Lesson Explainer: Kingdom Animalia: Invertebrates Biology

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

We will first take a look at some of the organisms that are considered invertebrates. This group is hugely diverse and incredibly large. Current estimates suggest that around 1.25 million different invertebrate species have been identified, but there could be as many as 30 million species in total. Invertebrates make up around 95% of the animal species on Earth, and they can be found everywhere, from deserts to oceans to even your back garden.

When studying living organisms, scientists often classify them into distinct groups, in a process known as taxonomy. Organisms within the same group share some key characteristics.

Definition: Taxonomy

Taxonomy is the scientific study of biological classification.

An example of a taxonomic group is a kingdom, which is a large group consisting of many different organisms that all share some key characteristics. An outline of the traditional five-kingdom system of classification is shown in Figure 1.

Almost all the organisms belonging to kingdom Animalia share some key characteristics. They are usually mobile and can freely move around. Animals are multicellular, eukaryotic organisms, which means their cells contain a membrane-bound nucleus. They are also heterotrophic, which means they need to consume other organisms to obtain their nutrition.

Definition: Heterotroph

A heterotroph is an organism that obtains its nutrition by consuming other organisms or organic matter.

If we take a closer look at kingdom Animalia, we can see that biologists have classified this kingdom into two different groups: vertebrates and invertebrates. The basic outline of this grouping, along with some key characteristics of each group, is shown in Figure 2.

As shown in Figure 2, organisms classified as invertebrates do not possess a vertebral column, or backbone. This is a characteristic that all invertebrates, no matter how complex or different they are, share.

In the taxonomic system, each kingdom is made up of many smaller groups called phyla. Organisms within the same phylum share even more key characteristics. Let’s take a closer look at some of the phyla that are classified as invertebrates in the animal kingdom.

Key Term: Kingdom

A kingdom is a taxonomic rank that is below domain and contains multiple different phyla.

Key Term: Phylum

A phylum is a taxonomic group that is below kingdom and contains multiple different classes.

Phylum Porifera comprises organisms that more commonly known as sponges. A picture of a sponge is shown below.

Coral reef with great yellow sea sponge


Sponges are found in aquatic environments. They consume other organisms for their nutrition and are made up of multiple different cells. Their structure is very simple; their cells do not have a cell wall, and they do not have true tissues or organs like other animals do. This means that they have no digestive, respiratory, or circulatory systems, and instead their biological processes are carried out by specialized cells. Unlike most other invertebrates, sponges lack an exoskeleton but instead have spicules (or spikes) that provide structural support. They are usually hermaphroditic and can reproduce either sexually using gametes or asexually via a process called budding.

However, you might think that it is slightly odd that we have placed this phylum into kingdom Animalia, as unlike many other animal species sponges are not mobile. Actually, the young and larval stages of sponges are mobile, and they swim in search for a suitable place to settle and develop into an adult. Although adult sponges are immobile, their other characteristics (e.g., being multicellular heterotrophs) imply that they are best classified as animals. Sometimes, organisms are not a perfect fit for the group they are placed in, but they are close enough.

Example 1: Recalling the Characteristics of Sponges Placing Them into Kingdom Animalia

The picture provided shows a member of phylum Porifera, more commonly known as sponges. Sponges are immobile but are still classified into kingdom Animalia. Which characteristics do they share with the rest of the animal kingdom?

Coral reef with great yellow sea sponge
  1. They are unicellular and reproduce sexually.
  2. They are eukaryotic and reproduce asexually.
  3. They are multicellular and autotrophic.
  4. They are multicellular and heterotrophic.


Adult sponges, like the one shown here, are immobile invertebrates that are often found in marine environments, such as oceans. Surprisingly, they are classified as animals. Let’s recap the main features of the organisms belonging to the animal kingdom to help us answer this question.

First, all organisms belonging to the animal kingdom are multicellular. This means that their bodies are made up of more than one cell.

Second, animals are eukaryotic. This means that their cells contain a nucleus, which contains the genetic material of the organism.

Third, all organisms belonging to kingdom Animalia are heterotrophs. This means that they cannot synthesize their own food, unlike plants, which produce their own sugars. Animals, however, must consume other organic material to obtain their nutrition. As adult sponges cannot move, they obtain their nutrition by filtering water through their bodies and extracting tiny organic particles and very small organisms like plankton.

Using our knowledge of the animal kingdom and looking back at our answer options, we can see that the only correct option is option D. Sponges are multicellular and heterotrophic.

Phylum Cnidaria is a group that is largely composed of aquatic animals, such as hydras, anemones, and jellies, which can live individually or in groups called colonies. A picture of a hydra is shown below.

Hydra is a genus of small, fresh-water animals of the phylum Cnidaria and class Hydrozoa


These organisms do not possess a head but do have appendages, called tentacles, that surround an opening that functions as both a mouth and an anus. The body of a cnidarian is formed of two layers, with the external layer possessing cnidocytes. Cnidocytes are commonly called stinging cells and are important in capturing prey and in helping the organism to defend itself against predators.

Organisms belonging to phylum Platyhelminthes are commonly called flatworms. All flatworms, as the name indicates, have a very flat body. The anterior (top) and posterior (bottom) areas of their bodies can be identified, but they do not have a distinct head. Their bodies are bilaterally symmetrical, which means that if they are split lengthways down the middle, each side will be the same. Their bodies are made up of three layers, so they are referred to as triploblastic. The prefix triplo- in this term means “three times.” While flatworms do have a central nervous system, they do not have a specialized circulatory system. Most flatworms rely on a single opening that acts as a mouth to take in food and as an anus to remove waste. Some flatworms are free living, and some are parasitic and rely on obtaining nutrients from other organisms to survive. The majority are hermaphroditic, so they contain both male and female reproductive cells.

Definition: Hermaphrodite

A hermaphrodite is an organism that contains both male and female reproductive cells or organs.

An example of flatworms is tapeworms. Tapeworms are particularly interesting as adult worms live inside vertebrate organisms and attach, using suckers and hooks, to the host organism’s intestines. This allows them to feed off of the food that is being digested by the host. The photo below shows an enlarged image of a tapeworm.

Tapeworm is a freshwater fish parasite under the microscope for education.


The organisms belonging to phylum Nematoda are commonly referred to as roundworms. These worms have unsegmented, bilaterally symmetrical bodies made up of 3 layers. Unlike flatworms, they have two distinct openings for the mouth and the anus connected by an alimentary canal. Also unlike flatworms, they are unisexual, so each individual organism has either male or female sex cells. They are widely distributed and found in many different environments. Examples of roundworms include Ascaris lumbricoides, which is the most common parasitic worm in humans, and filarial worms, which can cause a serious disease of the lymphatic system called lymphatic filariasis (commonly referred to as elephantiasis).

The organisms belonging to phylum Annelida are known as ringworms, or segmented worms. As the name suggests, their bodies are segmented. A common example of an annelid is the earthworm, which has a distinctly segmented body. These worms thrive in high-moisture environments, so they are most commonly found in the sea, fresh water, and muddy soils. These worms can be unisexual (either male or female) or hermaphroditic. Some of them are also parasites, such as leeches, like the one shown below.

Hirudotherapy, medical leech the suction blood of


Figure 7 compares and contrasts the characteristics of the three phyla of worms that we have discussed so far.

Example 2: Classifying Organisms into Phyla according to a Given Description

An unknown species of worm has been discovered. The body is cylindrical and bilaterally symmetrical. The organism found is determined to be a male. What phylum (Platyhelminthes, Nematoda, or Annelida) is this worm most likely to belong to?


To answer this question, we need to recall the features shared by the organisms belonging to the phyla given in the question and then determine which phylum the organism in the question would best fit into. Organisms belonging to phyla Platyhelminthes, Nematoda, and Annelida are all types of worms, so they are invertebrates with bilaterally symmetrical bodies. Phylum Platyhelminthes includes flatworms, which are organisms with a flattened body and no distinct head. They are also hermaphroditic, meaning that they contain both male and female sex cells. Phylum Nematoda includes roundworms, which are organisms with a cylindrical body and a separate mouth and anus. They are also unisexual, meaning that they are either a male or a female. Phylum Annelida includes segmented worms, which are characterized by distinct rings along their bodies. These worms can be either hermaphroditic or unisexual. The organism mentioned in the question has a cylindrical body and is a male, so it is unisexual.

Therefore, the species of worm that has been discovered is most likely to belong to phylum Nematoda.

Phylum Arthropoda contains all the invertebrates that we commonly refer to as insects. This phylum can be further divided into three classes—Crustacea, Arachnida, and Insecta—and the subphylum Myriapoda. All of these classes share the characteristics of having a bilaterally symmetrical body covered by a hard exoskeleton and divided into segments with pairs of appendages (e.g., claws). They are all unisexual and reproduce sexually. However, there are some differences between the classes, which are outlined in Table 1.

Example 3: Recalling the Defining Characteristics of Phylum Arthropoda

Which of the following is not a characteristic of an organism that belongs to the Arthropoda phylum?

  1. Being immobile
  2. Having a body divided into segments
  3. Being covered by an exoskeleton
  4. Having a bilaterally symmetrical body


Arthropods are an incredibly diverse range of organisms. However, they all have some characteristics in common. All arthropods have bilaterally symmetrical bodies covered by a hard exoskeleton. Their bodies are divided into segments, usually two regions and sometimes even three, and these segments have various specialized appendages attached to them. Examples of arthropods include lobsters, spiders, scorpions, ants, bees, and centipedes.

Therefore, the only characteristic listed that is not a characteristic of phylum Arthropoda is being immobile.

The organisms belonging to phylum Mollusca, known as mollusks, are generally found in salt water, with a few species living in fresh water or on land. All mollusks share a similar basic body plan, with the three main divisions being a muscular foot for movement, a soft mass containing most of the internal organs, and a fold of tissue that covers this mass, called a mantle, which may secrete a calcareous shell. A large proportion of mollusks are unisexual, and a few species are hermaphroditic. Organisms belonging to this group that you may recognize are snails and octopuses. The heads of these snails and octopuses are well developed compared to other invertebrate species, and they even contain sense organs, like the specialized antennae on a snail’s head.

Example 4: Using Our Knowledge of Invertebrates to Distinguish between Phyla

Which of the following characteristics would help distinguish an arthropod from a mollusk?

  1. A body divided into distinct segments
  2. The absence of a backbone
  3. Having a single opening acting as both the mouth and the anus
  4. Possessing extensions from the body or appendages
  5. The absence of a complex nervous system


Both arthropods and mollusks are organisms in phyla that belong to the invertebrate group of the animal kingdom. This means that they have some characteristics in common, namely, that they do not possess a backbone (vertebral column). As more complex invertebrates, both of these phyla have extensions or appendages attached to their body, and unlike simpler forms, they have separate openings for their mouth and anus.

However, there are some key differences between the two. Mollusks, common examples of which are snails, oysters, and octopuses, have a large, fleshy body. They are all composed of a muscular foot for movement, a soft mass for their internal organs, and a fold of tissue, called the mantle, that may or may not secrete a calcareous shell (e.g., a snail’s shell). Some mollusks have complex nervous systems, such as that of an octopus, and some have far simpler nervous systems. Arthropods, such as lobsters, spiders, and scorpions, are characterized by having a hard exoskeleton and bodies that are divided into distinct segments. For instance, they may have a distinct head, thorax, and abdomen, from which their appendages extend.

Therefore, we can see that, of the given options, the characteristic that best distinguishes between an arthropod and a mollusk is having a body divided into distinct segments.

Phylum Echinodermata contains organisms that have an unsegmented body and a hard endoskeleton covered by epidermis. This endoskeleton may possess bumps and spines, which make them appear prickly. Echinoderms are largely found in marine environments. Examples of echinoderms include sea stars (shown below), sea urchins, and sea cucumbers.

Pebble red seastar on colorful seafloor


Some echinoderms, such as the sea star shown, display radial symmetry; unlike other phyla of invertebrates, this means that their bodies are not symmetrical if they are split down the middle. Instead, their extensions symmetrically extend from a central point, or axis. These organisms are unisexual and can reproduce sexually. They also demonstrate regeneration, that is, the division of cells by asexual methods. This means that if an organism like a sea star is damaged and lost an arm, it can regrow this arm from the central part of its body. The bodies of echinoderms develop extensions called tube feet, which help them move. In contrast to invertebrates like roundworms and segmented worms, echinoderms do not have a front and back end. Instead, they have two surfaces: an oral surface, where the organism’s mouth is located, and an aboral surface, located on the opposite side to the oral surface. As these organisms are generally bottom-feeders, the oral surface is what we would picture the bottom of the organism to be.

Let’s review some of the key points that we have learned so far.

Key Points

  • Kingdom Animalia is divided into two distinct groups: vertebrates and invertebrates.
  • Invertebrates are organisms that do not possess a vertebral column, or backbone.
  • Invertebrate phyla include Porifera, Cnidaria, Platyhelminthes, Nematoda, Arthropoda, Mollusca, and Echinodermata.
  • Organisms can be classified into these phyla according to the key characteristics that they share.

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