Sorting Animals into Groups
FoundationScientists classify animals by body features: first vertebrates and invertebrates, then into mammals, birds, reptiles, amphibians and fish by body covering, breathing and reproduction.
- Animals are first split into vertebrates and invertebrates by the backbone.
- Vertebrates include mammals, birds, reptiles, amphibians and fish.
- Classification is based on body covering, breathing and reproduction.
文體小知識：自然觀察說明文教我們按特徵觀察和整理，學會像科學家一樣為動物分類。
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1There are more than one and a half million kinds of animals in the world. To make study easier, scientists sort them by body features. The first step is the backbone: animals with a backbone inside are vertebrates, such as dogs, sparrows and koi; those without are invertebrates, such as ants, earthworms and jellyfish.
2Vertebrates are then divided further. Mammals have hair, breathe with lungs, give birth to live young and feed them milk, and are warm-blooded. Birds have feathers and beaks, lay eggs, and are warm-blooded too. Reptiles are covered with dry scales, breathe with lungs, lay eggs on land, and are cold-blooded. Amphibians have moist skin; their young breathe with gills and the adults breathe with lungs and skin. Fish live in water, breathe with gills, and have scales and fins.
3Classification depends on body features, not on where an animal lives or what colour it is. For example, whales and dolphins live in the sea and look like fish, but they breathe with lungs and feed their young milk, so they are mammals. Bats can fly, but they have hair and suckle their young, so they are not birds either.
4Knowing how to classify helps us understand and remember animals, and makes looking things up easier. When observing an animal, ask questions in order: Does it have a backbone? What covers its body? How does it breathe? How does it reproduce? Following the features step by step puts every animal into the right group.
1Summary 重點摘要
Scientists classify animals by body features: first vertebrates and invertebrates, then into mammals, birds, reptiles, amphibians and fish by body covering, breathing and reproduction.
2Vocabulary cards Vocabulary
Sorting things into groups according to their features.
An animal with a backbone inside, such as a dog, sparrow or fish.
An animal without a backbone, such as an ant, earthworm or jellyfish.
A warm-blooded animal with hair that breathes with lungs, bears live young and feeds them milk.
A cold-blooded animal covered with dry scales that breathes with lungs and lays eggs on land, such as turtles and snakes.
An animal whose young breathe with gills in water and whose adults breathe with lungs and moist skin, such as frogs.
The breathing organ fish and young amphibians use to take oxygen from water.
An animal that keeps a steady body temperature, such as mammals and birds, staying active even in cold weather.
An animal whose body temperature changes with its surroundings, such as reptiles and fish; it basks in the sun to warm up.
3Step by step Steps
- Step 1: Split by backbone firstThe first step in classifying animals is to look for a backbone inside the body. Goldfish and birds have backbones, so they are vertebrates; ants and earthworms have none, so they are invertebrates. The backbone is like the pillar of the body. Splitting animals into these two big groups first makes the finer sorting much easier. There are more than one and a half million kinds of animals in the world; splitting them into two big groups first keeps the study orderly.
- Step 2: Look at the body coveringThe outer covering of the body gives important clues. A dog is covered with hair that keeps it warm, so it is a mammal; a sparrow is clothed in feathers that help it fly, so it is a bird; a turtle has dry scales that protect its body, so it is a reptile. A frog's skin is moist and smooth, with neither hair nor scales, so we must check its other features. A magnifying glass helps the observation; the more clearly we see, the more accurate the features we grasp.
- Step 3: Check how it breathesHow an animal breathes reveals its true identity. Although the whale lives in the sea, it must surface regularly to breathe air through its blowhole, which shows it breathes with lungs and is a mammal; the shark lives in water all its life and takes oxygen from the water with gills, so it is a fish. The closer we look, the more accurate our sorting is. Amphibians are a special case: the young breathe with gills, and the adults breathe mainly with lungs and moist skin.
- Step 4: Check how it reproducesHow an animal reproduces is another key piece of evidence. A mother cat bears live kittens and feeds them milk — this is called live birth, a feature of mammals; a hen lays eggs which hatch into chicks — this is called egg-laying, the way of birds. Most fish, amphibians and reptiles lay eggs, while mammals almost always bear live young. Reptiles and birds both lay eggs, but birds incubate them and care for the chicks, while most reptiles leave once the eggs are laid.
- Step 5: Verify with all the featuresOne feature alone can lead us astray, so put several features together to check. A frog has moist skin; as a young tadpole it lives in water and breathes with gills; as an adult it breathes mainly with lungs and skin and can live both in water and on land — exactly the features of an amphibian. Only when the features agree is the conclusion reliable. Bats fly but have hair and feed their young milk, so they are mammals — proof that one feature alone easily leads to a wrong conclusion.
- Step 6: Make your own classification chartOnce you know the method, make a classification chart of your own. Draw a table listing the body covering, breathing and reproduction of a cat, a pigeon, a snake, a frog and a goldfish, then write which group each belongs to. Making the chart helps you remember the knowledge in an orderly way, and you can check it against your classmates. After finishing, compare answers with classmates, and re-check the facts wherever you disagree — the learning works even better.
4Diagrams Diagrams
5Worked example Example
Example: Yan says, Dolphins live in the sea, so they are fish. Is she right? Explain using the features of mammals and fish.
- Fish take oxygen from water with gills; dolphins must surface regularly to breathe air with lungs.
- Most fish reproduce by laying eggs; dolphins bear live young and feed them milk.
- A dolphin's skin has no scales, and its tail fin is built quite differently from a fish's.
- Dolphins have sparse hair, a steady body temperature and milk-feeding — every feature of a mammal.
- Answer: Yan is not right. Living in the sea does not make an animal a fish: dolphins breathe with lungs, bear live young and feed them milk, and have no scales — every feature matches a mammal, so dolphins are mammals, not fish.
6Common mistakes Mistakes
7Quick check Quick check
Q1How do we tell vertebrates from invertebrates?
Q2What are the main features of mammals?
Q3What do fish breathe with?
Q4How do young and adult frogs breathe?
Q5Why is a bat not a bird?
8Recap Recap
- Animals are first split into vertebrates and invertebrates by the backbone.
- Vertebrates include mammals, birds, reptiles, amphibians and fish.
- Classification is based on body covering, breathing and reproduction.
- Whales and dolphins are mammals; bats are mammals too.
- Ask in order: backbone? body covering? breathing? reproduction?