Kingdom Animalia

A sycon, an earthworm, a cockroach, a starfish, a fish, a pigeon and a cow could hardly look more different. The sycon is a clump of yellow tubes fixed to a rock on the sea floor. The earthworm is a soft tube that burrows through damp soil. The cow is a large mammal grazing in a field. Still, all seven belong to one kingdom, Kingdom Animalia. They are grouped together because their bodies are built in the same basic way, even though their outer shapes are so different.

Every member of this kingdom shares three features:

• Their bodies are made of eukaryotic cells. A eukaryotic cell has a true nucleus wrapped in a nuclear membrane.
• Their cells have no cell wall. A plant cell is boxed in by a stiff wall of cellulose, but an animal cell has only a thin, flexible cell membrane around it.
• They are heterotrophs. No animal can make its own food, so every animal depends on plants or on other animals for food.

Inside this one kingdom there is enormous variety. Animals differ in body structure, shape and form. In some the body is very simple. A sponge, for example, has no tissues and no organs at all. In others, such as a cow or a human being, every organ system is fully developed, from the digestive system to the nervous system. Animals also differ in how their bodies are supported. Some have a hard frame inside the body, and some are covered by a hard shell on the outside.

Run your fingers down the middle of your back and you will feel a row of small bony bumps. That is your vertebral column, also called the backbone. It is a chain of small bones called vertebrae, and it protects the spinal cord that runs inside it. Whether an animal has a vertebral column or not is the first big division made inside Kingdom Animalia.

Sycon, earthworm, cockroach and starfish have no vertebral column in their bodies. Animals like these are called invertebrates. Most kinds of animals on Earth are invertebrates.

Fish, pigeon and cow all have a vertebral column. Animals like these are called vertebrates. Fishes, frogs, snakes, birds and mammals, including human beings, are all vertebrates.

The starfish and the fish show the difference clearly, even though one is named after the other. A fish has a backbone, a skull, fins for swimming and gills for breathing. A starfish has no backbone at all. Its body is a star of five arms spreading out from a centre, its skin is tough and spiny, and it creeps slowly over the sea floor on hundreds of tiny tube feet. It is called a starfish only because it is star shaped and lives in the sea.

Animals also differ in their skeleton. A fish, a pigeon and a cow have an endoskeleton, a framework of bones inside the body covered by muscles and skin. A cockroach has an exoskeleton, a hard outer covering that protects the soft body inside it like a suit of armour.

Kingdom Animalia holds millions of kinds of animals, so they are sorted into large groups called phyla. The singular is phylum. Animals in one phylum share the same basic body plan. According to the five kingdom system, the invertebrates are divided into eight phyla. Animals that have a notochord and a vertebral column are placed together in one phylum, Chordata. So there are nine phyla altogether in Kingdom Animalia.

The list runs roughly from the simplest body plan to the most complex. The first two phyla, Porifera and Coelenterata, have the simplest bodies of all animals, and they are described below.

Look closely at a sponge and you see that its whole body is riddled with holes. The name Porifera means pore bearer, and it fits: every member of this phylum has pores all over its body. Spongilla, Sycon and Euspongia are three such animals.

These animals are multicellular. They live in water, and most of them are marine, which means they live in the sea. A few, such as Spongilla, live in fresh water in ponds and streams. Water enters and leaves their bodies through their pores, and that is why they are placed in phylum Porifera. They are also called poriferans or sponges.

An adult sponge does not move from place to place. It stays attached to its substratum for the whole of its adult life, much as a plant stays in one spot. The soft bath sponge sold in shops is actually the dried skeleton of Euspongia, left behind after its living cells have been removed.

A sponge cannot chase its food, so it makes the food come to it. Its body wall is pierced by a great many tiny pores called ostia. The singular is ostium. At the top of the body there is one large opening called the osculum. Water is drawn in through the ostia, flows through narrow channels into a central cavity, and then leaves the body through the osculum.

This steady current of water is the sponge's lifeline. The water brings in tiny particles of food and dissolved oxygen. Cells lining the inner channels trap the food particles and take them in. The same current then carries carbon dioxide and waste out through the osculum. So nutrition, respiration and excretion in a sponge all depend on water flowing through its canal system.

The main features of the animals in phylum Porifera are these:

• They are the first multicellular animals.
• Tissues are not developed in them. Their cells do different jobs, but cells of the same kind are not grouped into tissues.
• Their body develops from two germ layers in the embryonic stage, so they are diploblastic animals.
• Their body has pores. The small pores are the ostia and the one large pore is the osculum. Water enters through the ostia and leaves through the osculum, forming the water canal system.
• They respire through the general body surface.
• Their adult forms are found attached to a substratum.
• They have a high power of regeneration. If the body breaks into pieces by accident, each piece can grow into a new sponge.
• They reproduce both asexually and sexually. Asexual reproduction takes place by budding and regeneration. Sexual reproduction takes place by the formation of gametes.
• Most of them are asymmetrical. Some forms are radially symmetrical.

Examples of Porifera: Sycon, Spongilla, Euspongia (the bath sponge), Leucosolenia, Hyalonema and Cliona.

Hydra, jellyfish and coral look nothing like sponges. Each has long, thin, finger-like outgrowths arising from its body, usually in a ring around the top. These outgrowths are called tentacles, and they help the animal to catch its food and to move.

Inside the body of these animals there is one large hollow space, like the inside of a bag. This cavity is called the coelenteron, and the phylum takes its name from it: Coelenterata means the animals with a coelenteron. The coelenteron does two jobs. Food is digested inside it, and from it the digested food is carried to all parts of the body. For this reason it is also called the gastrovascular cavity. Gastro refers to digestion and vascular refers to transport.

A hydra is a tiny freshwater animal, only about 1 centimetre long, that clings to water plants in ponds. Watching how it feeds shows every special part of a coelenterate at work. Its body has only one opening, the mouth, at the top. The mouth is surrounded by tentacles, and the tentacles are loaded with stinging cells called nematocysts.

When a small animal such as a water flea brushes against a tentacle, this is what happens:

• Step 1: The nematocysts shoot tiny coiled threads into the prey, and the sting paralyses or kills it.
• Step 2: The tentacles wrap around the prey and hold it tight.
• Step 3: The tentacles bend inwards and push the prey into the mouth.
• Step 4: The food is digested inside the coelenteron, and the digested food spreads to the cells of the body.
• Step 5: Undigested waste is thrown out through the same mouth, because there is no second opening.

The main features of the animals in phylum Coelenterata are these:

• They are multicellular and diploblastic.
• They are the first tissue grade animals. They have a hollow, vessel-like cavity inside the body called the coelenteron, which is also called the gastrovascular cavity because it carries out both digestion and transportation.
• The alimentary canal has a single opening, the mouth, which is surrounded by tentacles. Feeding and locomotion take place by the tentacles. The tentacles carry stinging cells, or nematocysts, which help to sting or kill the prey and capture it.
• They respire through the general body surface.
• They reproduce both sexually and asexually. Asexual reproduction takes place by budding and regeneration.
• They are found in ponds, lakes and seas.
• Their bodies show radial symmetry.

Examples of Coelenterata: Hydra, which lives in fresh water, and jellyfish, coral and sea anemone, which live in the sea. A coral reef is built from the hard skeletons of millions of tiny coral animals living together in one place.

The two simplest phyla share a few features, but they differ in almost everything that matters for how they live: