Video Transcript
In this video, we will learn what
it means to be an invertebrate. We will then take a closer look at
the characteristics of invertebrates that are classified into eight different
phyla.
All members of kingdom Animalia are
multicellular, eukaryotic organisms. They are mobile for at least one
stage of their life and are heterotrophic, which means they consume other organisms
to get their nutrition. Animals can be divided into two
major groups. Invertebrates are animals that do
not have a spinal column, like this unfortunate worm here, while vertebrates are
animals that do have a spinal column or backbone, like this bird. When we hear the term animal, most
of us tend to think of a vertebrate. After all, we’re vertebrates
ourselves. However, there are many, many more
invertebrate species than there are vertebrate species, possibly as many as 30
million. If we’re going to have time to
explore the vast diversity of invertebrates, we better get to it.
You may remember that a taxonomic
kingdom is divided into smaller groups called phyla. Scientists are still researching
exactly how to classify animals. But for today, we will go through
eight phyla in the approximate order of their evolutionary appearance, starting with
Porifera, moving to Cnidaria, then Platyhelminthes, Nematoda, Annelida, Arthropoda,
Mollusca, and Echinodermata.
The most primitive phyla is
Porifera, and its members are commonly called sea sponges. They are found in aquatic habitats,
where they filter their food from the water. Sponges have a simple asymmetric
body plane, with no true tissues or organs. Instead, all of their biological
processes are carried out by specialized cells. For example, specialized cells
called sclerocytes secrete spiky elements called spicules. These mineral-based spikes create a
sort of skeleton to provide structural support for the sponge. Sponges are hermaphrodites, meaning
they have both male and female reproductive cells. They can use these cells, called
gametes, to reproduce sexually or they can reproduce asexually through budding. Sponges can regenerate their adult
form even after being broken into single cells. The cells will clump back together
and eventually become a fully functioning new sponge.
Another phylum whose members are
all aquatic is Cnidaria. These include jellies, sea
anemones, hydras, and corals, all of which are predators. Cnidarians display radial symmetry,
which means their bodies can be divided into equal parts around a central axis or
point, kind of like pieces of a pie. Cnidarians have two layers of true
tissues which we refer to as being diploblastic because the prefix di- means
two. Their life cycle includes both a
sexual phase and an asexual phase of reproduction. They have no real organs and a
single opening serves as both the mouth and anus. Most cnidarians have a nerve net,
the simplest type of nervous system in which neurons are spread out instead of
grouped in one region, such as the brain.
The nerve net, shown as a network
of green lines on this hydra, allows them to sense and respond to stimuli, for
example, a fish brushing against their tentacles. This might be painful for the fish,
as most species have special stinging cells in their outer tissue layer called
cnidocytes. When cnidocytes are triggered by
touch, a coiled thread inside is fired. The thread lodges in the flesh of
the unlucky organism, releasing venom to stun or paralyze the prey. A handful of cnidarian species,
including some box jellies and the Portuguese man o’ war, have venom that’s powerful
enough to be fatal to humans.
There are three invertebrate phyla
whose members are commonly referred to as types of worms. Before we discuss these phyla, it’s
important to note that the term worm doesn’t have a taxonomic meaning. Instead, it is used to describe
many distantly related organisms that have soft, tube-shaped bodies. All members of the three worm phyla
are bilaterally symmetric. You may remember the prefix bi-
means two. So organisms with bilateral
symmetry can be divided into two equal parts along their long axis. All of the remaining phyla we will
discuss in this video have members that are triploblastic. The prefix tri- means three, which
refers to the three embryonic tissue layers present in these organisms, shown here
in a cross section of a generalized worm body.
The first of the worm phyla is
Platyhelminthes, commonly called flatworms. Flatworms are found in a wide
variety of habitats and can be parasites or free-living scavengers or predators. As their name implies, flatworms
have very flat bodies with clear anterior and posterior sides. Their flattened shape allows for
gas exchange to take place through simple diffusion across their bodies, a good
thing since they have no circulatory or respiratory organ systems. They have very primitive digestive
and nervous systems, which can include light sensitive organs called eye spots and
clusters of neurons called ganglia.
Flatworms are primarily
hermaphrodites that undergo sexual reproduction with other individuals, a process
called cross-fertilization. Because they have both male and
female gametes, some species can sexually reproduce with themselves, a process
called self-fertilization. One example of a group that can
self-fertilize is the tapeworms. Adult tapeworms reside in the
intestines of vertebrates. And species that infest very large
vertebrates like whales can reach up to 40 meters in length.
The second worm phyla we will cover
is Nematoda, referred to as nematodes or roundworms. They are found in every possible
terrestrial or aquatic habitat from the poles to the tropics, including several
kilometers underground. They are so abundant that
scientists estimate four out of every five animals on Earth is a nematode. And a research paper published in
2019 estimated that there are 60 billion nematodes for every one human. As you might expect from their
abundance and diversity of habitats, nematodes can obtain nutrition in a wide
variety of ways. They can be carnivores, herbivores,
parasites, scavengers, or decomposers.
Unlike flatworms, nematodes have a
complete digestive tract with separate openings for a mouth and anus connected by an
intestine. They have a simple central nervous
system with ganglia in the head region connected to nerve cords. Nematodes have no respiratory or
true circulatory system. And instead of a hard skeleton,
they’re supported by the outward pressure of their own body fluids, a system called
a hydrostatic skeleton. They reproduce sexually and are
unisexual, meaning an individual nematode is either male with sperm cells or female
with egg cells.
Our third worm phyla is Annelida,
commonly called segmented worms. Its members include earthworms,
leeches, and bristle worms. Segmented worms can be found in
salt or fresh water or in land, where they help to create healthy soil. Most members of Annelida are
predators or detritovores, which means they feed on decaying plant and animal
matter. Some groups, such as the
blood-sucking leeches, are parasites. As you may have guessed from their
common name, annelids have segmented bodies, a feature that distinguishes them from
the other two worm phyla, Platyhelminthes and Nematoda. The segments often have repeated
internal or external structures, such as the bristle-like hairs called setae on
earthworms or the parapodia on bristle worms, both of which are used for
locomotion.
Annelids have a complete digestive
tract and essential nervous system that includes a primitive brain composed of
ganglia. Some species have eyes. And earthworms can actually sense
light with special receptors in their skin, enabling them to move away from the
light and stay safely buried in dark, moist soil. They have a true circulatory system
that contains blood within vessels. Earthworms even have five pairs of
structures that function similar to human hearts. All annelids undergo sexual
reproduction and most are hermaphrodites.
Arthropoda is an extremely diverse
phyla and includes crustaceans, spiders, insects, and millipedes and centipedes. Many of its members are essential
to human life. Crustaceans, such as shrimp and
crabs, provide a direct food source. Insects pollinate our crops, and
spiders and silkworms produce silk. In fact, about 90 percent of the
world’s silk is produced by a single species of silkworm called Bombyx
mori. The species is not found in the
wild and is completely reliant on humans for its survival.
All arthropods have bilaterally
symmetric segmented bodies. The segments compose a hard outer
covering called an exoskeleton. The word “arthropod” comes from the
Greek terms for “jointed” and “foot,” which refers to the paired jointed appendages
that are characteristic of this phylum. Many insect species, like the
beetle drawn here, also have pairs of wings, while other arthropods, including the
centipede drawn here, are wingless. Although the internal anatomy of
arthropods varies among groups, all have a complete digestive tract, a circulatory
system, and a relatively advanced central nervous system that includes the
brain.
Members of Arthropoda can be
carnivores, herbivores, scavengers, or detritivores, and they’re found in virtually
every habitat. In fact, there’s probably some in
the room with you right now. Most arthropods undergo sexual
reproduction and are unisexual, although there are instances of asexual reproduction
such as female Aphids that can produce clones of themselves through live birth.
Members of the phylum Mollusca are
primarily aquatic, although there are some land-dwelling species. Mollusca includes bivalves such as
clams and scallops, which are filter feeders, snails and slugs, which are usually
herbivores, and octopuses and squid, which are predatory. Their bilaterally symmetric bodies
consist of three main structures, which we can see in this simplified diagram of a
clam. A muscular foot used for movement,
a soft mass containing most of their organs, and a thin layer of tissue that covers
this mass, called a mantle. The mantle is used in
respiration. And in species such as clams,
scallops, and snails, it also secretes a hard shell.
Molluscs have complex, complete
digestive systems that include stomachs. But most groups have a very
primitive type of circulation called an open circulatory system, which consists of a
heart, but no blood vessels. Their nervous systems vary widely,
from extremely simple in clams and scallops to very complex in the cephalopods,
which is the group that includes octopus, squids, and cuttlefish. In fact, this group, known as
Cephalopoda, contains both the largest invertebrate, the colossal squid add up to 13
meters long, and the most intelligent invertebrates. Octopuses in captivity have figured
out how to do puzzles, escape their tanks, and even short-circuit the lighting and
research labs. Nearly all molluscs are unisexual,
although some species of snails and slugs are hermaphroditic.
The final phylum we will cover in
this video is Echinodermata, which includes sea stars, urchins, and sea
cucumbers. The word “Echinodermata” comes from
the Greek terms for “echino,” meaning spine, and “dermata,” meaning skin. It refers to the bumpy or spiny
internal skeleton found beneath the epidermis. Although all echinoderms live in
marine habitats, they obtain nutrition in a variety of ways, including predation,
herbivory, and scavenging. This wide diversity in feeding
strategies leads to digestive systems that vary from a single opening in brittle
stars to complete and complex in sea stars.
Echinoderms have unsegmented
bodies. And the adult forms exhibit radial
symmetry. They have a reduced circulatory
system but instead have a unique network of tubes and openings called the water
vascular system. Moving fluid through these tubes
helps echinoderms with gas exchange, feeding, and movement. They have a simple radio nervous
system with no centralized brain or ganglia. Echinoderms are unisexual and
primarily undergo sexual reproduction. However, many species can also
reproduce asexually through regeneration. Sea cucumbers also use regeneration
as a form of self-defense. Under stressful conditions or when
threatened by a predator, they can actually expel their entire digestive tract. Even more amazing is the fact that
sea cucumbers can regrow their digestive tracts in just a few weeks. Studying this process could help
scientists develop new techniques for regenerating lost or damaged tissue in
humans.
Now let’s highlight the key points
from this video. Members of kingdom Animalia, the
animal kingdom, are multicellular, heterotrophic, and mobile for at least part of
their life cycle. The kingdom can be split into two
major groups: invertebrates, which have no spinal column, and vertebrates, which do
have a spinal column. The invertebrates are classified
into phyla based on shared characteristics. The phyla we covered in this video
are Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca,
and Echinodermata.
