The Amazing Variety of Animals
From the tiniest insect to the colossal blue whale, our planet is teeming with an incredible variety of animal life. So far, scientists have described over a million different species! To make sense of this staggering diversity, scientists classify, or group, animals based on their shared characteristics. This process helps us understand how different animals are related and what makes each group special. The key to this classification is looking at the most fundamental features of an animal’s body plan, such as how its cells are organized, its body symmetry, and its internal systems. Let’s explore some of the major animal groups, or phyla, to see how these features define them.
1. Phylum Porifera (The Sponges)
Members of this phylum are commonly known as sponges. They are considered primitive multicellular animals and are generally found in marine environments.
- Cellular Organization: Sponges have the simplest body plan in the animal kingdom. Their bodies are made of loose collections of cells, representing a cellular level of organization. This means they don’t have true tissues or organs like more complex animals.
- Water Canal System: Their most unique feature is a water transport system. Water enters the body through countless tiny pores called ostia, flows into a central cavity called the spongocoel, and then exits through a single large opening, the osculum. This constant flow of water is essential for gathering food, breathing, and removing waste. Think of this system as the sponge’s all-in-one kitchen, lungs, and plumbing!
- Support Structure: To hold their shape, sponges have a simple skeleton made of sharp, spike-like structures called spicules or flexible spongin fibres.
Examples:
- Sycon
- Spongilla (Fresh water sponge)
- Euspongia (Bath sponge)
While sponges are simple collections of cells, the next group on our journey developed a major innovation: organizing those cells into cooperative teams called tissues.
2. Phylum Coelenterata / Cnidaria (The Stingers)
This phylum includes animals that are aquatic (mostly marine), have radial symmetry, and can either be fixed in one place (sessile) or be free-swimming.
- Stinging Cells (Cnidoblasts): Cnidarians get their name from specialized cells called cnidoblasts, which are used for anchorage, defense, and capturing prey. These cells contain tiny stinging capsules, called nematocysts, that give animals like jellyfish their infamous sting.
- Tissue-Level Organization: As a step up from sponges, cnidarians exhibit a tissue level of organization. Their cells are grouped into distinct tissues to perform specific functions. They have a central digestive cavity with only a single opening that acts as both a mouth and an anus.
- Two Body Forms (Polyp and Medusa): These animals exist in one of two basic body forms. The polyp is a stationary, cylindrical form, like a sea anemone. The medusa is a free-swimming, umbrella-shaped form, like a jellyfish.
Examples:
- Physalia (Portuguese man-of-war)
- Adamsia (Sea anemone)
- Meandrina (Brain coral)
These stinging animals come in two main body styles, but the next group, while similar in some ways, shows off a truly mesmerizing way of moving through the ocean.
3. Phylum Ctenophora (The Comb Jellies)
Commonly known as sea walnuts or comb jellies, ctenophores are exclusively marine animals. Like the cnidarians, they are radially symmetrical and have a tissue level of organization, but they possess their own unique and fascinating features.
- Comb Plates: Their most distinctive feature is the presence of eight external rows of ciliated comb plates. The rhythmic beating of the cilia on these plates helps them move through the water.
- Bioluminescence: It is very common for ctenophores to be bioluminescent, meaning they have the ability to produce and emit their own light, creating a beautiful spectacle in the dark ocean waters.
Examples:
- Pleurobrachia
Up to this point, our animals could be divided into equal halves from many angles, like a pizza. But our next group introduces a revolutionary body plan that would change the course of animal evolution: bilateral symmetry, creating a distinct front and back, and a left and a right.
4. Phylum Platyhelminthes (The Flatworms)
Members of this phylum are called flatworms because their bodies are flattened from top to bottom (dorso-ventrally). Many are parasites found in other animals, including humans, and they represent a significant evolutionary step as the first group with an organ level of organization.
- Bilateral Symmetry: Flatworms are the first animals in our survey to have bilateral symmetry. This means their body can be divided into identical left and right halves along a single plane.
- Organ-Level Organization: They represent another step in complexity, with tissues grouped together to form specialized organs for particular functions.
- Parasitic Adaptations: To live inside a host, many flatworms have developed special structures like hooks and suckers for attachment. Some can absorb nutrients directly through their body surface, and they possess specialized flame cells that help with osmoregulation and excretion.
Examples:
- Taenia (Tapeworm)
- Fasciola (Liver fluke)
In contrast to the wafer-thin body of a flatworm, the next phylum we’ll meet has a body that’s round, like a piece of spaghetti.
5. Phylum Aschelminthes (The Roundworms)
The body of an aschelminth is circular when viewed in a cross-section, earning them the common name of roundworms. They can be found as free-living organisms in soil and water or as parasites in plants and animals.
- Complete Digestive System: Unlike the previous groups, which had no digestive tract or one with a single opening, roundworms have a complete alimentary canal. This means they have a tube-like digestive system with two separate openings: a mouth for intake and an anus for waste removal.
- Separate Sexes: In this phylum, the sexes are separate (dioecious), meaning there are distinct male and female individuals. Often, the females are noticeably longer than the males.
Examples:
- Ascaris (Roundworm)
- Wuchereria (Filaria worm)
While roundworms are smooth, the next group of animals developed a revolutionary new body plan based on a simple but powerful idea: repeating segments.
6. Phylum Annelida (The Segmented Worms)
The name Annelida comes from the Latin word annulus, meaning “little ring.” This perfectly describes their appearance, as their bodies are distinctly divided into repeating segments, a feature known as metameric segmentation.
- True Segmentation: The annelid body is marked out into segments, or metameres, both externally and internally. This repetition of body units allows for more specialized movement and function.
- Closed Circulatory System: This is the first phylum we have examined that possesses a closed circulatory system. In this system, blood is confined within a network of vessels (arteries, veins, and capillaries) as it circulates through the body.
- Organs for Excretion: Annelids have specialized organs called nephridia that are responsible for osmoregulation (balancing water levels) and excretion (removing waste).
Examples:
- Nereis
- Pheretima (Earthworm)
- Hirudinaria (Blood sucking leech)
This segmented body plan proved to be incredibly successful, and the next group of animals took it a step further by adding an external skeleton and jointed legs, a combination that allowed them to conquer every habitat on Earth.
7. Phylum Arthropoda (The Jointed Legs)
This is the largest and most diverse phylum in the entire animal kingdom, including all insects, spiders, and crustaceans, and accounting for over two-thirds of all named species on the planet, from the smallest mite to the largest crab.
- Chitinous Exoskeleton: The body of an arthropod is covered by a tough, protective external skeleton made of a substance called chitin. This exoskeleton provides support and defense.
- Jointed Appendages: The name Arthropoda literally means “jointed foot.” Their signature feature is the presence of jointed legs, antennae, and mouthparts, which allow for a wide range of complex movements and functions.
- Segmented Body: Like annelids, their bodies are segmented, though the segments are typically grouped into distinct regions, such as a head, thorax, and abdomen.
Examples:
- Apis (Honey bee)
- Locusta (Locust)
- Limulus (King crab)
While arthropods wear their skeletons on the outside, the next major group of animals is known for its soft body, which is often, but not always, protected by a hard shell.
8. Phylum Mollusca (The Soft-Bodied Animals)
As the second-largest animal phylum, Mollusca is a diverse group of soft-bodied animals. They are found in terrestrial, freshwater, and marine habitats and are often protected by a hard, calcareous shell. Their unsegmented body is typically organized into three distinct parts.
| Body Part | Primary Function |
| Head | Contains sensory tentacles. |
| Muscular Foot | Used for movement. |
| Visceral Hump | Contains the internal organs. |
A soft layer of skin called the mantle drapes over the visceral hump. The space between them, the mantle cavity, contains feather-like gills used for breathing. Many molluscs also have a unique, file-like feeding organ in their mouth called a radula.
Examples:
- Pila (Apple snail)
- Octopus (Devil fish)
From these soft-bodied marvels, we dive back into the ocean to meet a group of animals defined by their spiny skin and a truly unique way of getting around.
9. Phylum Echinodermata (The Spiny-Skinned Animals)
The name Echinodermata means “spiny bodied,” which refers to their endoskeleton (internal skeleton) made of hard, calcareous plates, or ossicles. All members of this phylum are marine. Their digestive system is complete, with a mouth on their underside and an anus on their upper surface.
- Radial Symmetry in Adults: A curious feature of echinoderms is their symmetry. While their larval forms are bilaterally symmetrical (with left and right sides), the adult animals have radial symmetry, often with five parts arranged around a central axis, like a starfish.
- Water Vascular System: Their most distinctive and unique feature is the water vascular system. This is a complex network of water-filled canals that they use for locomotion (a function used for locomotion), capturing and transporting food, and respiration.
Examples:
- Asterias (Star fish)
- Echinus (Sea urchin)
- Cucumaria (Sea cucumber)
After exploring these spiny-skinned wonders, we’ll meet a small group of worm-like animals that for a long time puzzled scientists, seeming to bridge the gap between animals without a backbone and those with one.
10. Phylum Hemichordata (The “Half-Chordates”)
Hemichordata is a small phylum of worm-like marine animals that were once classified as a sub-phylum of Chordata. This was because of a rudimentary structure in their collar region called a stomochord, which was thought to be a primitive notochord. However, they are now placed in their own separate phylum among the non-chordates. Their simple, cylindrical body is composed of three parts: an anterior proboscis, a collar, and a long trunk.
Examples:
- Balanoglossus
With the hemichordates reclassified, we can now turn our full attention to the phylum that includes the largest, fastest, and most complex animals on the planet—including ourselves.
11. Phylum Chordata (Animals with a Notochord)
Animals belonging to Phylum Chordata are defined by a set of fundamental features present at some stage of their life cycle. The most important of these is the notochord.
All chordates share these fundamental characteristics:
- A notochord (a flexible, rod-like support structure).
- A dorsal, hollow nerve cord.
- Paired pharyngeal gill slits.
- A post-anal tail.
- A closed circulatory system.
This massive and diverse phylum is divided into several groups, including Urochordata, Cephalochordata, and the large sub-phylum Vertebrata, which includes fishes, amphibians, reptiles, birds, and mammals. This leads to an important distinction: while all vertebrates are chordates, not all chordates are vertebrates, setting the stage for a more detailed study of this fascinating group.
