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Biology library
Course: Biology library > Unit 36
Lesson 1: Crash Course: Biology- Why carbon is everywhere
- Water - Liquid awesome
- Biological molecules - You are what you eat
- Eukaryopolis - The city of animal cells
- In da club - Membranes & transport
- Plant cells
- ATP & respiration
- Photosynthesis
- Heredity
- DNA, hot pockets, & the longest word ever
- Mitosis: Splitting up is complicated
- Meiosis: Where the sex starts
- Natural Selection
- Speciation: Of ligers & men
- Animal development: We're just tubes
- Evolutionary development: Chicken teeth
- Population genetics: When Darwin met Mendel
- Taxonomy: Life's filing system
- Evolution: It's a Thing
- Comparative anatomy: What makes us animals
- Simple animals: Sponges, jellies, & octopuses
- Complex animals: Annelids & arthropods
- Chordates
- Animal behavior
- The nervous system
- Circulatory & respiratory systems
- The digestive system
- The excretory system: From your heart to the toilet
- The skeletal system: It's ALIVE!
- Big Guns: The Muscular System
- Your immune system: Natural born killer
- Great glands - Your endocrine system
- The reproductive system: How gonads go
- Old & Odd: Archaea, Bacteria & Protists
- The sex lives of nonvascular plants
- Vascular plants = Winning!
- The plants & the bees: Plant reproduction
- Fungi: Death Becomes Them
- Ecology - Rules for living on earth
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Simple animals: Sponges, jellies, & octopuses
Hank introduces us to the "simplest" of the animals, complexity-wise: beginning with sponges (whose very inclusion in the list as "animals" has been called into question because they are so simple) and finishing with the most complex molluscs, octopuses and squid. We differentiate them by the number of tissue layers they have, and by the complexity of those layers. Created by EcoGeek.
Want to join the conversation?
- Is it true that octopuses have 8 brains - each one controls one tentacle? :))(18 votes)
- Octopuses have three hearts. Two branchial hearts pump blood through each of the two gills, while the third is a systemic heart that pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. :D(8 votes)
- What would a "germ layer" be? He doesn't say what it is.(8 votes)
- Athe says that they are layers that together form a tube that allow an animal to "ingest, digest, and get rid of stuff." 3:05
The individual germ layers are formed during embryogenesis and each one develops into a specific structure. This Wikipedia site goes into further detail about this stuff: https://en.wikipedia.org/wiki/Germ_layer(14 votes)
- athank says that there are 4 groups of Molluscs, but mentions 5 8:50
chitons, snails, bivalves, octopi and squids.so are there 4 or 5 groups of molluscs?(9 votes)- Molluscs are much more varied than just 5 groups. Hank was referring to basic body plans, of which octopi and squids share a body plan.
But in terms of actual taxonomy or clades, there are far more than 5 groups of molluscs.(9 votes)
- I heard that there is an "immortal" jellyfish species (yah literally immortal), is this actually true or false? Seems interesting...(6 votes)
- Believe it or not, it's true! The immortal jellyfish, a.k.a. turritopsis dohrnii, can go back from the adult stage to an immature state, which I find amazing. However, it isn't truly immortal. It can still die from getting eaten and getting diseases. Still, pretty amazing.(13 votes)
- This boggled me when I was a kid and it still kinda boggles me even today, since sponges doesn't have a brain what part of their body commands them to breath?(2 votes)
- they just have pores called ostia and oscula...the water just diffuses in/out ...(12 votes)
- at, What is a diplopod? 3:09(3 votes)
- A diplopod is a millipede that has two pairs of legs on each body segment.(5 votes)
- at, does the number of neurons you have determine your intelligence? 10:40(2 votes)
- Not necessarily larger animals tend to have a larger brain mass but that doesn't make them more intelligent. Intelligence is best analyzed by knowing the animal's encephalization quotient, which is a ratio between the actual brain mass of an animal and the predicted brain mass of an animal based on its size.(6 votes)
- Are cephalopods immune to other cephalopods poison too?(4 votes)
- Cephalopods are immune to their own poison (just like venomous snakes are).
However, I doubt they are immuen to other animal's poison.
https://www.theguardian.com/science/2017/aug/09/why-do-cephalopods-produce-ink-and-what-on-earth-is-it-anyway
https://www.sciencedaily.com/releases/2009/04/090415102215.htm(1 vote)
- What is the max number of layers of germ cells an animal can have?(4 votes)
- There are max. Of 3 germ layers. In the diploblastic organisms they are only 2-the ectoderm and the endoderm. But in the triploblastic they are 3-ectoderm, endoderm and the mesoderm(1 vote)
- AtHank says that plathelminthes' digestive cavities have two openings, but other soruces say it's only one. Which one is right? 5:12(3 votes)
- They only have one opening. ( for the ingestion and the excretion of food and other substances)(1 vote)
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. But 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. (upbeat music) Unlike those dullards that
we've all meet 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 animal cells are organized into tissues that perform specialized functions. The more different kinds
of specialized cells an animal has, the more complex it is. And this complexity is determined
in the embryonic phase. As embryos most animals
ither 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 bench marks in animal evolution. Let's start with the very simplest of animals, and 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 their so frickin' simple, they can't move, they just
hang out and filter water for food like bacteria, while some host photosynthesizing microbes and 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 organs, 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, but 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 their multi-cellular 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 them. Things get more interesting with cnidaria, which include jellies, sea
anemones, corals, and hydras. They got a couple of
sweet evolutionary breaks that made them animals you
do not want to mess with. First and most important break is that they developed two germ layers. You'll remember these layers are called the endoderm, or the inside derm, and the ectoderm, or the outside derm, 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 worlds first diploblast, which is the common ancestor
of all true animals, but still jellies, and
anemones, 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. Their second evolutionary
break is in there ectoderm which contains stinging
cells called cnidocysts. Think Portuguese man o' war. I once 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. (explosion booms) And we can see evidence
in platyhelminthes. The phylum of soft unsegmented worms that includes flatworms,
planaria, tapeworms, and flukes. Not super handsome, but
these guys are a big deal because they're the oldest existing phylum that is triploblastic,
or has three germ layers. So in addition to an
endoderm and an ectoderm. 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. Platyhelminthes 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 mesoderms
key 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. Coeloms are where all the action happens when it comes to organ systems, but not all triploblasts have them. From here on we can assess the complexity of an animal by whether
it has a coelum or not, and if so how complete it is. For instance because they're the simplest of the triploblasts, platyhelminthes have their
mouths and butt holes on opposite ends of their bodies,
which is awesome for them. But, they're acoelomates,
they don't have the ceolum 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 platyhelminthes 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 taunt you with talk of explosions
without giving you one. (lively piano music) 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 10 or 12 million years about half of the animal
phyla that exist today started to appear. It remains the most biologically productive period in history. Think of 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, but 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 1800s 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 metabolism. It's 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 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. So flukes are cool and all,
but things start to get more complex with another phylum of mostly nasty parasites, nematoda,
unsegmented round worms. These guys are pseudocoelomates, meaning that they have an
incomplete body cavity. Unlike a true coelomate who's body cavity is contained within the mesoderm, pseudocoelomates sorta improvise one between the mesoderm and the endoderm. The vast majority of
nematodes live in soil where they eat bacteria, or fungus, or parasitized plant roots, but humans host at least
fifty nematode species including hookworms, which
burrow into our intestines and treat us like some kind of food court. While most nematodes are very very small a single teaspoon of forest soil can have several hundred in it. Rotifera 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 pseudocoelomates
like nematodes and although they are way
smaller than most flatworms, the big honkin' rotifera 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 rotifera is that many of it's species are known to exist entirely of females, and they reproduce
through unfertilized eggs. Fossils of rotiferas 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 chitons, 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 coelum, a body cavity completely
within the mesoderm that contains most of the internal organs. Two, they also have a big muscular foot which takes different forms
in each class of mollusk. Three, they have a mantle, which in some mollusks makes a shell and in others just
covers the visceral mass. And four, finally all
mollusks except bivalves, have a radula, or a rasping
organ on their mouth that allows them to scrape up food. So chitons are these
headless marine animals covered with a plated shell on one side and they use their foot
to move around on rocks, scraping off algae with their radula. You know about bivalves, they have shells that are
divided into hinged halves like clams and scallops. They're filter feeders so
they trap particles of food in the mucus that covers their gills. Snails and slugs are the gastropods. One thing that sets
them apart is a process called torsion, in which the visceral mass twists to the side during
embryonic development so that by the end of it, it's anus is basically
right above it's head. Most gastropods also have
a single spiraled shell, and most use their radula to
graze on algae and plants. And last, be 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 use 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 and
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
peoples video cameras. They're 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.