Animals with backbones are called vertebrates. Vertebrates are classified into five groups: fish, amphibians, reptiles, birds, and mammals. Each group has characteristic habitats and body structures.
Basic Knowledge
The five groups are mainly distinguished by their method of respiration, body surface, temperature regulation, and way of giving birth.
① Fish: live in water, gill respiration, scales, ectothermic, oviparous (eggs without shells, laid in water). Examples: medaka, sharks.
② Amphibians: larvae use gill respiration in water; adults use lungs and skin, moist skin, ectothermic, oviparous (eggs without shells, laid in water). Examples: frogs, newts.
③ Reptiles: live on land, lung respiration, scales or shells, ectothermic, oviparous (eggs with shells, laid on land). Examples: snakes, lizards, turtles.
④ Birds: live on land and in the air, lung respiration, feathers, endothermic, oviparous (eggs with shells). Examples: pigeons, penguins, ostriches.
⑤ Mammals: live on land (some in water), lung respiration, body hair, endothermic, viviparous, and raise their young with milk. Examples: dogs, whales, bats, humans.
Vertebrates (the general term for animals with backbones)
Ectothermic animals (animals whose body temperature changes with the outside temperature; fish, amphibians, and reptiles)
Endothermic animals (animals that maintain a constant body temperature; birds and mammals)
Viviparity (young develop inside the mother's body to a certain extent before birth; mammals)
Oviparity (being born from eggs; fish, amphibians, reptiles, and birds)
Gill and lung respiration (gills are used in water and lungs on land; amphibians use both)
Further Study (Background and Significance)
Whales and dolphins look like fish, but they are mammals because they have all the characteristics of mammals: lung respiration, endothermy, viviparity, and raising their young with milk. Bats can fly, but they are mammals. Penguins swim well, but they are birds because they have feathers and lay eggs. Platypuses lay eggs, but they are mammals (a primitive group) because they raise their young with milk.
These examples show the importance of classifying animals by body structure and how they are born, not by appearance.
Amphibians represent an intermediate stage in evolution from “water to land” and provide clues for considering the history of life's evolution.
- Vertebrates=five groups: fish, amphibians, reptiles, birds, and mammals
- Respiration: fish=gills, amphibians=gills→lungs+skin, reptiles, birds, and mammals=lungs
- Body temperature: fish, amphibians, and reptiles=ectothermic, birds and mammals=endothermic
- Way of giving birth: mammals=viviparity, all others=oviparity
- Body surface: fish=scales, amphibians=moist skin, reptiles=scales and shells, birds=feathers, mammals=body hair
- Eggshells: fish and amphibians=none (water), reptiles and birds=present (land)
- Whales, dolphins, and bats are mammals; penguins are birds
Points of Caution (Easily Confused)
① Whales and dolphins are not fish; they are mammals. ② Newts are amphibians, while geckos are reptiles (their names are similar, but they belong to different groups). ③ Platypuses are mammals but are oviparous (primitive). ④ Amphibian eggs have no shells, so they must be laid in water.
Practice
- Name all five groups of vertebrates.
- What group are whales classified into? Give the reason as well.
- How does the method of respiration change between amphibian larvae and adults?