Basis of animal classification: levels of organisation and symmetry
A sandcastle, a hut and a multi-storey building are all 'buildings', but a bigger one has more complicated parts: rooms, wiring, lifts. Animals too differ in how complex their bodies are and how the body is balanced.
Animals are classified by their level of organisation (cellular, tissue, organ, organ system) and by symmetry: asymmetrical (no plane divides the body into equal halves), radial (any plane through the central axis gives equal halves) and bilateral (only one plane gives right and left halves).
In the figure : Levels of organisation increase in complexity from sponges to the higher animals.
Rule 1: Levels of organisation — Sponges have cells loosely arranged without forming tissues (cellular level). Cnidarians have tissues (tissue level). Flatworms have tissues grouped into organs, but no organ systems (organ level). From aschelminthes onwards the organs work together in organ systems.
- Easy : A sponge has no true tissues.
- Medium : A flatworm has organs such as a gut, but no complete systems.
- Exam-type : Q: Which phylum shows the cellular level? A: Porifera.
Rule 2: Symmetry — Sponges are mostly asymmetrical. Cnidarians, ctenophores and adult echinoderms show radial symmetry. Most other animals (annelids, arthropods, molluscs and chordates) are bilaterally symmetrical.
- Easy : A starfish can be cut into equal halves in more than one plane.
- Medium : Humans can be cut into similar right and left halves in only one plane.
- Exam-type : Q: Name an animal with radial symmetry. A: Hydra, jellyfish or starfish (adult).
Level of organisation goes cell → tissue → organ → organ system as complexity rises.
Saying that all echinoderms are radial at every stage. Adults are radial, but their larvae are bilateral.
Memory clue: C-T-O-S — Cellular, Tissue, Organ, System.
Spot the difference
| Symmetry | Meaning | Example |
|---|---|---|
| Asymmetrical | No plane gives equal halves | Most sponges |
| Radial | Many planes give equal halves | Hydra, jellyfish |
| Bilateral | One plane gives equal halves | Earthworm, human |
Practice questions
- 1What is meant by the level of organisation?
- 2Which animals show tissue-level organisation?
- 3Name an asymmetrical animal.
- 4What is radial symmetry?
- 5Give an example of bilateral symmetry.
- 6Which level of organisation do flatworms have?
- 7Why are echinoderms called both radial and bilateral?
- 8Arrange in rising complexity: organ system, cellular, organ, tissue.
Show answers (try them yourself first)
- 1The way cells, tissues, organs and organ systems are arranged in an animal body.
- 2Cnidarians and ctenophores.
- 3A sponge such as Sycon or Spongilla.
- 4A body that can be divided into equal halves by any plane passing through the central axis.
- 5Earthworm, cockroach or human.
- 6The organ level.
- 7Adults are radially symmetrical but their larvae are bilaterally symmetrical.
- 8Cellular, tissue, organ, organ system.
Germ layers, coelom, segmentation and notochord
Think of a sandwich. Some animals are like a two-slice sandwich, some like a three-layer one, and some have a hollow space inside where the filling should be. These differences help us sort animals.
Embryos have germ layers: diploblastic animals have two (ectoderm and endoderm) and triploblastic animals have three (with the mesoderm between). The body cavity lined by mesoderm is the coelom. Notochord is a rod-like mesoderm structure on the back during development.
In the figure : Three kinds of body cavity in animals.
Rule 1: Germ layers and coelom — Cnidarians are diploblastic with mesoglea between the layers. Flatworms to chordates are triploblastic. Annelids, molluscs, arthropods, echinoderms, hemichordates and chordates are coelomates; aschelminthes are pseudocoelomates, where the mesoderm is only scattered pouches; flatworms are acoelomates.
- Easy : Hydra is diploblastic; a frog is triploblastic.
- Medium : A round worm has a cavity that is not fully lined by mesoderm.
- Exam-type : Q: Name an acoelomate group. A: Platyhelminthes.
Rule 2: Segmentation and notochord — In some animals the body is divided into segments and the organs repeat. This is called metamerism, as in the earthworm. Animals with a notochord are chordates; those without it (porifera to echinoderms) are non-chordates.
- Easy : An earthworm's body shows ring-like segments.
- Medium : A sponge has no notochord, so it is a non-chordate.
- Exam-type : Q: What is metamerism? A: Repetition of body segments internally and externally.
Diploblastic: ectoderm + endoderm. Triploblastic: ectoderm + mesoderm + endoderm. Coelom is lined by mesoderm.
Calling the pseudocoelom a true coelom. In a pseudocoelom the mesoderm is not a complete lining.
Memory clue: A-P-C — Acoelomate (no), Pseudocoelomate (patchy), Coelomate (complete).
Spot the difference
| Type | Cavity | Example |
|---|---|---|
| Acoelomate | Absent | Flatworms |
| Pseudocoelomate | Present, not lined fully by mesoderm | Roundworms |
| Coelomate | Lined by mesoderm | Annelids, chordates |
Practice questions
- 1Name the germ layers of a diploblastic animal.
- 2What lies between the two layers in cnidarians?
- 3What is a coelom?
- 4Give an example of a pseudocoelomate.
- 5Define metamerism.
- 6What is a notochord?
- 7Why are flatworms called acoelomates?
- 8Classify the following as chordates or non-chordates: sponge, frog, starfish, fish.
Show answers (try them yourself first)
- 1Ectoderm and endoderm.
- 2Mesoglea (an undifferentiated layer).
- 3The body cavity lined by mesoderm.
- 4A roundworm such as Ascaris.
- 5The serial repetition of segments of the body, as in the earthworm.
- 6A rod-like mesodermal structure formed on the dorsal side during development in some animals.
- 7They have no body cavity between the body wall and the gut.
- 8Non-chordates: sponge and starfish; chordates: frog and fish.
Phyla Porifera, Coelenterata and Ctenophora
A bath sponge looks like a soft object, but it is the dried skeleton of a sea animal. Jellyfish drifting at the beach can sting a swimmer with cells on their tentacles.
Porifera (sponges) are asymmetrical animals with a canal system and cellular organisation. Coelenterates (Cnidaria) are radially symmetrical, diploblastic, with stinging cells. Ctenophores (comb jellies) are marine animals with eight rows of comb plates.
In the figure : Key feature of each of the three phyla.
Rule 1: Porifera — Water enters through pores (ostia), passes into the central cavity (spongocoel) and leaves through the osculum. Choanocytes (collar cells) line the canals. Digestion is intracellular. The skeleton has spicules or spongin fibres. Sponges are hermaphrodite. Examples: Sycon, Spongilla, Euspongia.
- Easy : A bath sponge is made of spongin fibres.
- Medium : Water entering through pores brings food and oxygen and carries wastes out through the osculum.
- Exam-type : Q: Name the cells that line the spongocoel. A: Choanocytes (collar cells).
Rule 2: Coelenterata and Ctenophora — Cnidarians have cnidoblasts with nematocysts for anchorage, defence and catching prey. They show two body forms: polyp (sessile, as in Hydra) and medusa (free-swimming, as in Aurelia). Obelia shows alternation of polyp and medusa forms. Ctenophores have eight rows of ciliated comb plates, show bioluminescence, and reproduce only sexually with external fertilisation. Examples: Pleurobrachia and Ctenoplana.
- Easy : Jellyfish is a medusa; Hydra is a polyp.
- Medium : A glowing sea walnut in the dark sea is a ctenophore.
- Exam-type : Q: Which cells of cnidarians help in capturing prey? A: Cnidoblasts (cnidocytes) with nematocysts.
Sponges have a water canal system; cnidarians have stinging cells; ctenophores have comb plates and glow.
Calling sponges plants because they do not move. They are multicellular animals.
Memory clue: P-C-C — Pores, Cnidoblasts, Combs.
Spot the difference
| Point | Porifera | Coelenterata | Ctenophora |
|---|---|---|---|
| Symmetry | Asymmetrical | Radial | Radial |
| Organisation | Cellular | Tissue | Tissue |
| Special feature | Canal system | Cnidoblasts | Comb plates |
| Example | Sycon | Aurelia, Hydra | Pleurobrachia |
Practice questions
- 1What is the osculum?
- 2Name the two body forms of cnidarians.
- 3What are nematocysts?
- 4Give one example of Ctenophora.
- 5What makes up the skeleton of sponges?
- 6What is bioluminescence?
- 7How does the canal system help a sponge?
- 8Why is Obelia said to show alternation of generations?
Show answers (try them yourself first)
- 1The large opening of a sponge through which water goes out.
- 2Polyp and medusa.
- 3Stinging capsules in the cnidoblasts of cnidarians.
- 4Pleurobrachia (or Ctenoplana).
- 5Spicules or spongin fibres.
- 6The ability of a living organism to give out light.
- 7Water flow brings in food and oxygen, and carries out wastes.
- 8It has both polyp and medusa forms; polyps produce medusae asexually and medusae produce polyps sexually.
Phyla Platyhelminthes, Aschelminthes and Annelida
A farmer's best friend is the earthworm that loosens soil. In contrast, a liver fluke in cattle or a round worm in a child's gut takes food from its host. These are all worm-like animals, but their bodies are very different.
Platyhelminthes are flat, unsegmented acoelomate worms. Aschelminthes are round, pseudocoelomate worms. Annelida are segmented coelomate worms with a closed circulatory system.
In the figure : Three groups of worms and their key difference.
Rule 1: Platyhelminthes and Aschelminthes — Flatworms are dorso-ventrally flattened, bilateral and triploblastic, with hooks and suckers in parasitic forms and flame cells for excretion; examples: Taenia (tapeworm), Fasciola (liver fluke), and the regenerating Planaria. Roundworms are circular in cross-section with a complete alimentary canal; sexes are separate; examples: Ascaris, Wuchereria (filarial worm) and Ancylostoma (hookworm).
- Easy : Taenia lives inside the gut of a host.
- Medium : Wuchereria causes filariasis; Ascaris is an intestinal roundworm.
- Exam-type : Q: Which cells help flatworms in excretion? A: Flame cells.
Rule 2: Annelida — Annelids have a body divided into rings (metameric segmentation), longitudinal and circular muscles, and a closed circulatory system. Nephridia remove wastes and also help in osmoregulation. Nereis is an aquatic form with parapodia and has separate sexes; earthworm (Pheretima) and leech (Hirudinaria) are hermaphrodite.
- Easy : An earthworm is a hermaphrodite.
- Medium : Nereis has paddle-like parapodia for swimming.
- Exam-type : Q: What are the excretory organs of annelids? A: Nephridia.
Flat = Platyhelminthes, round = Aschelminthes, ringed = Annelida.
Calling the roundworm segmented. Roundworms have no true segmentation.
Memory clue: P-A-A — Flat, round, ringed.
Spot the difference
| Point | Platyhelminthes | Aschelminthes | Annelida |
|---|---|---|---|
| Shape | Flat | Round | Ringed |
| Body cavity | Absent | Pseudocoelom | True coelom |
| Example | Taenia | Ascaris | Pheretima |
Practice questions
- 1Why are Platyhelminthes called flatworms?
- 2Name the excretory cells of flatworms.
- 3Give two examples of roundworms.
- 4What is the body cavity of annelids called?
- 5Name the organ that removes wastes in annelids.
- 6What are parapodia?
- 7Why are parasites like Taenia provided with hooks and suckers?
- 8State two points of difference between Aschelminthes and Annelida.
Show answers (try them yourself first)
- 1Their body is flattened from top to bottom (dorso-ventrally).
- 2Flame cells.
- 3Ascaris and Wuchereria (or Ancylostoma).
- 4True coelom.
- 5Nephridia.
- 6Paddle-like lateral outgrowths of Nereis used for swimming.
- 7They help the parasite hold on to the host's gut.
- 8Aschelminthes are unsegmented with a pseudocoelom, while annelids are segmented with a true coelom and a closed circulatory system.
Phyla Arthropoda and Mollusca
A honey bee, a prawn and a spider all have jointed legs, while a snail, an octopus and an oyster have soft bodies. Together these two phyla include the largest number of animal species.
Arthropoda is the largest phylum (more than two-thirds of all named species). Its members have a chitinous exoskeleton and jointed appendages. Mollusca is the second largest phylum; members have a soft body usually covered by a calcareous shell.
In the figure : Arthropoda have jointed legs; Mollusca have a soft body with a shell.
Rule 1: Arthropoda — The body has a head, thorax and abdomen, and a chitinous exoskeleton. The circulatory system is open. Respiration is by gills, book gills, book lungs or tracheal system. Excretion is by Malpighian tubules. Examples: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect), Anopheles, Culex and Aedes (vectors of diseases), Locusta (locust) and Limulus (king crab, a living fossil).
- Easy : A mosquito has three pairs of jointed legs.
- Medium : Honey bee and silkworm are useful insects; locust is a pest.
- Exam-type : Q: Name the excretory organs in insects. A: Malpighian tubules.
Rule 2: Mollusca — The body has a head, a muscular foot and a visceral hump covered by the mantle. Feather-like gills (ctenidia) are used for respiration, and the mouth has a rasping organ, the radula. Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia and Dentalium.
- Easy : A pearl oyster makes pearls inside its shell.
- Medium : An octopus is a mollusc without an external shell.
- Exam-type : Q: Which structure in molluscs is used for scraping food? A: Radula.
Arthropods: jointed appendages and exoskeleton of chitin. Molluscs: soft body, mantle, shell, radula.
Thinking that spiders are insects. Insects have three pairs of legs, whereas spiders are arachnids with four pairs.
Memory clue: A-J / M-S — Arthropod = Jointed, Mollusc = Soft.
Spot the difference
| Point | Arthropoda | Mollusca |
|---|---|---|
| Skeleton | Chitinous exoskeleton | Calcareous shell |
| Circulation | Open | Open |
| Special organ | Malpighian tubules | Radula |
| Example | Apis, Locusta | Pila, Octopus |
Practice questions
- 1Which is the largest animal phylum?
- 2What is the exoskeleton of arthropods made of?
- 3Name two vectors of disease in Arthropoda.
- 4Which phylum has a mantle and radula?
- 5Give two economically important insects.
- 6What is the living fossil in Arthropoda?
- 7How does the mantle help a mollusc?
- 8State two differences between arthropods and molluscs.
Show answers (try them yourself first)
- 1Arthropoda.
- 2Chitin.
- 3Anopheles and Culex (or Aedes).
- 4Mollusca.
- 5Apis (honey bee) and Bombyx (silkworm); Laccifer (lac insect).
- 6Limulus (king crab).
- 7It covers the visceral hump and secretes the shell.
- 8Arthropods have jointed legs and a chitinous exoskeleton; molluscs have a soft body with a calcareous shell and a muscular foot.
Phyla Echinodermata and Hemichordata
A starfish at the beach is rough to touch because of tiny spines in its skin. It moves with hundreds of tiny tube feet filled with water.
Echinodermata (spiny-skinned animals) are marine animals with an endoskeleton of calcareous ossicles and a water vascular system. Hemichordata are worm-like marine animals with a proboscis, collar and trunk.
In the figure : The three parts of the body of Balanoglossus, a hemichordate.
Rule 1: Echinodermata — They are exclusively marine. Adults are radially symmetrical but larvae are bilateral. A water vascular system helps in locomotion, capturing and transporting food, and respiration. There is no excretory system. Sexes are separate, fertilisation is external and development is indirect with a free-swimming larva. Examples: Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).
- Easy : A starfish moves by tube feet that work with water pressure.
- Medium : A sea urchin has a spiny body.
- Exam-type : Q: What is the function of the water vascular system? A: Locomotion, food capture and transport, and respiration.
Rule 2: Hemichordata — Hemichordates were once part of Chordata but are now a separate non-chordate phylum. They have a stomochord, a rudimentary structure in the collar region similar to the notochord. The circulatory system is open, respiration is by gills and the excretory organ is the proboscis gland. Examples: Balanoglossus and Saccoglossus.
- Easy : Balanoglossus lives in sand burrows of the sea.
- Medium : The stomochord looks a little like a notochord but is not one.
- Exam-type : Q: Name the excretory organ of hemichordates. A: Proboscis gland.
Echinoderms have spiny skin and a water vascular system. Hemichordates have proboscis, collar, trunk.
Treating the stomochord as a true notochord. It is only similar to a notochord.
Memory clue: E-W — Echinoderms use Water for tube feet.
Spot the difference
| Point | Echinodermata | Hemichordata |
|---|---|---|
| Skeleton | Calcareous ossicles | None specific |
| Symmetry | Radial (adult) | Bilateral |
| Example | Asterias, Echinus | Balanoglossus |
Practice questions
- 1What does Echinodermata mean?
- 2Name two echinoderms.
- 3What is the stomochord?
- 4Name the three body parts of Balanoglossus.
- 5What is the habitat of echinoderms?
- 6What type of fertilisation do echinoderms have?
- 7Why were hemichordates removed from Chordata?
- 8What is meant by saying that the larva of a starfish is bilateral and the adult is radial?
Show answers (try them yourself first)
- 1Spiny-skinned.
- 2Asterias (starfish) and Echinus (sea urchin).
- 3A rudimentary structure in the collar region of hemichordates, similar to a notochord.
- 4Proboscis, collar and trunk.
- 5Exclusively marine.
- 6External fertilisation.
- 7They do not have a true notochord, only a stomochord, so they are placed under non-chordates.
- 8The larval stage is bilaterally symmetrical and the adult becomes radially symmetrical.
Phylum Chordata: protochordates, jawless fishes and fishes
A tiny lancelet in the sea and a huge whale seem to have little in common, yet both have a rod along the back at some stage of life. That rod, the notochord, is the mark of all chordates.
Chordates have a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits and a post-anal tail at some stage. The phylum has three subphyla: Urochordata, Cephalochordata and Vertebrata.
In the figure : The three subphyla of Chordata.
Rule 1: Protochordates and vertebrates — In Urochordata the notochord is only in the larval tail; in Cephalochordata it extends from head to tail and lasts through life. In Vertebrata the notochord is replaced in the adult by a cartilaginous or bony vertebral column, so all vertebrates are chordates, but not all chordates are vertebrates. Vertebrates have a ventral muscular heart with 2, 3 or 4 chambers, kidneys and paired fins or limbs.
- Easy : Ascidia is a urochordate.
- Medium : A fish and a frog are vertebrates and also chordates.
- Exam-type : Q: Are all chordates vertebrates? A: No, urochordates and cephalochordates are not vertebrates.
Rule 2: Cyclostomata, Chondrichthyes and Osteichthyes — Cyclostomes (Petromyzon, Myxine) are jawless, circular-mouthed, sucking parasites with no scales. Chondrichthyes are marine cartilaginous fishes with a ventral mouth, placoid scales and no air bladder; examples: Scoliodon, Carcharodon (great white shark), Trygon (sting ray). Osteichthyes are bony fishes with an operculum covering the gills and an air bladder; examples: Exocoetus (flying fish), Hippocampus (sea horse), Labeo (rohu), Catla, Clarias (magur), Betta and Pterophyllum.
- Easy : A shark has a skeleton of cartilage.
- Medium : Rohu has bony scales and a gill cover.
- Exam-type : Q: Which class has the air bladder? A: Osteichthyes (bony fishes).
Four chordate features: notochord, dorsal hollow nerve cord, gill slits, post-anal tail.
Saying that all chordates keep the notochord for life. In vertebrates it is replaced by the backbone.
Memory clue: N-D-G-T — Notochord, Dorsal nerve cord, Gill slits, Tail.
Spot the difference
| Point | Chondrichthyes | Osteichthyes |
|---|---|---|
| Skeleton | Cartilage | Bone |
| Gill cover | Absent | Operculum present |
| Air bladder | Absent | Present |
| Example | Shark, sting ray | Rohu, sea horse |
Practice questions
- 1Name the four chief features of chordates.
- 2Which subphylum has a vertebral column?
- 3Give one example of Urochordata.
- 4What type of skeleton does a shark have?
- 5Name two jawless vertebrates.
- 6Give two examples of bony fishes.
- 7Why are protochordates not called vertebrates?
- 8Compare chordates and non-chordates in terms of nerve cord and heart.
Show answers (try them yourself first)
- 1Notochord, dorsal hollow nerve cord, paired pharyngeal gill slits and a post-anal tail.
- 2Vertebrata.
- 3Ascidia (or Salpa, Doliolum).
- 4A cartilaginous skeleton.
- 5Petromyzon (lamprey) and Myxine (hagfish).
- 6Labeo (rohu) and Exocoetus (flying fish); Catla, Clarias or Hippocampus also.
- 7They lack a backbone; the notochord is only in the tail of Urochordata or runs the length of the body in Cephalochordata.
- 8Chordates have a dorsal, hollow, single nerve cord and a ventral heart; non-chordates have a ventral, solid, double nerve cord and a dorsal heart if present.
Amphibia, Reptilia, Aves and Mammalia
A frog in a monsoon puddle, a lizard on a wall, a crow on the roof and a cat at your door each belong to a different class of vertebrates. Their skin gives it away: moist, scaly, feathered and hairy.
Four classes of land vertebrates (Tetrapoda): Amphibia (live in water and on land), Reptilia (creeping animals with dry scaly skin), Aves (birds with feathers and wings) and Mammalia (animals with mammary glands and hair).
In the figure : Skin covering is the quick clue to each class of land vertebrates.
Rule 1: Amphibia and Reptilia — Amphibians have a moist skin without scales, a three-chambered heart and a cloaca; fertilisation is external, and they lay eggs in water. Examples: Rana (frog), Bufo (toad), Hyla and Salamandra. Reptiles have dry cornified skin with scales, lay shelled eggs and have a three-chambered heart (four in crocodiles). Examples: Chelone (turtle), Testudo (tortoise), Calotes, Naja (cobra), Bangarus (krait) and Crocodilus.
- Easy : A frog has a moist skin that helps breathing.
- Medium : A snake sheds its dry scaly skin.
- Exam-type : Q: How many chambers does the heart of a frog have? A: Three.
Rule 2: Aves and Mammalia — Birds have feathers, forelimbs modified into wings, a beak without teeth, air-filled (pneumatic) bones and a four-chambered heart; examples: Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich). Mammals have mammary glands, hair, external ears (pinnae), a four-chambered heart and are mostly viviparous; examples: Ornithorhynchus (platypus, egg-laying), Macropus (kangaroo), Pteropus (flying fox), Canis (dog), Delphinus (dolphin), Balaenoptera (blue whale).
- Easy : A pigeon has hollow bones that make flight easier.
- Medium : A whale lives in the sea but is a mammal as it feeds milk to its young.
- Exam-type : Q: Which mammal lays eggs? A: Ornithorhynchus (platypus).
Heart chambers: fishes 2, amphibians 3, reptiles 3 (crocodile 4), birds 4, mammals 4.
Calling a bat a bird or a whale a fish. Bats and whales are mammals because they have mammary glands.
Memory clue: 2-3-3-4-4 — Heart chambers: Fish-Amphibian-Reptile-Bird-Mammal.
Spot the difference
| Class | Skin or cover | Heart chambers |
|---|---|---|
| Amphibia | Moist, no scales | 3 |
| Reptilia | Dry, scaly | 3 (4 in crocodile) |
| Aves | Feathers | 4 |
| Mammalia | Hair | 4 |
Practice questions
- 1What is the body covering of birds?
- 2Name the class of animals that has mammary glands.
- 3How many chambers does the heart of a bird have?
- 4Give two examples of reptiles.
- 5Which egg-laying mammal is found in Australia?
- 6What does the cloaca do in amphibians?
- 7Why is a bat not a bird although it flies?
- 8Which features of birds help them to fly?
Show answers (try them yourself first)
- 1Feathers.
- 2Mammalia.
- 3Four.
- 4Naja (cobra) and Calotes (garden lizard); turtle, tortoise or crocodile also.
- 5Ornithorhynchus (the platypus).
- 6It is a common opening for the digestive, urinary and reproductive tracts.
- 7It has hair, mammary glands and external ears, which are mammalian features.
- 8Feathers, wings, a beak, and air-filled (pneumatic) bones that reduce body weight.