grow like nitrogen, and calcium, and magnesium, and phosphorous.발음듣기
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Now on to the class hirudinea, the leeches, a lot of which are parasitic, and eat blood and other bodily fluids.발음듣기
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They've got them on both ends of their bodies; the posterior one being used to anchor itself, while the anterior one that surrounds its mouth attaches to its host. All leech species are carnivorous.발음듣기
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The polychaetes are bristly worms, hence a synapomorphy of this class is their poly, many, chaete, bristles.발음듣기
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The ones you've probably seen evidence of are lugworms, the ones that dig holes at the beach and leave piles of castings on the sand.발음듣기
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Just to put things into perspective, there are more insects in a square mile of rural land than there are human beings on the earth.발음듣기
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One reason the scientists think arthropods do so well has to do with their, you guessed it, segmented bodies!발음듣기
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Fossils of early arthropods showed that there used to be very little variation between the segments.발음듣기
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As they evolved, segments fused and became specialized for different functions, which led to crazy amounts of diversity.발음듣기
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So much diversity that arthropoda include stuff like scorpions, butterflies, and lobsters, which seems like maybe a bit of a stretch.발음듣기
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One, they have segmented bodies that in most cases are broken up into three segments, head, thorax, and abdomen.발음듣기
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It's a basic ancient trait that they share witth annelids, and us chordates for that matter, dating back to that softball game our fore bearers played some 600 million years ago.발음듣기
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Two, all arthropods have an exoskeleton, a hard outer shell made out of chitin, which is a really tough carbohydrate that's chemically similar to the cellulose that you find in plants.발음듣기
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Be glad that you don't have to do that because it looks like a traumatic experience for them.발음듣기
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Finally, they've all got paired and jointed appendages, which is actually where their name comes from. Arthropod means jointed feet.발음듣기
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That's what arthropods have in common, but they are grouped into four sub-phyla based on how they differ from each other.발음듣기
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First, and perhaps most terrifying, you have your cheliceriformes, which include spiders, and scorpions, but also horseshoe crabs, which are nice, and ticks, which aren't, and mites, which don't bother you at all, probably. They might.발음듣기
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Cheliceriformes comes from the Greek for arm lips, which, whatever Greeks referred to their long fang-like pincers.발음듣기
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Unlike a lot of arthropods, they have simple eyes with just one lens, and not compound eyes like flies, and they lack antennae. Most cheliceriformes are land lovers.발음듣기
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The largest class of cheliceriformes are the arachnids, the group that includes spiders, and scorpions, and ticks and mites.발음듣기
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They have what's called a cephalothorax, which is a head segment and a thorax segment fused together with eight legs and an abdomen behind.발음듣기
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They're very skittery, just sayin'. Next, myriapoda, or many feet, includes what you would expect, the millipedes, and centipedes.발음듣기
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All myriapods are terrestrial and have antennae, and scary jaw-like mandibles. Millipedes are vegetarians.발음듣기
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They may have been some of the very first animals to live on land, where there were mosses and primitive vascular plants for them to munch.발음듣기
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Although millipedes have a crap-ton of legs, they don't have as many as a thousand as their name implies.발음듣기
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Centipedes, whose name is a little more apt as they generally have between 20 and 350 legs are carnivores, and have poisoned claws to paralyze their prey.발음듣기
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If you're looking to cuddle with a myriapod, I"m going to advise you to go with a millipede.발음듣기
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The synapomorphies they share are three-part bodies consisting of a head, a thorax, and an abdomen, three pairs of jointed legs that come off the thorax, compound eyes, and two antennae.발음듣기
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Think of any random way you could put these characteristics together, and you'd probably come up with something that exists.발음듣기
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There are more species of insects than there are all other animal species combined, which is why I'm taking this opportunity to do a biolography, the insects edition.발음듣기
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Scientists think that they probably split off from their crustacean cousins about 410 million years ago.발음듣기
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For tens of millions of years, insects and some little skittery invertebrates were about the only land-dwelling animals.발음듣기
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About 320 million years ago, thanks to the high oxygen levels of the carboniferous period, some insects grew to be terrifyingly big like the Meganeura, which looked like a dragonfly with a two-foot wing span.발음듣기
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Since an insect's size is restrained by its respiratory system, as oxygen levels went down, these massive insects couldn't circulate enough oxygen to keep their gigantic bodies going and they died off.발음듣기
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The next major milestone of insect history occurred around 120 million years ago, which is when most flowering plants evolved, and with it, the sweet spirit of cooperation that insects and flowering plants still share to this day.발음듣기
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In fact, some insects and flowering plants have co-evolved really neat pollination strategies so that they basically evolved together, which I think is really sweet.발음듣기
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As a result of insect pollination, flowering plants are now the overlords of the plant world.발음듣기
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Insects are the only arthropods that have developed the ability to fly. It has served them well.발음듣기
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Unlike birds and bats, which have to sacrifice walking legs in order to evolve wings, insects are just as graceful on the ground as they are in the air.발음듣기
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In partial metamorphosis, the young, called nymphs, look exactly like the adult of the species, and undergo a series of molts, which allows them to get bigger, and bigger.발음듣기
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Most of the types of arthropods do this and some insects including grasshoppers and cockroaches.발음듣기
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However, complete metamorphosis is a process unique to some insects that lets them completely change shape.발음듣기
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This is a rhinoceros beetle larva, pretty gross, hangs out and eats until it's time to build a little case around itself called a pupa.발음듣기
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They have 19 pairs of appendages, some of which are claws, some are mouth parts, some are regular old walking legs.발음듣기
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Some shrimp have evolved bio luminescence, which is pretty much the most amazing thing any animal can do as far as I can tell.발음듣기
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