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!