Classify organisms using the features identified for vertebrates and arthropods.

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IGCSE Biology 0610 – Features of Organisms

Learning objective

Classify organisms by using the diagnostic features that distinguish the major animal groups covered in the syllabus – especially vertebrates and arthropods – and place them in the correct kingdom.

Key biological concepts

Species – a group of organisms that can inter‑breed and produce fertile offspring under natural conditions.
Binomial nomenclature – the two‑part scientific name of a species (Genus + specific epithet), e.g. Homo sapiens.

Why we classify

  • To organise the immense diversity of life.
  • To reflect evolutionary relationships (common ancestry).
  • Modern classification increasingly uses DNA sequencing together with morphological traits.

Overview of the five kingdoms (plus viruses)

Kingdom Cellular organisation Key diagnostic features Examples
Monera (Prokaryota) Unicellular, no nucleus Cell wall of peptidoglycan; reproduce by binary fission Escherichia coli, cyanobacteria
Protoctista Unicellular or simple multicellular, nucleus present Chloroplasts (or other pigments) in many; often motile Algae, amoeba, slime moulds
Fungi Multicellular (except yeasts), nucleus present Cell walls of chitin; absorptive nutrition Mushrooms, moulds, yeast
Plantae Multicellular, nucleus present Cell walls of cellulose; chloroplasts with chlorophyll a & b; alternation of generations Flowering plants, ferns, mosses
Animalia Multicellular, nucleus present Heterotrophic, no cell walls; specialised tissues & organs; usually motile at some life stage Vertebrates, arthropods, molluscs, etc.
Viruses (non‑cellular) Acellular; nucleic acid surrounded by a protein coat Obligate intracellular parasites; replicate only inside host cells Influenza virus, HIV

Animal kingdom – groups required for the syllabus

Vertebrates (sub‑phylum Vertebrata)

  • Mammals – hair or fur, mammary glands, three middle‑ear bones.
  • Birds – feathers, beak, lightweight hollow bones, hard‑shelled eggs.
  • Reptiles – dry scaly skin, ectothermic, usually lay amniotic eggs.
  • Amphibians – moist permeable skin, life‑cycle includes an aquatic larva (tadpole) and a terrestrial adult.
  • Fish – gills for aquatic respiration, fins, usually covered with scales.

Arthropods (phylum Arthropoda)

  • Myriapods – many‑legged (centipedes, millipedes); one pair of legs per body segment.
  • Insects – three distinct body regions (head, thorax, abdomen), three pairs of legs, usually two pairs of wings.
  • Arachnids – two body regions (cephalothorax & abdomen), four pairs of legs, no antennae.
  • Crustaceans – primarily aquatic, two pairs of antennae, often have gills and a hard carapace.

Diagnostic features of vertebrates

  • Endoskeleton of bone or cartilage (true internal skeleton).
  • Dorsal, hollow nerve cord (part of the central nervous system).
  • Closed circulatory system with a multi‑chambered heart.
  • True coelom (body cavity lined by mesoderm).
  • Complex organ systems – lungs or gills, specialised kidneys, liver, etc.
  • Development of a notochord at some embryonic stage.
  • Four pairs of limbs in tetrapods (or paired fins in fish) and a well‑developed brain.

Diagnostic features of arthropods

  • Exoskeleton of chitin that must be shed (ecdysis) for growth.
  • Segmented body with a ventral nerve cord and paired, jointed appendages.
  • Open circulatory system; dorsal heart pumps haemolymph into body cavities (haemocoel).
  • True coelom (although largely reduced by the haemocoel).
  • Respiratory structures vary:
    • Tracheae (insects)
    • Book lungs (arachnids)
    • External gills attached to the cuticle (many crustaceans)
  • Highly developed sensory organs – compound eyes, antennae, setae.

Side‑by‑side comparison

Characteristic Vertebrates Arthropods
Skeletal type Endoskeleton (bone or cartilage) Exoskeleton (chitin)
Nervous system Dorsal hollow nerve cord Ventral nerve cord with segmental ganglia
Circulatory system Closed; multi‑chambered heart Open; dorsal heart
Respiratory organs Lungs or gills within a body cavity Tracheae, book lungs, or external gills attached to the cuticle
Body segmentation Limited; vertebral column Clear segmentation – head, thorax, abdomen (or cephalothorax + abdomen)
Growth Continuous; no molting Growth by ecdysis (molting)
Limbs Typically four pairs (tetrapods) or paired fins Jointed appendages; number varies with group
Reproductive strategy Mostly internal fertilisation; live birth or egg laying Often external fertilisation; eggs protected by shells or gelatinous coats

Using features to classify an unknown animal

  1. Examine the external covering
    • Hard, chitinous exoskeleton that must be shed → Arthropod
    • Internal bony or cartilaginous skeleton → Vertebrate
  2. Locate the nerve cord
    • Dorsal, hollow → vertebrate
    • Ventral with segmental ganglia → arthropod
  3. Determine the circulatory type
    • Closed system, multi‑chambered heart → vertebrate
    • Open system, dorsal heart → arthropod
  4. Identify respiratory structures
    • Lungs or gills inside the body cavity → vertebrate
    • Tracheae, book lungs, or external gills attached to the cuticle → arthropod
  5. Observe growth pattern
    • Molting required → arthropod
    • Continuous growth without shedding → vertebrate
  6. Refine to lower taxonomic groups using the hallmark features listed in the “Vertebrate groups” and “Arthropod groups” sections (e.g., hair + mammary glands → Mammalia; three pairs of legs + wings → Insecta).

Example dichotomous key (vertebrate vs. arthropod)

1a. Body covered by a hard, chitinous exoskeleton that is shed …… Arthropod → go to 2
1b. Body supported by an internal skeleton of bone or cartilage …… Vertebrate → go to 3

2a. Two pairs of antennae, usually aquatic, with gills …… Crustacea
2b. One pair of antennae, terrestrial, three body regions (head‑thorax‑abdomen) …… Insecta / Arachnida / Myriapoda (use further couplets)

3a. Warm‑blooded, feathers, beak …… Bird
3b. Not warm‑blooded … go to 4

4a. Hair or fur, mammary glands …… Mammal
4b. Moist skin, life‑cycle includes aquatic larvae …… Amphibian
4c. Dry scaly skin, lay amniotic eggs …… Reptile
4d. Gills throughout life, fins, no limbs …… Fish

Suggested practical activities

  • Collect a small range of specimens (e.g., goldfish, frog, beetle, spider). Record the diagnostic features listed above.
  • Use the comparison table to decide whether each specimen is a vertebrate or an arthropod, then place it in the appropriate lower group.
  • Develop a more detailed dichotomous key for the five vertebrate classes and the four arthropod classes; test it with classmates.
  • Model molting in an arthropod (e.g., observe a house‑fly larva shedding its cuticle) and contrast it with growth in a vertebrate (e.g., fish fin development).
Suggested diagram: a side‑by‑side illustration of a typical vertebrate (frog) and an arthropod (beetle) with labelled features – skeleton type, nerve‑cord position, heart type, respiratory organ, and growth method.

Action‑oriented review against Cambridge IGCSE Biology 0610 (2026‑28)

Syllabus requirement How the notes meet it Targeted improvement (what to add / adjust)
1.3 Features of organisms – core: state the main features used to place animals and plants into the appropriate kingdoms Kingdom table lists cellular organisation and key diagnostic features for all five kingdoms and viruses. Emphasise the *three* core animal‑kingdom features (heterotrophic, no cell walls, specialised tissues) in a separate bullet list for quick recall.
1.4 Features of organisms – vertebrates: list at least six diagnostic characteristics Six‑plus bullet points provided under “Diagnostic features of vertebrates”. Number each feature (1‑7) and add a short one‑sentence justification (e.g., “Endoskeleton – provides internal support and attachment for muscles”).
1.5 Features of organisms – arthropods: list at least six diagnostic characteristics Six‑plus bullet points provided under “Diagnostic features of arthropods”. Number the points and include a quick note on why each is useful for classification (e.g., “Exoskeleton – must be shed, therefore indicates an arthropod”).
1.6 Classification – use a dichotomous key to identify an unknown animal as a vertebrate or arthropod and then to a lower group Step‑by‑step “Using features to classify an unknown animal” and a simple dichotomous key are included. Provide a printable two‑column key for the four arthropod classes and the five vertebrate classes; add a short activity prompt for students to complete it.
1.7 Practical skills – observation of external features, identification of skeleton type, molting, etc. Practical activity list covers specimen collection, feature recording, and molting observation. Insert a checklist of observable features (e.g., exoskeleton, antennae, number of limbs) that students can tick during the activity.
1.8 Examination technique – present information in clear tables/diagrams and answer structured questions All information is organised in tables, bullet points and numbered steps; a diagram suggestion is given. Add a concise “Exam‑style question” box at the end with a model answer, reinforcing how to use the tables under timed conditions.

Exam‑style practice question (example)

Question: A student finds an animal with a chitinous exoskeleton, jointed legs, and two pairs of antennae. Using the key provided, identify the phylum and class of the animal.

Answer (model): The hard chitinous exoskeleton that must be shed indicates an Arthropod. Two pairs of antennae and the presence of gills point to the class Crustacea.

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