ICSE Class 6 Biology Chapter 4 Classification of Animals

Classification of Animals

Learning Objectives

After completing this chapter you will be able to

Classify animals into invertebrates and vertebrates.

Classify invertebrates into important groups.

Give characteristic features of each invertebrate group with examples.

Classify vertebrates into important classes.

Give characteristic features of each vertebrate class with examples.

Differentiate between invertebrates and vertebrates.

Introduction

We see many animals around us. One thing which is common to all animals is that they depend on plants and other animals for their food. Most animals can move from one place to another. Animals can be classified into two broad groups - invertebrates and vertebrates, based on the absence or presence of backbone or the vertebral column.

Animals without a backbone are called invertebrates. Animals with a backbone are called vertebrates. There are more invertebrates than vertebrates. However, most of the animals we see around us are probably vertebrates.

Classification of Invertebrates

Based on their different characteristics, invertebrates are classified into many groups (Fig. 4.1). Let us study each group separately.

Sponges (Poriferans)

Sponges or poriferans are animals of the Phylum Porifera. These animals are also called porous animals. Sponges (Fig. 4.2) are mostly found in salty sea water, however, a few are found in fresh water also. Most sponges remain fixed to the bottom of the sea or the pond or to some object.

Sponges do not have nervous, digestive or circulatory systems. Instead most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. They have many small pores all over the body. Water along with food enters the body cavity through these pores. There is also a large pore present at the top from where the water passes out. Other than sponges, some examples of poriferans are Sycon and Spongilla.

Cnidarians

These animals are found in aquatic, mostly marine environment. They have radially symmetrical body, that is, the body can be divided into two identical halves along any plane. The body is tube-like and has only one opening called mouth. The mouth is surrounded by a number of finger-like structures called tentacles. Tentacles are used for capturing prey. Some examples are Hydra, jellyfish (Fig. 4.3) and sea anemone.

Platyhelminthes (Flatworms)

Flatworms are the simplest worms and found in marine and fresh water. The most common flatworm is the tapeworm (Fig. 4.4). The other example includes Planaria and liver fluke. The body is bilaterally symmetrical, that is, it can be divided into two identical halves only through one plane. They have only one opening through which they take in food and get rid of wastes. Tapeworms and liver flukes are parasites, that is, they live inside or on another organism (called a host), and get their food from it. Tapeworms live in the digestive tract of many vertebrates including human beings.

Nemathelminthes (Roundworms)

Roundworms are found in fresh water, marine and terrestrial environments. Some are thread-like as filarial worm while others such as hookworm are more rounded. The body is unsegmented and bilaterally symmetrical. They have a mouth opening where food is pulled in and crushed. Another opening called anus is present near the tip of the body. More of these are parasites and live in human intestines. There they reproduce sexually by forming eggs. Some examples are pinworm, Ascaris (Fig. 4.5) and whipworm.

Annelids (Segmented worms)

Worms of this group are found in marine and fresh water. They have a soft, segmented and bilaterally symmetrical body. They have a body cavity. They have special organs of excretion called nephridia, for example, earthworm and leech (Fig. 4.6). Earthworms are considered as farmers' friends because while moving in the soil, they improve its fertility. Leech is a parasite and attaches itself to the body of the host and draws blood from there.

Arthropods (Animals with jointed legs)

Animals belonging to this group have a segmented body. The body is bilaterally symmetrical and divided into three regions - head, thorax and abdomen. The body of most animals has a tough outer covering called exoskeleton. All arthropods have paired appendages with many joints. Arthropods are further classified into four subgroups or classes (Fig. 4.7).

Arachnids

Head and thorax are fused.

Have four pairs of legs, and wings are absent. Some examples are scorpions and spiders.

Crustaceans

Head and thorax are fused.

Appendages are present on all segments.

Have variable number of jointed legs. Some examples are shrimps and lobsters.

Myriapods

Body is divided into many segments.

Some myriapods have two pairs of legs per segment (for example, millipede).

Some myriapods have one pair of legs per segment (for example, centipede). Some examples are millipedes and centipedes.

Insects

Body is divided into three regions - head, thorax and abdomen.

Most insects have three pairs of legs.

Have two pairs of wings. Some insects are wingless. Some examples are ants and butterflies.

Molluscs (Soft-bodied shelled animals)

This group consists of animals which have a soft, unsegmented body, often with an external hard shell. Some molluscs have a single shell. Others may have two shells that are attached on one side. Some molluscs, like octopus, have a shell inside their body. A snail has a coiled shell. Some examples are octopus, mussel, cuttlefish, squid, snail (Fig. 4.8) and oyster.

Echinoderms (Spiny-skinned animals)

This group consists of marine animals like starfish and sea urchins (Fig. 4.9). They have sharp spines all over the body. They have a radially symmetrical body. They have a mouth on the lower side and an anus on the upper side. They also have a large number of tiny tube-like feet that end in suckers.

Classification of Vertebrates

The animals belonging to this group have a distinct backbone. The backbone of vertebrates is not just one bone. It is made up of a number of separate bones called vertebrae. All vertebrates have a well-developed closed circulatory system which means that blood travels inside blood vessels to all parts of the body. The nervous system of vertebrates is more complex than that of invertebrates. Brain that controls many body functions is present in vertebrates. Vertebrates are further classified into five classes (Fig. 4.10).

Pisces (Fishes)

They have a streamlined body, that is, smooth and narrow at the front and the back. Such a body offers very little resistance while swimming (Fig. 4.11). They breathe with the help of gills. Majority of fish have scales which are overlapping flat plates that cover the animal's body and provide protection. They use fins to swim. Fish are cold-blooded animals, that is, the temperature of their body changes with the temperature of their surroundings. For example, seahorse, carp, shark, ray fish, pomphret and eel.

Amphibia (Frogs, toads and salamanders)

These animals can live both on land and in water. They are cold-blooded animals. They have thin, moist and slippery skin. They breathe through lungs and the skin. Amphibians return to water in order to reproduce and always lay their eggs in water. Some examples are frog, toad and salamander (Fig. 4.12).

Reptilia (Lizards and snakes)

Reptiles are cold-blooded vertebrates. Most of them spend their life on land. They have dry skin covered with scales. They breathe through lungs. They lay eggs that have leathery shells. For example, snake (Fig. 4.13), lizard, turtle, tortoise, alligator and crocodile.

Aves (Birds)

They are warm-blooded animals, that is, their body temperature remains same and is not affected by the temperature of their surroundings. They are the only animals whose body is covered with feathers. Their forelimbs are modified into wings and hindlimbs are adapted for walking, perching or swimming (Fig. 4.14). They breathe through lungs. The skeleton of birds is very light because their bones have large air cavities. They have horny beak and do not have teeth. They lay eggs which have hard shell. For example, crow, ostrich, peacock, parrot and pigeon.

Mammalia (Animals with mammary glands)

They are warm-blooded and have hair on their body. They give birth to babies and are the only animals that produce milk for their young ones. They have mammary glands to produce milk. Most mammals have external ears, nose and possess specialized teeth. They have specialized teeth. They breathe through lungs. They use their limbs to walk, climb, swim, and fly. Some land mammals have toes that produce claws and hooves for climbing and running. Aquatic mammals such as whales and dolphins have fins which evolved from legs. For example, mouse, human, dog, tiger and bat (Fig. 4.15).

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