Animal & Plant Kingdom

Zoologists group animals into phyla using shared body-plan features - symmetry, presence of a coelom, segmentation, type of skeleton, and details of embryonic development. The nine phyla below cover the main branches of the animal tree.

• Porifera (sponges) - asymmetrical, sessile filter feeders. No true tissues or organs.
• Cnidaria (jellyfish, corals, anemones) - radial symmetry, specialised stinging cells (cnidocytes), a simple sac-like gut.
• Platyhelminthes (flatworms, tapeworms) - bilaterally symmetrical, flattened bodies, no body cavity, single gut opening. Many are parasites.
• Nematoda (roundworms, pinworms) - long, cylindrical, bilateral bodies with a pseudocoelom. Many are parasites of plants or animals.
• Annelida (earthworms, leeches) - bilateral, segmented bodies with a true coelom and a complete gut.
• Arthropoda (insects, spiders, crustaceans) - jointed limbs and a hard exoskeleton made of chitin that is moulted for growth. Largest phylum by species number.
• Mollusca (snails, slugs, octopus, mussels) - soft-bodied, most with a shell of calcium carbonate. Many have a rasping tongue called a radula.
• Echinodermata (starfish, sea urchins) - radial symmetry as adults (bilateral as larvae), internal skeleton of calcium carbonate plates, unique water-vascular system driving tube feet.
• Chordata (fish, amphibians, reptiles, birds, mammals) - bilateral, complete coelom, and a stiff nerve cord along the back (in vertebrates this becomes the spinal cord protected by a vertebral column).

The full taxonomic hierarchy runs: Kingdom → Phylum → Class → Order → Family → Genus → Species. A dog, for example, is Animalia → Chordata → Mammalia → Carnivora → Canidae → Canis → familiaris.

Within Class Mammalia, three sub-classes are recognised, based on reproduction:

• Monotremes - egg-laying mammals. Only five living species, including the platypus and echidnas.
• Marsupials - young are born tiny and undeveloped, then complete development in a pouch. Examples: kangaroos, koalas, opossums.
• Placentals - young develop fully inside the uterus, nourished through a placenta. The largest group, including humans, whales, bats and mice.

Rough timeline of animal evolution (from the fossil record):

• ~600 million years ago - first simple multicellular animals
• ~400 million years ago - insects appear on land
• ~300 million years ago - reptiles diversify
• ~200 million years ago - first mammals
• ~150 million years ago - earliest birds
• ~60 million years ago - primates diversify after dinosaur extinction
• ~300,000 years ago - anatomically modern Homo sapiens

Body plans are described using three things: the symmetry of the body, what happens during early embryonic development (from the blastula onwards), and whether a body cavity is present. Together these patterns tell biologists a great deal about how animal groups are related.

The three types of symmetry

Asymmetrical animals have no plane that divides them into equal parts. Sponges are the classic example: they are among the simplest and oldest animals, with several cell types but no true tissues or organs.

Radially symmetrical animals can be cut into several equal parts through a central point, like slicing a pizza. Jellyfish, sea anemones and corals belong here. They are more organised than sponges - they have real tissues, but no organs.

Bilaterally symmetrical animals can be divided into two mirror-image halves along one plane only. They have a definite head (anterior) and tail (posterior), a dorsal and ventral surface, and left and right sides. Most animals, including humans, are bilateral.

Why bilateral symmetry matters

Bilateral symmetry goes hand in hand with cephalisation - the concentration of nerve tissue and sense organs at the head end. An animal that moves head-first benefits from meeting its environment with its eyes, ears and brain, and this allowed complex nervous systems, organs and organ systems to evolve.

The coelom is a fluid-filled cavity lying between the gut and the body wall, fully lined with tissue derived from the mesoderm. Animals that have one are coelomates; animals that lack one are acoelomates. A third arrangement, the pseudocoelom, is a cavity that is not fully lined - roundworms (Nematoda) are built this way.

What the coelom does

It cushions and protects the internal organs, gives them space to grow, develop and move independently of the body wall, and the fluid inside can act as a hydrostatic skeleton that supports movement.

Acoelomates

Acoelomates have a solid body between the gut and the body wall. They usually have an incomplete digestive system, with a single opening acting as both mouth and anus. Flatworms and tapeworms (Platyhelminthes) are the examples to learn.

Coelomates

Coelomates have a complete digestive tract with a separate mouth and anus, and room for complex organ systems. Earthworms, insects, fish, birds and humans are all coelomates. Some are segmented (earthworms, insects) and some have a vertebral column (fish, birds, mammals).

The nine phyla

Porifera (sponges): asymmetrical, sessile filter feeders, reproduce sexually and asexually.
Cnidaria (jellyfish, corals, sea anemones): radial symmetry, stinging cells, sessile or free-swimming.
Platyhelminthes (flatworms, tapeworms): bilateral, flattened, no coelom.
Nematoda (roundworms, pinworms): bilateral, many parasitic, simple unlined cavity.
Annelida (earthworms, leeches): bilateral, segmented, true coelom.
Arthropoda (insects, spiders, crabs): exoskeleton, jointed limbs, segmented body - the largest phylum.
Mollusca (snails, octopus): soft bodies, usually a shell, most have a radula for rasping food.
Echinodermata (starfish, sea urchins): radial as adults but bilateral as larvae, internal skeleton of calcium carbonate.
Chordata (fish, amphibians, reptiles, birds, mammals): bilateral, complete coelom, a nerve cord running along the back; vertebrates have a bony vertebral column.

Reading the phylogenetic tree

Porifera branch off first, then Cnidaria. The remaining phyla share a common ancestor and are bilateral at some point in their life cycle - which is why echinoderms sit with them despite adult radial symmetry. Later branches are separated by differences in embryonic development.

Classification below phylum

Each phylum divides into classes, and classes into orders, families, genera and species. Class Mammalia, for example, contains warm-blooded animals with hair that produce milk, and splits into monotremes (egg-laying, platypus), marsupials (pouched, kangaroo) and placentals (humans, whales).

Rough timeline

First animals about 600 million years ago; the Cambrian explosion around 541-500 mya produced sponges, annelids, arthropods, molluscs and echinoderms in a short geological window. Fish followed about 420 mya, amphibians 400 mya, reptiles 300 mya, mammals about 200 mya, birds about 150 mya, bees about 100 mya, primates about 60 mya, early humans about 1-2 mya and modern humans roughly 50,000 years ago.

Three big developments drove this: the move from water onto land, the evolution of specialised tissues, and the evolution of internal body cavities.

Levels of organisation

Cells to tissues to organs to organ systems to organism. Specialised cell types include muscle cells for movement, nerve cells for communication, epithelial cells for protection and blood cells for transport.

Adaptations

Animals survive in their habitat through structural adaptations (body form, camouflage), behavioural adaptations (migration, hunting patterns) and physiological adaptations (internal processes such as temperature regulation).

There are close to half a million plant species, found in almost every habitat on Earth.

Shared features of all plants

They are multicellular and eukaryotic, their cells have a prominent nucleus and a cell wall made of cellulose, they make their own food by photosynthesis (they are autotrophic), their cells contain chloroplasts holding chlorophyll, they respire aerobically, and they reproduce both sexually and asexually.

How plants are classified

Four features do most of the work: the level of differentiation (how many specialised cell types), the presence of vascular tissue (xylem to carry water, phloem to carry food), whether the plant produces seeds, and whether it produces flowers and fruit.

Modern classification also uses DNA analysis, which gives a much clearer picture of how species are related and is the basis of the phylogenetic trees (cladograms) used today. Whether algae belong in the plant kingdom is still debated.

1. Bryophytes (mosses)

Non-vascular: no xylem or phloem, so water moves slowly by diffusion and the plants stay small. They need damp habitats. They have no true roots - instead rhizoids anchor them but cannot absorb water or minerals. They reproduce by spores.

2. Pteridophytes (ferns)

Vascular: xylem carries water up from the roots and phloem carries food from the leaves. They have true stems, leaves and roots that absorb water and minerals - so they can grow much larger than mosses. They still reproduce by spores, not seeds.

3. Gymnosperms (conifers)

Vascular plants that produce seeds. "Gymnosperm" means naked seed: the seeds sit in cones rather than inside a fruit. Most have needle-like leaves that are retained through winter.

4. Angiosperms (flowering plants)

Vascular plants whose seeds are enclosed in a fruit. They make up roughly 94% of plant species (about 370,000 identified) and include broadleaf trees, grasses, roses, daisies and orchids. Flowers are the reproductive organs and transfer pollen between plants. Insect-pollinated flowers have bright petals, nectar and scent; wind-pollinated flowers usually lack all three.

Origin and evolution

Land plants evolved from green algae roughly 500 million years ago, around the same time animals began to colonise land. Mosses were among the first and dominated early landscapes. Vascular plants appeared about 420-400 mya and gymnosperms about 350-320 mya. Angiosperms appeared about 150 mya, alongside bees and other insects - a clear case of co-evolution, where two groups evolve together into an interdependent relationship. Grasses evolved about 60 mya after the dinosaurs died out, at the same time as grazing ruminants such as horses, cattle and sheep - co-evolution again.

Monocots and dicots

Angiosperms are simplified into two main types: monocots (one seed leaf, parallel leaf veins, e.g. grasses, lilies) and dicots (two seed leaves, branched veins, e.g. roses, daisies).

Ten families worth knowing

Asteraceae (daisy): flower heads made of many small florets. Fabaceae (pea): complex flowers and seeds in pods. Poaceae (grasses): wind-pollinated. Rosaceae (rose): five petals or rows of five. Brassicaceae (cabbage): four petals, broad leaves. Orchidaceae (orchid): bilaterally symmetrical flowers, pollen in sacs called pollinia. Solanaceae (potato): many contain toxic alkaloids. Umbelliferae (hogweed): umbrella-shaped flower heads. Liliaceae (lily): fused flower parts, long narrow leaves with parallel veins. Lamiaceae (lavender): aromatic, square stems, floral foliage.