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The summer before I was going to take biology
in high school, I thought most of biology
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would be about animals.
Or, at least, a pretty big portion of it.
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Imagine my surprise when biology turned out
to be a lot more than animals, and we actually
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didn’t get to animals until
close to the end of the year.
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I’ve found that to be pretty typical.
In fact, even continuing into college when
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I majored in biology, I really didn’t have
much exposure to learning about animals
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specifically – although to
be fair, there are just so
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many courses to select from after you finish
general bio courses and I’m naturally drawn
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to cells so those always took over my scheduling.
Grad school: yes, I had a class on animals.
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It was a zoology course.
And it was AMAZING.
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I find that most people start out pretty
fascinated with animals from a young age
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and that really
never goes away.
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I’m grateful for all the content I’ve
learned in biology: cells, cell division,
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cell processes, genetics, mutations, evolution,
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ecology – all of that – because it gives
me a greater understanding of animals.
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Which is the focus of this video.
So, what is an animal?
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Seems simple enough - you might think of this
or this but you might not think about this
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animal here: an ant.
Or this sponge.
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But they’re both animals.
Generally, animals have
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some shared characteristics: they’re
multicellular and made of eukaryotic
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cells.
They have specialized cells that do
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certain functions and most animals – but not all
- have them arranged in specialized tissues.
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Animals are heterotrophs which means they
don’t make their own food like autotrophs;
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instead, they must consume it.
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And unlike fungi – which are also heterotrophs
by the way- animals generally ingest organic
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matter in some form whereas fungi
externally digest and absorb their food.
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Most animals are motile – meaning they can
move- at least at some point during their
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life cycle.
So how do these animals fit in?
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We’ll explore some major characteristics
of 9 different animal phyla.
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If we’re going to do that, let’s talk
about some vocab that will involve animal
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characteristics.
Symmetry – when talking about animal structure.
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If the symmetry is like slices of a pie – or
to say it fancier, if you can have more than
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two planes dividing similar portions
– well that’s radial symmetry.
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Some animals have that; it can be useful
if you’re sessile- meaning you don’t move-
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because you can respond to your
environment from many directions.
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Or bilateral symmetry - that means if there
was a line going down the middle, the right
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and the left halves would be very similar.
Humans, for example,
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are animals with bilateral symmetry.
Some advantages with this: well, moving forward
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is easier than it is for an animal that has
radial symmetry.
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Ok, focusing on animals with bilateral symmetry
- cephalization –it means the nervous system
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tissue is concentrated in a head region.
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Some advantages here because the mouth and
then a lot of the sensing organs can be all
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there together in the head
region, which is useful.
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And---this is not an animal development video
so it’s hard to talk about these words without
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going into the fascinating study of animal
development but we’ll do our best: animals
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that are considered triploblastic, which we’ll
get to in just a minute – are often categorized
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as protostomes or deuterostomes.
There are multiple characteristics in development
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that each of these have – see our further
reading- we’re just focusing on one characteristic
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that is often discussed but please know that
exceptions exist.
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Protostomes generally have their first embryonic
opening - called a blastopore – that develops
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into the mouth.
Then a second embryonic opening forms the anus.
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BUT if the second embryonic opening instead
becomes the mouth ---and the 1st opening,
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that blastopore, becomes the anus
---generally this is for deuterostomes.
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So, using this general definition, if the
1st opening blastopore develops into a mouth:
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protostome.
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If the 1st opening blastopore
develops into the anus – deuterostome.
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I’m really into alphabetical mnemonics
so “m” (mouth) and “p” (protostome) are
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close together.
And “a” and “d” are close together.
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So we mentioned that protostomes and deuterostomes
are triploblastic animals and said that we'd
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get to that - what does triploblastic mean?
Triploblastic animals have 3 germs layers.
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Germ layers are layers that eventually develop
into different structures of the animal.
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There’s an outside germ layer called the
ectoderm, an inner germ layer called the endoderm,
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and a mesoderm – the middle germ layer.
Some animals don't have that mesoderm layer
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and are considered diploblastic – they’re
neither protostomes nor deuterostomes.
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Many but not all triploblastic animals
can have something called a coelom.
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A true coelom is a body cavity derived from
the mesoderm that tends to be filled with
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fluid and acts as a designated space for
the animal’s organs – it can provide shock
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absorption, cushioning, and
space for organ development.
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We can classify animals depending on whether
or not they have a true coelom, and for each
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phylum, we’ll mention whether the
animals in it have a coelom or not.
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So now finishing some important vocab that’s
used in classifying animals, let’s get started
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into a tour of 9 major animal phyla – please
remember our tour is general and exceptions
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can and do exist.
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Phylum Porifera: the sponges.
They’re aquatic; most are saltwater and adults
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are generally sessile which means they
don’t move.
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They have a porous body – they’re sponges
after all - and oxygen and food in the water
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passes through these pores.
They have special cells that
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do intracellular digestion –
that means, digestion inside
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the cells.
They don’t have a gut so no gut
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opening; they don’t have organs or true tissue.
Most sponges do not have symmetry although some
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exceptions can have radial symmetry.
No cephalization and no coelom.
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Phlyum Cnidaria: that includes
jellies, sea anemones, and hydras!
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They’re aquatic and can be
saltwater or freshwater.
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They do have one gut opening acting as both the
mouth and anus, and they can have intracellular
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digestion with certain specialized cells but
they can also have extracellular digestion
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---that is, outside of their
cells in a gastrovascular cavity.
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Cnidarians can generally have
two forms: a polyp and a medusa.
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They can have specialized cells with
fascinating organelles that can be used
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in stinging their
prey.
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Most cnidarians have radial symmetry.
They have a no cephalization and no coelom.
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Phylum Platyhelminthes.
I like to think “plat”
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rhymes with “flat” – these are the flatworms.
Many are aquatic – freshwater or saltwater;
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some are terrestrial; planarians and tapeworms
are in this phylum.
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There are quite a few that
are parasitic in this phylum.
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Most in this phylum have one gut opening.
This phylum has bilateral symmetry and they
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do have cephalization.
But no coelom.
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It is now where we can add
the descriptor of whether the
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phylum are protostomes or deuterostomes
– animals in this phylum are protostomes.
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Phylum Nematoda: another worm-
but specifically the phylum has
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nematodes- hookworms and pinworms
are examples of nematodes.
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Ask a person what a nematode is
and they’ll likely be puzzled.
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But these animals have HUGE ecological impacts:
nematodes can infest crops, they can be parasites
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of humans, of your pets – actually they
can be parasites of nearly all animals.
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Not all nematodes are parasites – they can
actually be very beneficial to soil ecology
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and they’re popular for study in labs.
Most nematodes are very small – they can
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be microscopic – and they can live in aquatic
environments (both saltwater and freshwater)
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as well as terrestrial environments.
Symmetry is bilateral, they do have
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cephalization, and while they have
something called a pseudocoelom,
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they don’t have a true coelom.
They’re protostomes.
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And unlike most in the previous phylum, phylum
Nematoda have both a mouth and an anus so
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they have two gut openings.
And fyi, the remaining phyla we will
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cover after this generally will as well.
Phylum Mollusca!
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The mollusks.
Think of tiny micromullusks to the giant squid.
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Snails, clams, octopuses – these are all mollusks.
Many mollusks are aquatic – saltwater
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or freshwater; some are terrestrial.
Many – but NOT all - mollusks have a shell that
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is secreted by a structure called the
mantle.
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Many mollusks have a radula which is kind
of like a tongue-like and used to scrape or
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rasp food.
Mollusks have a muscular foot to help them move.
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Symmetry is bilateral, they do have
cephalization, and they do have a coelom.
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Also, they’re protostomes.
Phylum Annelida: lots of worms.
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Earthworms, leeches, tubeworms!
Animals in annelida can be aquatic:
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freshwater or saltwater and
they can be terrestrial.
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Most in this phylum are segmented which means
they have repeating body parts– they often
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also have external rings- and most but not
all have setae, little hair-like structures
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that can help them move or
swim or even anchor themselves.
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Symmetry is bilateral, they do have
cephalization, and they do have a coelom.
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Also, they’re protostomes.
Phylum Arthropoda: Oh here we are with the ant.
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And other insects.
And spiders which are not insects.
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And crustaceans.
All examples of arthropoda.
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Arthropods can live in aquatic
environments: freshwater and saltwater
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and they can be terrestrial.
As a bonus, many fly.
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These animals have jointed
appendages and a segmented bodies.
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Their exoskeleton is tough and protective
but it still lets them move around easily.
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For some, the metamorphosis or the change
they go to from larva to adult can let them
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have the benefit of different resources.
Symmetry is bilateral, they do have
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cephalization, and a coelom.
And they’re protostomes.
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Phylum Echinodermata.
Sea stars, sea urchins,
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sea cucumbers– these are examples of echinoderms.
Animals in this phylum are aquatic – and
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live in specifically saltwater.
Most larvae of enchinoderms have bilateral
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symmetry but as adults, most have radial symmetry.
Many echinoderms have the ability to regenerate
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portions of their structure: for example,
many sea stars can regenerate a lost arm.
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Some can even reproduce
asexually from that lost part.
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What about cephalization?
Ok so the phyla we’ve mentioned so
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far that did have cephalizationp–recall that means
animals in those phyla generally have a head (an
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anterior region) with a brain or ganglia
that function similar to a brain.
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But animals in Echinodermata do not have a brain,
most do not have ganglia either – echinoderms
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do not have cephalization.
However, they do have a coelom.
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They’re deuterostomes – interesting as
the other phyla that we’ve covered to this
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point have only had protostomes.
Ok we’re nearly there
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to the last phylum on our list.
But something to point out: up to this point,
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all these phyla have contained animals that
are invertebrates.
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Meaning all the animals we’ve discussed so
far do NOT have a VERTEBRAL column or spine.
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It might surprise you that if you were to
consider all animal species – it’s estimated
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that approximately 97% of all
animal species ARE invertebrates.
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Funny that’s often not what we
picture when we think of an “animal.”
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But vertebrate animals will be
in this last phylum: Chordata.
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Chordata contains the vertebrate animals like
fish, amphibians, reptiles, birds, and mammals
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– a reminder that humans are examples of
mammals - AND Chordata even includes a few
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invertebrates too like this lancelet because
animals in Chordata are actually defined as
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having a notochord.
The notochord is a flexible rod like structure
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that runs along the back – dorsal area – and
for vertebrates, it’s there during embryonic
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development but it often gets replaced by
vertebrate although often remnants of
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it can still be found.
Some chordates like that
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lancelet keep the notochord throughout their life.
Other characteristics you’ll find in chordates
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that will be present during a stage of embryonic
development - or beyond - include having a
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dorsal nerve cord, pharyngeal slits or pouches,
postanal tail, and a thyroid gland (or endostyle
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gland in some of the invertebrate chordates).
Animals in this phylum can be found in aquatic
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environments – saltwater or freshwater – as
well as terrestrial environments –some can fly.
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Like arthropoda and annelida,
chordates are segmented.
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Animals in chordata have bilateral symmetry,
they do have cephalization, and a coelom.
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And, like Echinodermata, they’re deuterostomes.
Phew.
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So that’s a brief overview of 9 animal
phyla with some major characteristics vocab.
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Are there more animal phyla
that we didn’t include?
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Oh yes.
Are there more characteristics
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of each of these phyla to learn?
For sure.
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And so from the sponge of Porifera to a puma in
Chordata – we hope you will keep on exploring.
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Well, that’s it for the Amoeba Sisters,
and we remind you to stay curious!
