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