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Simple animals: Sponges, jellies, & octopuses

Video transcript

you and I both know people or dogs that we don't consider particularly sophisticated we sometimes refer to them as simple or real housewives so when it comes to truly simple animals we shouldn't underestimate them because the animal phyla that we describe as being the least complex actually offer us a vivid way of understanding how animals are structured and also how they evolved simple doesn't always mean dumb unlike those dullards that we've all met in our lives animals aren't considered simple because they apparently take things for granted or they think that reality TV is reality their simplicity has to do with their tissue complexity as you know almost all animals cells are organized into tissues that perform specialized functions the more different kinds of specialized cells an animal has the more complexities and this complexity is determined in the embryonic phase as embryos most animals either form two layers of early tissue called germ layers or they form three by exploring the very simplest phyla from animals with no layers at all aka sponges to the most basic of three layer animals like mollusks you can see how a not totally amazing sounding change in tissue results in truly fundamental and amazing changes so the places in the animal family tree where these transitions take place from no layers to two layers and from two layers to three are some of the most important benchmarks and animal evolution let's start with the very simplest of animals in the phylum porifera the sponges they diverged from the protists probably 600 million years ago and not a whole lot has changed for them since then if you've been paying attention you've noticed by now that almost nothing that applies to other animals applies to sponges that's because they're so freakin simple again move they just hang out in filter water for food like bacteria while some hosts photosynthesizing microbes in mooch off of them more important sponge embryos don't have any layers they just have cells this means that sponges don't have specialized tissues or organ and their cells can take different forms some have flagella to force water into the sponge some are more amoeba-like and wander around distributing nutrients these cells can transform into whatever type of cell the sponge needs for this reason some scientists argue that sponges aren't even animals at all they're actually colonies of cells that depend on each other to function but for our purposes mainly because they're multicellular eukaryotic organisms that can't make their own food they still count and they've managed to diversify into nearly 10,000 different species so good for that things get more interesting with Nigeria which include jellies cien enemies corals and hydras say got a couple of sweet evolutionary breaks that made them animals that you do not want to mess with first and most important brick is that they develop to germ layers you'll remember these layers are called the endoderm or the insider min the ectoderm the outside term and they form a tube that allows an animal to ingest digest and get rid of stuff this makes Cnidaria among the oldest living descendants of the world's first type low blast which is the common ancestor of all true animals but still jellies and an enemies and other cnidarians have only one hole that serves as both mouth and anus and they don't have any organs so still pretty simple your second evolutionary break is in their ectoderm which contains stinging cells called Naito cysts think Portuguese Man O'War I want stepped on a dead one it was dead long dead and I wanted someone to cut my foot off it hurt so much so now we've got two layer animals swimming around able to move and eat and poop and defend themselves the animal kingdom is just one evolutionary breakthrough away from a huge like explosion and we could see evidence of this breakthrough in platyhelminthes the phylum of soft unsegmented worms that includes flat worms planaria tapeworms and flukes not super handsome but these guys are a big deal because they're the oldest existing phylum that is tripe low blastic or has three germ layers so in addition to an endo term and an ecto term the embryos form a mesoderm i know it's starting to sound like just another piece of toast and turkey on a club sandwich but this development changes everything bloody hell meant these themselves are pretty simple but a couple of phyla up the ranks this new layer allows animals to form true organ systems the ectoderm forming the brain and nervous system and skin the mesoderm forming muscles and bones and cartilage the heart blood and other very useful stuff and the endoderm forming the digestive and respiratory systems and this kind of complexity is only possible because one of the mesoderm skee features the coelom a fluid-filled cavity that stores and protects the major organs it allows the internal organs to move independent of the body wall and the fluid can provide some shock resistance see limbs are where all the action happens when it comes to organ systems but not all tripe low blasts have them from here on we can assess the complexity of an animal by whether it has a coelom or not and if so how completed it for instance because they're the simplest of the try blow blasts platyhelminthes have their mouths and buttholes on opposite ends of their bodies which is awesome for them but they're a sila mates they don't have a coelom which tells us that they're still on the shallow end of the pool complexity wise to give you an idea of how simple you can cut a plot II helment these in half and both of the pieces will happily continue on with their wormy business that my friends is simplicity now you probably haven't forgotten that I mentioned an explosion a minute ago well I'm not going to talk to you with talk of explosions without giving you one the Cambrian explosion not long after germ layers became a thing say 535 million years ago life on Earth was undergoing some pretty terrific and rapid innovations over about ten or twelve million years about half of the animal phyla that exist today started to appear it remains the most biologically productive period in history the most creative and vibrant and dangerous experience and then invite all of kingdom Animalia to the party like burning man and comic-con and Coachella all at once this is when animals started to look and behave as we know them today before the Cambrian most of the big animals were slow and soft bodied and ate algae or scavenged this explosion of diversity brought all kinds of new adaptations including predatory ones like claws and defensive ones like spikes and armored plates shells and mineral skeletons made their first appearances in fact the adaptations were so many and so abrupt that in the 1800's the abundance of fossils from this period was used to argue against evolution scientists offer a lot of different theories about what caused this explosion it was probably a combination of a few of these things for one oxygen levels became very high in Cambrian seas which allowed for larger bodies and higher metabolisms also thought that ocean chemistry changed with more minerals becoming available for the production of shells and skeletons and of course with more diversity comes more competition and predation which drove selective pressures on animals to become either better at hunting or better at defending themselves it's pretty near at the top of my list of places I want to go once I put the finishing touches on my time machine but for now we still have many modern animal phyla to remind us of this time of crazy awesomeness if looks are cool and all but things start to get more complex with another phylum of mostly nasty parasites nematoda unsegmented roundworms these guys are pseudo Silla mates meaning that they have an incomplete body cavity unlike a true SI labate whose body cavity is contained within the message m su de seal awaits sort of improvised one between the mesoderm in the endoderm the vast majority of nematodes live in soil where they eat bacteria or fungus or parasitize plant roots but humans hosts at least 50 nematode species including hook worms which burrow into our intestines and treat us like some kind of food court but most nematodes are very very small a single teaspoon of forest soil can have several hundred in it rotifer meanwhile our tiny filter feeding animals that live mostly in fresh or saltwater though some of them can live in damp soil they're also pseudo seal mates like nematodes and although they are way smaller than most flat worms a big honkin rotifer is like two millimeters long they're anatomically more complex as they have a stomach jaws and a tiny little anus my favorite fun fact about rotifer ax is that many of its species are known to exist entirely of females and they reproduce through unfertilized eggs fossils of rotifers have been found as old as 35 million years and in many cases there's not a dude to be found you go girls okay so now for some of the big dogs the phylum Mollusca mollusks might be kind of simple but they're amazing and some of them are incredibly smart they take four different basic forms the chitin x' the snails bivalves and octopi and squid now I realize it can be hard to see how an oyster and an octopus might be related but mollusks have some important similarities one they all have a visceral mass which is a true coelom a body cavity completely within the mesoderm that contains most of the internal organs too they also have a big muscular foot which takes different forms in each class of maules 3 they have a mantle which in some mollusks makes a shell and in others just covers the visceral mass and for finally all mollusks except bivalves have a radula or a rasping organ on their mouth that allows them to scrape up food soaked itens are these headless mean animals covered with a plated shell on one side and they used their foot to move around on rocks scraping off algae with their rajala you know about bivalves they have shells that are divided into two hinged halves like clams and scallops they're filter feeders so they trap particles of food and the mucus that covers their gills snails and slugs are the gastropods one thing that sets them apart is process called torsion in which the visceral mass twists to the side during embryonic development so that by the end of it its anus is basically right above its head most gastropods also have a single spiraled shell and most use their rajala to graze on algae in plants and last but certainly not least we have the cephalopods which are the kings of the mollusks as far as I'm concerned cephalopods include octopi and squid and they are obviously a lot different from other mollusks for starters they have tentacles that they used to grab their prey which they then bite with their beaks and immobilize with poisonous saliva and the foot of a cephalopod has been modified into a really powerful muscle that shoots out water to help it move into steer through the water but probably the coolest thing about cephalopods is how smart they are while a typical mollusk might have 20,000 neurons an octopus has half a billion if you just do a youtube search for octopus you'll find all kinds of videos of them opening jars and stealing people's video cameras that like freakin ocean ninjas cephalopods got skills so remember simple doesn't equal dumb there is a lot to learn from our less developed cousins next time we'll talk about even more complex animals and what we have to learn from them