Defining the Animal Kingdom
What makes an animal an animal
Kingdom Animalia is defined by a specific combination of traits: animals are multicellular, eukaryotic, heterotrophic (they must consume other organisms for energy and nutrients, unlike photosynthetic plants), and lack the rigid cell walls that plants and fungi have — animal cells are bounded only by a flexible plasma membrane, which is part of what allows animal tissues to move and change shape. Most animals are also motile — capable of self-directed movement — at least during some stage of their life cycle, even species like sponges and corals that are sessile (fixed in place) as adults but have a motile larval stage.
With over a million described species (and likely millions more undescribed, especially among insects), Animalia is enormously diverse, but taxonomists organize that diversity using a relatively small number of body-plan criteria that separate the roughly 30-35 recognized animal phyla from one another. Three of the most important criteria are: body symmetry, the presence and type of a body cavity (coelom), and the developmental fate of an embryonic opening called the blastopore.
The vast majority of animal diversity — an estimated 95% of all animal species — belongs to invertebrates, animals without a backbone. Vertebrates (animals with a backbone, all within phylum Chordata) are a comparatively small slice of animal diversity but include the most anatomically complex and, to humans, the most familiar animals: fish, amphibians, reptiles, birds, and mammals.
💡 Body Symmetry as a First Sorting Criterion
One of the very first questions taxonomists ask when classifying an animal is: what kind of body symmetry does it have? Radial symmetry means the body can be divided into similar halves along many planes passing through a central axis, like slices of a pie — seen in Cnidaria (jellyfish, sea anemones) and echinoderms as adults (though echinoderm larvae are actually bilaterally symmetric, a clue to their evolutionary origin). Radial symmetry works well for animals that encounter their environment from all directions equally, such as a jellyfish drifting in open water or a sea anemone anchored to a rock, since threats and food can arrive from any direction.
Bilateral symmetry means the body has a single plane of symmetry, dividing it into distinct left and right mirror-image halves, along with a clear head end (cephalization) and tail end. This is the symmetry type of the vast majority of animals, including all vertebrates, arthropods, and mollusks, and it's strongly associated with active, directional movement — a bilaterally symmetric animal moving forward can concentrate sensory organs (eyes, antennae) and a brain at its leading end, where they'll encounter new stimuli first. A small number of animals, including most sponges, have no true symmetry at all (asymmetry).
Coel
Body cavity types (coelom)
The presence, absence, and type of internal body cavity is a major classification criterion. Acoelomate animals have no body cavity at all between the digestive tract and the outer body wall — the space is filled with solid tissue (seen in flatworms, phylum Platyhelminthes). Pseudocoelomate animals have a body cavity, but it is not completely lined by tissue derived from mesoderm (one of the three embryonic germ layers) — seen in roundworms, phylum Nematoda. Coelomate animals have a true coelom: a fluid-filled body cavity completely lined by mesoderm-derived tissue on all sides, which provides several key advantages — cushioning for internal organs, room for the digestive tract to move and specialize independently of body wall movement, and (in many groups) a hydrostatic skeleton that muscles can push against for movement.
Most complex animal phyla — Annelida, Mollusca, Arthropoda, Echinodermata, and Chordata — are coelomates, suggesting a true coelom was a major evolutionary advantage that was retained and elaborated on across many separate lineages.
P/D
Protostomes vs deuterostomes
During early embryonic development, a coelomate animal forms an opening called the blastopore. What that opening becomes is one of the most fundamental splits in the animal kingdom. In protostomes ("mouth first"), the blastopore develops into the mouth, and the anus forms separately later — this pattern is seen in Annelida, Mollusca, Arthropoda, and Nematoda. In deuterostomes ("mouth second"), the blastopore develops into the anus, and the mouth forms separately later — this pattern is seen in Echinodermata and Chordata, the phylum that includes all vertebrates.
This developmental split also correlates with other embryonic differences: protostomes typically undergo spiral, determinate cleavage of the early embryo (where each cell's fate is fixed early), while deuterostomes typically undergo radial, indeterminate cleavage (where early cells retain more developmental flexibility — the biological basis of identical twins forming from a single early-stage human embryo splitting).
Echinoderms (sea stars) are deuterostomes, making them, developmentally speaking, closer relatives of vertebrates than of the protostome invertebrates (insects, snails) they superficially resemble more in daily life.
Phyla
Overview of the major phyla
Working roughly from structurally simplest to most complex: Porifera (sponges) — no true tissues or organs, filter feeders, asymmetric or radial. Cnidaria (jellyfish, corals, sea anemones) — radial symmetry, specialized stinging cells (cnidocytes), simple nerve net. Platyhelminthes (flatworms) — bilateral, acoelomate, simplest animals with a true head. Nematoda (roundworms) — bilateral, pseudocoelomate, among the most numerous animals on Earth by individual count. Annelida (segmented worms) — bilateral, coelomate, body divided into repeating segments. Mollusca (clams, snails, octopuses) — bilateral, coelomate, soft body often with a hard shell, second-largest phylum by species count. Arthropoda (insects, crustaceans, arachnids) — bilateral, coelomate, jointed appendages and a hard exoskeleton, by far the largest animal phylum. Echinodermata (sea stars, sea urchins) — pentaradial symmetry as adults, deuterostome, unique water vascular system. Chordata (vertebrates, plus tunicates and lancelets) — defined by a notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail present at some life stage.
Arthropoda alone accounts for roughly 80% of all described animal species — a single phylum containing more diversity than all other animal phyla combined.
🔬 Applied Scenario — Classifying Animals by Body Plan
Working through how a taxonomist assigns an unfamiliar animal to a phylum shows how these criteria are used together, not in isolation.
A
Starting with symmetry. The very first observation is body symmetry — radial symmetry immediately points toward Cnidaria or the echinoderms (as adults), while bilateral symmetry narrows the field to the large majority of remaining phyla, and asymmetry points strongly toward Porifera.
B
Checking for a coelom. Next, does the animal have a body cavity, and if so, is it fully lined with mesoderm? A flatworm-like animal with no cavity suggests Platyhelminthes; a cavity that's not fully mesoderm-lined suggests Nematoda; a true coelom narrows things to the more structurally complex phyla.
C
Protostome vs deuterostome as a tiebreaker. Among coelomates, tracking blastopore fate during development (or, more commonly today, comparing DNA sequences that correlate with this developmental pattern) separates the protostome phyla (Annelida, Mollusca, Arthropoda) from the deuterostome phyla (Echinodermata, Chordata) — explaining why, despite outward appearance, a sea star's development pattern links it more closely to a fish than to an insect.
D
Segmentation and appendages as finer detail. Once phylum is established, finer distinctions — is the body segmented (Annelida, Arthropoda) or not (Mollusca)? Are there jointed, paired appendages (Arthropoda) or not? — refine the classification down toward class and order, using the same nested logic covered in the classification hierarchy lesson.
📌 Exam Application
1. Core animal traits: multicellular, heterotrophic, no cell wall, motile at some life stage.
2. Symmetry types: radial (Cnidaria, adult echinoderms), bilateral (most phyla), asymmetric (most sponges).
3. Coelom types: acoelomate (Platyhelminthes), pseudocoelomate (Nematoda), coelomate (Annelida, Mollusca, Arthropoda, Echinodermata, Chordata).
4. Protostome vs deuterostome: blastopore becomes mouth (protostomes) vs anus (deuterostomes) — Echinodermata and Chordata are deuterostomes, making echinoderms closer relatives of vertebrates than of most other invertebrates.
5. Largest phylum: Arthropoda, roughly 80% of all described animal species.
⚠️ Most Common Kingdom Animalia Mistakes
Radial symmetry in adult echinoderms is misleading — their larvae are bilateral. Students sometimes classify echinoderms purely by their adult radial symmetry and miss that this is a secondarily evolved trait; their bilaterally symmetric larval stage, and their deuterostome development, are what actually link them evolutionarily to Chordata rather than to Cnidaria.
Pseudocoelomate does not mean "no body cavity" — it means an incompletely lined one. A frequent mix-up is confusing pseudocoelomate (has a cavity, just not fully mesoderm-lined — Nematoda) with acoelomate (no cavity at all — Platyhelminthes). These are two different conditions, not the same thing worded differently.
Most animal species are invertebrates, not vertebrates. Because vertebrates (fish, amphibians, reptiles, birds, mammals) are the animals humans interact with and study most, students often underestimate how small a slice of animal diversity they represent — roughly 95% of all animal species are invertebrates, and the majority of those are arthropods.
✓ Quick Self-Test
1. What four traits define Kingdom Animalia?
2. What are the three types of body symmetry seen in animals, and which phyla show each?
3. What is the difference between acoelomate, pseudocoelomate, and coelomate body plans?
4. What is the difference between protostome and deuterostome development, and which major phyla fall into each group?
5. Which animal phylum contains the most described species, and roughly what percentage of all animal species does it represent?
Answers:
1. Animals are multicellular, eukaryotic, heterotrophic (must consume other organisms for energy), and lack rigid cell walls, being bounded only by a flexible plasma membrane; most are also motile during at least one life stage.
2. Radial symmetry (body divisible into similar halves along many planes through a central axis — Cnidaria, and echinoderms as adults), bilateral symmetry (one plane of symmetry, distinct left/right halves and head/tail ends — the majority of animal phyla), and asymmetry (no symmetry at all — most sponges).
3. Acoelomate animals have no body cavity between the digestive tract and body wall (Platyhelminthes). Pseudocoelomate animals have a body cavity that is not completely lined with mesoderm-derived tissue (Nematoda). Coelomate animals have a true coelom — a fluid-filled cavity completely lined with mesoderm on all sides (Annelida, Mollusca, Arthropoda, Echinodermata, Chordata).
4. In protostomes, the blastopore (an opening formed during early embryonic development) becomes the mouth, with the anus forming separately later — seen in Annelida, Mollusca, Arthropoda, and Nematoda. In deuterostomes, the blastopore becomes the anus, with the mouth forming separately later — seen in Echinodermata and Chordata.
5. Arthropoda (insects, crustaceans, arachnids) contains the most described species, accounting for roughly 80% of all described animal species.