Classification of Kingdom Animalia
Senior 2 · Biology
Classification of Kingdom Animalia
Lesson 2 of 5
20 min
Let's begin
On dark nights on Lake Kivu, fishers row out in three-hulled boats and hang bright lamps over the water. Shoals of small silver fish called isambaza swim up towards the light and are scooped into nets. Meanwhile, in the rice fields of the Bugarama valley, frogs call loudly after the rain.
Isambaza and frogs look nothing alike, yet both have a backbone. They belong to the same phylum as you: Chordata. This lesson explains what a chordate is, and how to tell fish from amphibians.
At some stage of life, every chordate has these four features:
You had all four as an early embryo. In vertebrates, the notochord is replaced during development by a backbone made of small bones called vertebrae, and the brain is protected by a skull.
Vertebrates are divided into five classes: fish (Pisces), amphibians, reptiles, birds (Aves) and mammals. One more pair of words helps here:
Fish fall into two main groups. Cartilaginous fish (sharks and rays) have a skeleton of cartilage and no operculum; almost all live in the sea. Bony fish have a skeleton of bone, an operculum and usually a swim bladder, a gas-filled bag that lets them float at a chosen depth. Every fish in Rwanda's lakes and rivers is a bony fish, including Nile tilapia, which is farmed in ponds and cages; isambaza; and the African catfish found in marshes.
| Fin | Paired or single? | Main job |
|---|---|---|
| Caudal (tail) fin | single | pushes the fish forward and helps it steer |
| Dorsal and anal fins | single | keep the fish upright and stop it rolling or swinging from side to side |
| Pectoral and pelvic fins | paired | steering, braking, and moving up or down |
Question: An animal from Lake Kivu has scales, fins, a flap covering its gills, and a line of small pores along each side. Name its class and group, and explain.
Solution: Fins and gills tell you it is a fish (class Pisces). The flap over the gills is an operculum, which only bony fish have, so it is a bony fish. The line of pores is the lateral line, which fish use to sense vibrations.
The name amphibian means "double life": most amphibians begin life in water and move to land as adults.
There are three groups: frogs and toads (no tail as adults, long hind legs for jumping, webbed feet); salamanders and newts (a tail as adults); and caecilians (legless and worm-like, living in damp soil).
| Feature | Fish | Amphibian (adult) |
|---|---|---|
| Skin | scales and mucus | moist, no scales |
| Breathing | gills | lungs, skin and mouth lining |
| Limbs | fins | usually four legs |
| Habitat | water only | water and land |
| Eggs | laid in water | laid in water, jelly coat, no shell |
Tip
Memory hook: a frog needs water twice, once to keep its skin moist so it can breathe, and once to breed.
Exam technique: "operculum" is a bony-fish word. Sharks have gill slits but no operculum, so do not write it for them.
Name (a) the tail fin and (b) the single fin on the back of a fish.
Solution: (a) the caudal fin; (b) the dorsal fin.
Explain how the lateral line helps a fish survive.
Solution: The lateral line senses vibrations and pressure changes in the water. This lets the fish detect prey and approaching predators, even in dark or muddy water, and keep its place in a shoal.
Give three differences between a tadpole and an adult frog.
Solution: Any three: the tadpole breathes with gills, the adult with lungs and skin; the tadpole has a tail, the adult has none; the tadpole has no legs at first, the adult has four; the tadpole lives only in water, the adult lives in water and on land; the tadpole mainly eats algae, the adult eats insects.
A frog left in hot sun dries out and dies, even though its lungs are healthy. Explain why.
Solution: A frog takes in much of its oxygen through its skin. Oxygen must first dissolve in the moisture on the skin before it can diffuse into the blood. When the skin dries, this gas exchange stops, and the lungs alone cannot supply enough oxygen.
Summary
Sources and credits
The Rwandan examples, worked examples and practice questions are written by Ganzaa.
