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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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Chordates
Hank introduces us to ourselves by taking us on a journey through the fascinatingly diverse phyla known as chordata. And the next time someone asks you who you are, you can give them the facts: you're a mammalian amniotic tetrapodal sarcopterygian osteichthyen gnathostomal vertebrate cranial chordate. Created by EcoGeek.
Want to join the conversation?
- Why would animals develop without a brain? E.g. a Lancelet.(9 votes)
- A downside of a brain is that it takes up an incredible amount of energy to function. Some animals that you see, like jellyfish, have certain life styles and niches where having a brain may not be selected for.(10 votes)
- Hank said at the very end atthat all mammals evolved from one species. What species was that? 11:33(8 votes)
- It is some ancestor species. We don't know that species however.(5 votes)
- athank mentions that the hagfish doesn't have a vertebrae yet it is a vertebrate. how? 4:45(5 votes)
- He actually doesn't classify it as a vertebrate, just a chordate. Chordates are known to have spinal chords be their main attraction. But to be classified you also need a skull, a post-anus tail at one point in one's life cycle, the ability to make mucus, and other minor details. The hagfish, while not containing most of these has at least a skull and a post-anus tail so it can be grouped into this section because there nowhere better to put it.(9 votes)
- Can an animal survive without a heart ? If so how?(5 votes)
- There are many animals without a proper "heart" organ like in humans. They are either very small, live in the water or both e.g. jellyfish. They are small enough and not too complex that all the oxygen they need can defuse right into them and all the nutrients they eat can easily spread through out the body without any pumping involved.(8 votes)
- Athe mentions the brain. Does this mean no other animals previously mentioned had a brain? Do insects have a brain, etc.? 4:26(5 votes)
- Not all animals have a brain. Some have a simplified central nervous system with no actual brain -- just nerve cords.(7 votes)
- are all chordates vertebrates?(2 votes)
- The phylum Chordata contains the sub phyla of Vertebrata, Cephalochordata (lancelets) and Tunicata.
So not every chordate is a vertebrate but every vertebrate is a chordate.(6 votes)
- Why the Chondrichthyes ( a taxonomic class of chordates) excrete Urea how nitrogen waste. It´s equal to mammals. However, they are fishes and do not need convert Ammonia in Urea, i suppose. So, why they make that? Are there some special reason? I know bony fishes ( Osteichthyes class ) excrete more Ammonia than Urea. Can you explain to me the reason? Thanks in advance :)(5 votes)
- All fishes would produce urea (just liek Teratpods) but that is metabolically expensive process so in most fishes ornithin urea cycle is suppressed and they excrete ammonia whihc is toxic in high concnetrations.
There are some fishes whihc produce urea (despite its cost) because it is susrvival tactic in highly acidic lakes.
https://www.ncbi.nlm.nih.gov/pubmed/2911349
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3059970/
Urea is also eaisre diffusing through gills of fishes.(1 vote)
- So do Mammals evolve from Osteichthyes? Don't Osteichthyes and Mammals belong to different super-classes?(2 votes)
- Yes, both are "Super-Classes" Although Mammals would likely have evolved from Reptiles, think of the monotremes! Although reptiles evolved from amphibians which evolved from the Bony Fish.(3 votes)
- Yes or no ?: Does the human have 2 chambers In the heart? PLZ answer.(0 votes)
- we are classified into the class mammalia and thus we have four chambers, the left ventricle, the left atrium, the right ventricle, the right atrium. Apart from us mammals the class aves, which are mostly birds also have four chambers.
Hope this helped! ;)(1 vote)
- myxini has no vertebral column but has the skull ! does it mean it has the brain with medulla oblongata which extends to form the spinal cord?(2 votes)
- Yes, myxini does have a medulla oblongata that extends down into an uncovered spinal cord.(2 votes)
Video transcript
- Next time someone asks
you who you think you are, just give em the facts. You're a mammalian, amniotic,
tetrapodal, sarcopterygian, osteichthyan, gnathostomal
vertebrate cranial chordate. Yeah, it's a mouthful, and in order to understand what it means, you're gonna have to understand
the most complex group of animals on earth, and what
it takes to get from this, to this. (upbeat music) The phylum Chordata
accounts for all 52,000 species of vertebrates on Earth, and several thousand
species of invertebrates. Together, they range from
tiny brainless filter feeders, all the way up to Scarlett Johansson. Now, you know by now that when we talk about classifying animals,
we're really talking about their shared ancestry. Each new branch on this
tree, marking an important new evolutionary milestone. And just like with tissue
layers and segmentation in simpler animals, there
are traits we can look for to track the evolution of chordates. By the time all of those
traits appear in one organism, we'll have arrived at
the most complex class within the most complex phylum: the mammals. But first, let's start
with the fundamentals. We've talked before about
synapomorphic traits, traits that set a group of
animals apart from it's ancestors and from other groups that
came from the same ancestors. Chordates share four synapomorphies
that make us who we are. Each of them is present at some point in every chordates life cycle. How about a volunteer to
demonstrate these traits? Ah, I see that the
lancelets are raising their mouthparts. Lancelets, also known as cephalochordata, literally, "head-chords", are one of the three
sub-phyla of chordates. And unlike almost all other chordates, these tiny, brainless
invertebrate filter feeders retain all four of these characteristics for their entire lives. You probably already know
where most of these traits are gonna appear, since the
phylum is named after it: The spinal cord, or at least something that resembles a spinal cord. First, there's the notochord,
a structure made of cartilage that runs between a
animal's digestive tube and it's nerve cord. In most vertebrates, a skeleton develops around the notochord and
allows the muscles to attach. In humans, the notochord
is reduced to the disks of cartilage that we have
between our vertebrae. Second, we have the nerve cord itself, called the dorsal hollow nerve cord, a tube made of nerve fibers that develops into the central nervous system. This is what makes chordates different from other animal phyla, which have solid, ventral nerve cords, meaning they run along
the front or stomach side. Third, all chordates
have pharyngeal slits. In invertebrates like the lancelet here, they function as filters for feeding. In fish and other aquatic
animals, they're gill slits, and in the land-dwelling
vertebrates like us, they disappear before we're born. But that tissue develops
into areas around our jaws, ears and other structures
in the head and neck. And finally, we can't forget
our fourth synapomorphy, the post-anal tail, which is
exactly what it sounds like. It helps propel aquatic
animals through the water, makes our dog look happy when she wags it, and in humans, it shrinks
during embryonic development into what is known as
the coccyx or tailbone. It's right here. And trust me, when it
comes to tail placement, post-anal is the way to go. These four traits all began to appear during the Cambrian explosion more than 500 million years ago, and today they're shared by members of all three chordate subphyla, even if the animals in those subphyla look pretty much nothing like each other. For instance, our new friends
here in cephalochordata are the oldest living subphylum, but you can't forget the
other invertebrate group of chordates, the Urochordata,
literally "tail-chords". There are over 2,000 species
here, including sea squirts. And if you're confused about why this ended up in a phylum with us, it's because they have tadpole-like larvae with all four chordate characteristics. The adults, which actually
have a highly developed internal structure with
a heart and other organs, retain the pharyngeal slits, but all the other chordate
features disappear, or reform into other structures. The third and last, and
most complex subphylum is the vertebrata, and
has the most species in it because its members have a hard backbone, which has allowed for an
explosion in diversity, From tiny minnows, to
the great blue whale. You can see how fantastic
this diversity really is when you break down vertebrata into its many, many classes, from slimy sea-snakey things, to us warm and fuzzy mammals. And as these classes become more complex, you can identify the
traits they each developed that gave them an evolutionary edge over the ones that came before. For example, how's this
for an awesome trait: A brain! Vertebrates with a head,
that contains sensory organs and a brain are called craniates. They also always have a heart
with at least two chambers. So since this is science,
you're gonna have to know that there's gonna be an
exception for every rule that you're gonna have to remember. And the exception in this case
is the Myxini, or Hagfish, the only vertebrate class
that has no vertebrae, but is classified with us
because it has a skull. This snake-like creature swims
by using segmented muscles to exert force against its notochord. Whatever, hagfish. Closely related to it is
the class petrimyzontida, otherwise known as lampreys, the oldest living lineage of vertebrates. Now these have a backbone,
made of cartilage, and maybe even more important, a more complex nervous system. With the advent of the backbone, we see vertebrates getting larger, developing more complex skeletons, and becoming more
effective at catching food and avoiding predators. But do you notice anything missing? Lampreys and other early
vertebrates are agnathans, literally, "no-jaws". And if you want to be able to chew food, it really helps to have a jaw and teeth. Most scientists think that the jaw evolved from structures that
supported the first two pharyngeal slits near the mouth. And teeth, well the current
theory is that they evolved from sharp scales on the face. Gnathostomes, or, "jaw-mouths", arrived on the scene
470 million years ago, and one of the oldest and
most successful groups of gnathostomes that have
survived to the present day are the class chondrichthyes,
the "cartilage fish". You know them as the
sharks, skates and rays, and as their names says,
their skeleton is made up mostly of cartilage, but
they show the beginnings of a calcified skeleton. Chondrichthyans haven't
changed much over the past 300 million years or so, and their success stems
from the paired fins that allow for efficient swimming, and those jaws for biting
off delicious hunks of flesh. If we're gonna eventually get
to the mammals, we need bones, and we find those with
the evolution of fish. Meet Osteichthyes, which
technically means, "the bony fish". Unlike cartilaginous fish,
members of this group have a mineralized endoskeleton. Now, Osteichthyes is sometimes
considered a superclass, because it includes a whole
slew of diverse classes that descended from it. There's actually some
controversy among taxonomists about what to call it. The main thing to know
is that the majority of all vertebrates fall
under Osteichthyes, and that includes you. It's broken up into two main groups, which themselves include
a bunch of classes. The first is the Actinopterygii,
or ray-finned fishes, and with 27,000 species,
pretty much every fish you've ever heard of is in this group. Ray-finned fishes evolved in fresh water, spread out into the oceans, and some eventually then
came back to fresh water. In the second group, things
start to get really strange and interesting. These are the lobe-finned
fishes, or sarcopterygii, a name derived from bones
surrounded by muscle found in their pectoral and pelvic fins. And that sounds like something that could be used for walking. Lobe-fins include the coelacanths, which consist of one living species; lungfish, which gulp air into their lungs; and tetrapods, which have
adapted to land with four limbs. So this is weird, right? Even though land animals
clearly are not fish, since tetrapods evolved from bony fish, they are filed under this group. These taxonomists, man. I wanna party with them sometime. But first: (upbeat piano) Imagine that you're a fisherman off the coast of South Africa
in the Western Indian Ocean about 75 years ago. Put that in your brain, hold on to it. And you've just pulled up a
fish that no one has ever seen. Not only that, you've caught
a fish that was thought to have become extinct 75 million years ago. This is exactly what happened in 1938, when Captain Hendrick Goosen hauled up a Coelacanth, and it has mystified
scientists ever since. A second population has since
been identified near Indonesia in 1999, but the deep sea creatures
remain extremely rare. The coelacanth fascinates scientists because of it's paired-lobe fins. They extend from the body like legs, and move in an alternating pattern, in other words, they
move more like a horse than like a fish. And in fact, those
paired fins are supported by the very same bones that
we have in our arms and legs. The coelacanth also has a
hinge joint in the skull so it can widen it's
mouth to eat large prey, as well as thick scales that
don't exist on any living fish. It's not good eating, but
why would you wanna eat what's essentially a living fossil. Alright, now we're
talking about tetrapods, which of course means "four feet", and getting those four feet
onto land was really awesome for those early creatures,
because that meant that they could escape
the increasingly brutal and predatory world of the ocean. Tetrapods gradually replaced
their fins with limbs, and developed entirely new body parts that were never seen before, like necks, with the help of additional vertebrae, that separated the body from the head. The first tetrapods are today
found in the class amphibia, which were the first creatures to develop a 3-chambered heart. There are more than 6,000
known species of amphibians, like frogs and salamanders, most of which begin life
as tadpoles in water, and then later develop legs and lungs and a digestive system, and often migrate to
the land for adulthood. But amphibians lay eggs
that don't have shells, so they dehydrate quickly, so
they have to be laid in water. So this leads us to our
next evolutionary milestone for the chordates: the amniotic egg. Amniotes are tetrapods that have eggs adapted for life on land. A group that includes
reptiles, birds and mammals. The amniotic egg was crucial for the success of land-dwellers, allowing embryos to develop
in their own private pond of the amniotic sac, Often surrounded by a hard shell in the case of reptiles and birds. The class Reptilia represents
the earliest amniotes. Like amphibians, they
have a 3-chambered heart, but they're totally terrestrial. And here's where we find our dinosaurs, and our snakes and turtles and lizards. You often hear reptiles
described as "cold-blooded". This does not mean that
their blood is cold, they're actually ectothermic,
which means that they absorb external heat as their
main source of body heat. Hence, the lizard that likes
to lay in the sun all day. The oldest group of
reptiles, the Archosaurs, most disappeared when most
of the dinosaurs died out 65 million years ago, but two lineages of archosaurs survived. One includes the modern reptiles,
crocodiles and alligators, and the other is a type of
dinosaur that we now call birds, the class Aves. There are big, obvious
differences between these two surviving archosaurs. One is designed for eating
and fighting big animals, while the other is
designed for flying around and being graceful and stuff. The not so obvious, but
equally important difference is that birds are endotherms, which means that they can
crank up their metabolism to regulate their body temperature. Making all that heat
requires a big furnace, which is provided thanks to the evolution of a 4-chambered heart. There's only one other group of animals that develop this trait, independently of birds by the way, and it allowed them to
spread through the planet, and I'm talking of course
about the class Mammalia, otherwise known as
amniotes that have hair, three special ear bones,
and mammary glands. And most mammals have evolved to dispense with the hard egg shell altogether, the embryo avoiding predation and other environmental dangers by developing inside the mother's body. In this class of chordates you'll find me, Dame Judi Dench, your dog,
your cat, Shamu the orca, African elephants, the
South American pudu, and 5,300 other known species of mammals. It all began with the simple ancestor more than 500 million years ago in this crazy chordate phylum, and we've finally made it! And now you know exactly who you are.