NSB31 Recap

Classification: animal groups and keys

Vertebrates and invertebrates

Animals split first by whether they have a backbone. Vertebrates have an internal skeleton with a backbone; invertebrates do not. Around 97% of animal species are invertebrates.

The five vertebrate classes

  • Fish — live in water, breathe with gills, have scales and fins, lay eggs in water, cold-blooded.
  • Amphibians — frogs, toads, salamanders. Moist skin without scales, lay jelly-covered eggs in water, young breathe with gills and adults with lungs, cold-blooded.
  • Reptiles — snakes, lizards, turtles, crocodiles. Dry scaly skin, breathe with lungs, lay leathery-shelled eggs on land, cold-blooded.
  • Birds — feathers, beak, wings, lay hard-shelled eggs, warm-blooded.
  • Mammals — hair or fur, feed young on milk, breathe with lungs, warm-blooded. Almost all give birth to live young; the platypus and echidna (monotremes) lay eggs, and kangaroos (marsupials) raise tiny young in a pouch.

Cold-blooded (ectothermic) animals take their body temperature from their surroundings. Warm-blooded (endothermic) animals keep a constant temperature using energy from food.

Invertebrate groups

The main invertebrate phyla you need:

  • Arthropods — jointed legs and a hard exoskeleton. Includes insects (6 legs, 3 body parts), arachnids (8 legs, 2 body parts — spiders, scorpions), crustaceans (crabs, prawns) and myriapods (centipedes, millipedes).
  • Molluscs — soft body, often with a shell: snails, slugs, octopus.
  • Annelids — segmented worms such as earthworms.
  • Cnidarians — jellyfish, coral and sea anemones, with stinging tentacles.
  • Echinoderms — spiny-skinned sea creatures such as starfish and sea urchins.

Plants can be grouped too: flowering plants, conifers (cones), ferns (spores, with true roots and stems) and mosses (spores, no true roots).

Dichotomous keys

A dichotomous key identifies an organism by a series of questions, each with exactly two choices (“di” means two). Every answer either names the organism or sends you to another numbered question. To use one, start at question 1 every time, look carefully at the organism, and follow the numbers — never skip ahead.

To write a key, pick a feature that splits the group into two, then keep splitting each smaller group until every organism is on its own. Use clear, structural, yes-or-no features (“has six legs” rather than “is big”). A key for nn organisms needs n−1n - 1 questions.