Evolutionary Trends, Body Plans & Symmetry
This is the key to the rest of the course's animals. Before meeting sponges, jellyfish, worms and crabs one phylum at a time, you need the toolkit biologists use to compare them all: how many tissue layers, what kind of body cavity, what kind of symmetry — and the nine life patterns every animal has to solve to stay alive.
Look into a rock pool at Cape Recife. A sea anemone has no head, no front, no back, no left and no right. A rock lobster has all of them — a head packed with sense organs, a front that leads, a back, and two matching sides. Which of the two is the more advanced animal? And is "advanced" even the right word?
Each body suits its lifestyle exactly. The anemone sits in one place and waits; food and danger can arrive from any direction, so it is built the same all the way round — radial symmetry. The lobster moves; whatever it meets, it meets front first, so its body has a front end with eyes, antennae and a brain-like ganglion — bilateral symmetry and cephalisation.
The lobster's body plan is more complex, and its group appeared later on the tree of life. Biologists — and your booklet — sometimes call that "more advanced" or "higher". But careful: cnidarians — the group anemones belong to — have been succeeding in the sea for more than 500 million years. In evolution, "later and more complex" does not mean "better". It means suited to a different way of life.
This topic is the toolkit for seeing those differences — in every animal from here to the end of the course.
One Continuum, Not a List
Animal phyla are not separate boxes. They are branches of one tree, and as you move along it you see bodies becoming more complex and specialised in how they do the same basic jobs.
Form and function. CAPS sets the depth of this topic as understanding form and function to survive: why a body is shaped and built the way it is, given how the animal lives. Each phylum should be seen "as part of an evolutionary continuum" — compare it with the ones before and after, and ask what each difference makes possible.
The three structural questions. For any animal, ask: how many tissue layers does it grow from (two or three)? What kind of body cavity does it have, if any? What kind of symmetry? Those three answers place it on the tree.
The nine life patterns. Every animal must support itself, move, coordinate, feed, transport materials, exchange gases, excrete and osmoregulate, control its temperature and reproduce. How it solves each one is a life pattern — and the solutions become more elaborate as you move up the tree.
Evolution in three sentences — the idea this topic rests on
Your booklet says that "Topic 41 introduces the theory of evolution" — that is the Two Oceans series numbering. Evolution is CAPS block 14, which your school sequence has not covered as a topic of its own, so here is the short version.
① Individuals of a species vary, and much of that variation is inherited. ② More young are born than can survive, so those whose features suit their environment are more likely to survive and breed — natural selection, the "survival of the fittest". ③ Over many generations the helpful features become common, populations change, and given enough time one ancestral species can give rise to many — which is why all animals can be drawn on one family tree.
The Phylogenetic Tree
A phylogenetic tree is a diagram showing the evolutionary relationships between groups of organisms and their appearance in time. CAPS wants one introduced now and used through every zoology topic after this. Read it from the root on the left. Tap the five numbered branch points.
Read the branch points
Each branch point is a common ancestor. Two groups are close relatives if they share a recent branch point. How close two names are printed along the edge means nothing — the branches can be rotated like a mobile.
A clade is a whole branch
A clade is a group made of one ancestor and all its descendants — snip any branch and everything that falls off is a clade. Biologists build clades from similar appearance, anatomy, physiology, biochemistry and, when it is available, genetic makeup.
No tip is "the top"
Every living group is at the tip of a branch exactly as long as ours. Sponges are not "unfinished" animals — they are modern animals whose ancestors branched off early and kept a body plan that still works.
Your booklet's tree (Fig 28.1), read carefully
The booklet's tree has more branches than this one — bryozoans, protochordates and the vertebrate classes — and a timeline. Three points to know. ① It says branch lengths are "drawn to scale to indicate geologic time". In that tree they are; in many trees they are not, so check the axis before reading times off one. ② Its dates are older rounded values: the Permian mass extinction is now dated to about 252 million years ago (it shows 245), and the Cretaceous one to 66 (it shows 65). ③ Its "first land animals, 350 mya" means the first land vertebrates; small arthropods were walking on land tens of millions of years earlier.
Why the five protostome phyla branch from one point on this page. The order in which flatworms, roundworms, segmented worms, molluscs and arthropods split is still being worked out. Body plans suggest one order; DNA suggests another — for example, that segmentation evolved more than once. Rather than choose, this tree shows them as one fan, which is how a careful biologist draws an unresolved group. The practical sheet lets you build a tree from body characters and then ask why DNA might disagree.
Levels of Organisation
A revision from Topic 14, because it is where the tree begins. Each level is built from the one before.
Look ahead: sponges (Topic 29) never get beyond cells working cooperatively — they have specialised cells but no true tissues. Cnidarians (Topic 30) are built of true tissues but have few real organs. Everything else on the tree builds organs and systems.
Body Plans — Two Layers or Three, and the Cavity Inside
Embryology — the study of how a multicellular animal develops from one fertilised cell — shows that simpler animals grow from two initial tissue layers and more complex ones from three. Those layers, and what lies between them, are the animal's body plan.
Diploblastic
Animals with two distinct tissue layers. The outer ectoderm is essentially the skin — it isolates the body, protects it, and helps with food collection and gaseous exchange. The inner endoderm encloses a chamber and helps with digestion, hydrostatic movement and gaseous exchange. Between them there may be a jelly-like layer with scattered cells, the mesogloea. Example: cnidarians.
Triploblastic
Animals with three distinct tissue layers: ectoderm, endoderm and, between them, the mesoderm, which gives rise to more specialised tissues and organs — muscles, blood, reproductive organs. Often there is also a body cavity as well as the gut. Examples: every animal from flatworms onwards.
Four cross-sections — tap through them
Imagine slicing across each animal's middle. CAPS asks you to explain these terms with the aid of diagrams, so learn what each looks like: which colour is which layer, and where the cavity sits.
Why a body cavity is such an advantage
A coelom — a fluid-filled cavity inside the mesoderm — separates the body wall from the gut. That one gap changes a great deal. The gut and organs can move independently of the body wall, so swallowing and digesting carry on while the animal moves. There is room for organs to grow large and complex. The fluid can carry materials around the body. And squeezed by muscles, the fluid becomes a hydrostatic skeleton to push against. The booklet calls these animals "more advanced" for exactly this reason: the cavity opens up possibilities a solid body cannot have.
📌 Enrichment — the ladder is a learning aid, not history. Acoelomate → pseudocoelomate → coelomate is the right order for learning the plans. But DNA suggests it is not one staircase: flatworms are probably descended from ancestors that had a cavity and lost it, and the pseudocoelom seems to have evolved more than once. Bodies can lose features as well as gain them.
A booklet error to correct — in the examinable definitions
The booklet's definition list gives ectoderm as "the inner layer of body tissue" — word for word the same as its definition of endoderm. That is a copying slip: its own body text (and its Fig 28.3) says the ectoderm is the outer layer, "essentially the skin of the animal". Learn: ectoderm = outer · mesoderm = middle · endoderm = inner. "Ecto-" means outside, "meso-" middle, "endo-" within.
Symmetry — and What It Says About a Life
Symmetry is the arrangement of an animal's body parts around its central plane. Try to cut each animal into two mirror-image halves. How many ways can you do it? Tap each one.
| Symmetry | Mirror cuts possible | Typical lifestyle | Example |
|---|---|---|---|
| Asymmetrical | none | attached, growing all its life, filtering water from all around | sponges |
| Radial | many, through the central axis | sedentary predators that must sense and respond equally in all directions | sea anemones, jellyfish |
| Pentaradial | five | slow-moving or sedentary, feeding on food slower than themselves | sea stars, sea urchins |
| Bilateral | one, along the longitudinal plane | active movers that hunt prey or escape predators | worms, crabs, fish, us |
The bilateral vocabulary. A bilateral animal has a dorsal (upper) and a ventral (lower) surface, two matching lateral (side) surfaces, an anterior (front) end and a posterior (back) end. Moving front-first is what makes cephalisation worthwhile: put the brain and sense organs at the end that meets the world first.
Two definitions to tighten — and a surprise about sea stars
Radial symmetry. The booklet's examinable definition is "having both halves identical either side of a central axis" — but that also describes a bilateral animal cut once. The key word is any: a radially symmetrical animal can be cut into mirror halves along many planes through its central axis, as the body text says. Write the definition as given and add "in many planes".
Sea stars are bilateral underneath. A sea star's larva is a tiny bilateral swimmer. It rebuilds itself into a five-armed adult. So pentaradial symmetry is a later adaptation by an animal whose ancestors were bilateral — one reason echinoderms sit on the same branch of the tree as us.
Your turn — draw the diagrams CAPS asks for
On blank paper, without looking: ① draw the four cross-sections — diploblastic, acoelomate, pseudocoelomate, coelomate — as concentric rings, shading each layer and labelling ectoderm, mesoderm, endoderm, mesogloea, gut, pseudocoelom and coelom where they appear; ② draw an anemone, a sea star and a fish from above and add every plane of symmetry as a dashed line; ③ label a bilateral animal's dorsal, ventral, anterior, posterior and lateral surfaces. Then check against this page. The practical sheet's Investigation 2 has you build the cross-sections in modelling dough first, which makes the layers far easier to remember.
The Nine Life Patterns
As animals adapt to new and more demanding environments, they become more complex and specialised in how they carry out their life functions. These are the nine patterns you will trace through every phylum from here on. For now this is an overview — the detail comes phylum by phylum.
A framework to pull against
Muscles need something to act on. A hydrostatic skeleton is enclosed fluid that muscles squeeze to make the body long and thin or short and fat (anemones, worms, most molluscs). An exoskeleton is a hard, jointed outer case (crabs, rock lobsters) — very successful, but the animal must moult to grow, and is vulnerable while soft. An endoskeleton of bone or cartilage sits inside the muscles (fish, birds, seals), and can enclose and protect organs, like a skull or rib cage. fluid → outer case → inner frameStaying put, or moving
Some animals are sessile (fixed, like a fan worm or a sponge) or sedentary (moving very little), and let food come to them. Most move to find food and avoid danger, using muscles and specialised appendages to wriggle, burrow, swim, walk or fly. sessile → wriggling → walking and swimmingSensing and responding
Single cells react to strong chemical, touch or light stimuli. Simple multicellular animals have a nerve net of interconnected cells. Later phyla show cephalisation — a cerebral ganglion or brain at the front, with sense organs — linked to the body by nerve cords with ganglia along them. An endocrine system of ductless glands releases hormones, often triggered by the nerves. simple response → nerve net → brain and cordFood: what, how, and where it is digested
Source: herbivore, carnivore, omnivore, detritivore. Mode: grazing, browsing, scavenging, hunting, filter feeding. Digestion: inside cells (phagocytosis engulfs food into the cell), in a simple gut with one opening, or in a through gut with a mouth at one end and an anus at the other — which lets an animal eat again before the last meal has been digested and removed. inside cells → simple sac → through gutMoving things around inside
In single cells and simple animals, diffusion is enough. A gut or body cavity brings the outside world deeper inside. Larger animals need blood, usually carrying a respiratory pigment such as red haemoglobin. In an open system blood leaves the vessels and bathes the organs in spaces called haemocoels; in a closed system it stays in arteries, veins and capillaries, pumped by a heart. diffusion → open system → closed systemThe ideal surface is…
Broad (enough area for the whole animal) · thin (gases diffuse fast) · moist (gases must dissolve first) · well ventilated (fresh O₂ in, CO₂ out) · close to the cells or well supplied with blood · enclosed or protected, because a surface that is thin, moist and broad is also easy to damage. Surfaces: skin, gills, tracheal tubes, lungs. whole skin → gills → protected lungsRemoving waste, balancing water
Excretion removes the (often toxic) waste of the cells' chemistry. Osmoregulation keeps the body's fluids at the right concentration, often different from the sea's. The structures: the endoplasmic reticulum of cells; contractile vacuoles in freshwater protists; flame cells in flatworms; nephridia in each segment of annelids; kidneys. Specials: sharks keep hyperosmotic blood; seabirds and turtles have salt glands. cell surfaces → flame cells → nephridia → kidneysKeeping enzymes in range
Enzymes work only within a temperature range — too cold and the animal is sluggish, too hot and it can die. Most marine habitats are temperature-stable, so many marine animals simply have enzymes suited to that range. Land animals face bigger swings and need ways to regulate. Two separate questions: where does the heat come from (ecto- or endothermic), and how narrowly is it held (poikilo- or homoeothermic)? See the table below. tolerate it → behave → generate heatMaking more
Asexual: one cell divides, or an animal buds or splits. Sexual: gametes unite, giving the variation a population needs in a changing world. Sexes may be separate (unisexual), combined (hermaphrodite), or change partway through life. Fertilisation is external (spawning) or internal. Embryos develop oviparously, ovoviviparously or viviparously — table below. splitting → spawning → internal fertilisation and careFour booklet wordings to read carefully
Haemocoels are not a coelom. The booklet defines an open blood system as blood circulating through "coelomic spaces (haemocoels)". A haemocoel is a blood-filled body space; in arthropods and molluscs the true coelom is small, and most of the body cavity is haemocoel. Say "blood-filled spaces called haemocoels".
Exoskeletons are mostly chitin. The booklet says the casing is calcium carbonate and protein. An arthropod exoskeleton is built of chitin and protein; crustaceans such as crabs and rock lobsters harden theirs with calcium carbonate. Mention chitin.
"Internal" and "external" digestion. The booklet and CAPS use internal for digestion inside cells and external for digestion in a gut. Most textbooks call these intracellular and extracellular, and keep "external digestion" for food digested outside the body — as when a sea star pushes its stomach out over a mussel. Use the booklet's words in its exam, and know the other pair exists.
"Flagellae". The correct plural is flagella (one flagellum).
| Poikilothermic — wide range | Homoeothermic — narrow range | |
|---|---|---|
| Ectothermic — heat from outside | rock-pool animals, which bake at low tide and chill at night; land lizards basking and sheltering | most open-ocean and deep-sea animals: the water barely changes, so their temperature barely changes either |
| Endothermic — heat from inside | mammals that hibernate, letting their temperature fall for the winter | seals, dolphins, penguins and gannets, insulated by fat, fur or feathers |
Counter-current exchange (named by CAPS): blood flowing out to a flipper or fin runs right alongside blood flowing back, so the warm outgoing blood hands its heat to the cold returning blood instead of losing it to the sea. Dolphins' flippers and penguins' feet use it — and so do tunas and great white sharks, keeping their swimming muscles warmer than the water. That is why research now finds some endothermic ability in large fish, as the booklet notes.
| Strategy | What happens | Example |
|---|---|---|
| Oviparous | eggs laid and hatch outside the mother's body | most bony fish; sea turtles; the endemic puffadder shyshark, whose leathery egg cases ("mermaid's purses") wash up on our beaches |
| Ovoviviparous | internally fertilised eggs develop and hatch inside the mother, who then gives birth | the ragged-tooth shark — see section seven |
| Viviparous | the embryo develops in a uterus or similar organ, nourished through the mother's blood until birth | dolphins, whales and seals; some sharks, such as hammerheads |
Sex change is common in our waters: many South African sea breams, such as the roman, start life as females and become males later. All the processes above serve one end — homeostasis, keeping the internal environment of the cells stable.
Body Plans on Our Shore
Local connection — a kelp holdfast zoo, and a shark that eats its siblings
A whole tree of life in one fist-sized lump. After a storm, look along the high-tide line in Algoa Bay for a kelp holdfast torn off the reef — CAPS lists one as a resource for this topic. Its tangle of root-like branches shelters encrusting sponges (asymmetrical), tiny anemones (radial), brittle stars and baby urchins (pentaradial), and bristleworms, flatworms, amphipods, small crabs, chitons and even sea squirts and klipfish (all bilateral). Most of the tree on this page, in one wash-up. The practical sheet shows you how to sort it.
The ragged-tooth shark, our ovoviviparous neighbour. Pregnant "raggies" spend much of their pregnancy in warm KwaZulu-Natal water, then travel south to give birth along the Eastern Cape coast, south of the Kei River, between September and November. Inside each of the mother's two uteri, the first embryo to develop eats the other eggs and embryos — so a ragged-tooth shark gives birth to only two large, well-developed pups, one from each side. Internal fertilisation, eggs hatching inside the mother, and live birth: ovoviviparity in its most dramatic form.
In the Lab — the CAPS Investigations
CAPS sets two investigations: "Study cladograms and phylogenetic trees of the 7 kingdoms and major animal phyla, to see how they work", and "Study photos of marine heterotrophs; name and identify them" — with preserved animals or a kelp holdfast as resources. The practical sheet does both, and adds the two hands-on tasks that make body plans stick.
Four investigations · modelling dough, jelly cubes and a washed-up holdfast
- 🟢 desk Build a cladogram — from a table of body characters for six marine animals, then read the booklet's tree and compare your tree with the DNA evidence.
- 🟢 kitchen Body plans in modelling dough — build the four cross-sections in three colours, slice them, then draw and label each one, as CAPS asks.
- 🟢 kitchen Why big animals need gills and blood — jelly cubes of three sizes, coloured with red cabbage, dropped into vinegar. Measure how far the change reaches, and calculate surface area to volume.
- 🌊 shore 🟢 desk The holdfast zoo — sort the animals living in a washed-up kelp holdfast by symmetry and body plan. A described holdfast is supplied if you can't find one.
→ Open the Topic 28 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. Your booklet's examinable list for this topic has fifty-one terms — the longest in the course. All fifty-one are here, in the booklet's own wording, grouped so you can learn them a set at a time. These words will be used in every remaining zoology topic, so they repay the effort.
Trees and groups
A diagram illustrating the relatedness between groups of organisms and their appearance in the geological timeline.
Groups of organisms having similar appearance, internal anatomy, physiology, biochemistry and genetic makeup (when available) — on a tree, an ancestor and all its descendants.
Body plans
Animals having a body wall made up of two tissue layers. e.g. cnidarians.
Animals with three tissue or body layers. Flatworms onwards.
The outer layer of body tissue — essentially the skin. ⚠️ The booklet's list prints "the inner layer" here by mistake; its own text and diagram say outer.
The inner layer of body tissue, lining the gut cavity.
A third embryonic layer which forms between the ectoderm and endoderm, giving rise to muscles and other specialised tissues and organs.
A jelly-like layer with loosely arranged cells sometimes found between the ectoderm and endoderm — the "jelly" of a jellyfish.
Having no body cavity/coelom — the mesoderm fills the space between body wall and gut. e.g. flatworms.
A body cavity, within the mesoderm, normally housing organs.
Having a distinct body cavity/coelom within a split mesoderm. e.g. annelids, molluscs, echinoderms, chordates.
A body cavity lying between the mesoderm and the endoderm — "false" coelom, because mesoderm lines only its outer side.
When the body cavity is between the mesoderm and endoderm, as opposed to within a split mesoderm. e.g. roundworms.
Symmetry
An animal which can only be bisected along one line in the longitudinal plane to give identical halves.
Having both halves identical either side of a central axis — in many planes through that axis, unlike bilateral symmetry.
(Five-pointed) adaptation of radial symmetry, seen in echinoderms such as sea stars.
Support and coordination
A means of body support, using enclosed fluid.
A hard, jointed, external skeleton outside the ectoderm of an animal. Built of chitin and protein; hardened with calcium carbonate in crustaceans.
A structural support system consisting of hard material inside the ectoderm of an animal — bone or cartilage.
The concentration of nervous tissue and sense organs at the anterior of an animal.
A large ganglion at the anterior of an animal, associated with sense organs — a simple brain.
A concentrated pocket of nerve cells (neurons). One is a ganglion.
A system of ductless glands which assist in regulating metabolic functions by means of hormones.
Feeding and transport
An animal that feeds only on plant matter.
An animal that feeds only on animal matter.
An animal that feeds on plants and animals.
An animal that feeds on decomposing organic matter.
To obtain microscopic food items from the water column by means of an adapted filtration organ.
To absorb and incorporate chemical compounds into the body tissue of an organism.
A tube-like gut with openings at each end — mouth on one side and anus on the other.
The movement of molecules from areas of higher concentration to areas of lower concentration — a form of passive transport.
A system in which blood circulates through spaces (haemocoels) interconnected by blood vessels. The booklet says "coelomic spaces"; haemocoels are blood-filled spaces, not a true coelom.
A system in which blood circulates through vessels.
Excretion, balance and temperature
A membrane-bound osmoregulatory organelle; collects and expels excess water accumulating in the cells of freshwater protists.
Whip-like structures used by microorganisms to swim; such cells can be referred to as flagellated cells. (Correct spelling: flagella; one flagellum.)
Small bulbs attached to the excretory tubules of flatworms, lined with many flagellated cells — the flickering flagella look like a flame.
A specialised but simple excretory organ in the segments of annelids and related phyla.
A cluster of cells in the body of an animal, which extract salt from the blood stream and excrete it — in seabirds and marine turtles.
The maintenance of the correct concentration of all fluids inside an animal.
The maintenance of a constantly favourable internal environment in multicellular organisms.
The ability to regulate and maintain a favourable body temperature.
Adapted to maintain a favourable body temperature through behavioural responses to the external environment.
Adapted to maintain a favourable body temperature by means of internal cellular metabolism.
Animals which maintain their body temperatures over a relatively broad range.
Animals which can maintain their body temperatures within a very narrow range. Also spelt homeothermic.
Reproduction
A form of reproduction where the offspring derive from the mitotic division of part of the adult.
A form of reproduction where the offspring derive from the union of male and female gametes.
An organism having both male and female reproductive organs.
Animals which hatch from eggs which are outside the body of the mother.
Animals which develop in and hatch from eggs which have been kept within the body of the mother.
Animals which undergo development inside a specialised organ within the body of the mother.
General terms to know
Anterior the front end · posterior the back end · dorsal the upper surface · ventral the lower surface · lateral the side surfaces (all of a bilateral animal) · appendages limbs or functional outgrowths · asymmetrical irregular form, no symmetry · symmetry the arrangement of body parts around the central plane · browsing feeding on leaves, shoots or other soft vegetation · grazing eating a little at a time, but constantly · hunting chasing and catching prey · scavenging feeding on dead or decaying matter · digest break food down mechanically and chemically so it can be absorbed · ingest take in food · moulting stage when an animal sheds its outer layer · retractable able to be withdrawn into the body · sedentary not moving · sessile immobile or attached.
Also from the booklet's questions: secretion (releasing a useful substance made by a cell, such as an enzyme or hormone), excretion (removing waste made by cell metabolism) and egestion (getting rid of undigested food that never entered the cells). Only excretion removes metabolic waste.
Quick Check
Question 1
Well done!
You have the toolkit. Next: the first branch on the tree — the sponges.