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Lesson Video: Kingdom Animalia: Invertebrates Biology

In this video, we will learn how to recall the characteristics of different invertebrates within the animal kingdom.


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.

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