What is an animal?
Animals are living things. The things that distinguish an animal from other living things (such as plants, fungi and so on) are as follows :
- Unlike bacteria, the body of an animal is usually made of many cells.
- Unlike plants, an animal cannot generate its own food – it must eat to survive. This is related to the fact that most animals breathe Oxygen. (Plants breathe Carbon Dioxide).
- Unlike plants and fungi, animal cells have a flexible membrane, rather than a solid wall.
In addition, animals can usually move around, and respond to the environment. The world’s largest animal is the Blue Whale.
What kinds of animals are there?
The animal kingdom is divided into a number of groups, called phyla. These include :
- Acanthocephala, meaning “Thorny head”, contains 850 species of ‘thorny-headed worms’. These parasites have a spiny mouthpiece with which they hold onto the stomach lining of their victim.
- Acoelomorpha, meaning “Without gut”, have no stomach. Instead, these tiny creatures form a jelly-like surface into which their food is absorbed.
- Annelida, meaning “Little ring”, contain the 15000 species of segmented worms, including the common earthworm. If you look at these worms it’s as if their body is made up of a whole bunch of circular rings.
- Arthropoda means “jointed foot”. This is the biggest phylum by far, with over one million species. They include insects, spiders, crabs – and many other animals with a hard ‘exoskeleton’.
- Brachiopoda, meaning “arm foot”, is a small phylum of marine animals. They live in a shell, like clams and other bivalves, but internally they are very different from the bivalves. They typically have a bunch of tentacles around their mouths, and attach themselves to rocks.
- Bryozoa means “Moss animals”. These creatures are usually very small, and live in colonies, forming mats or rocky masses. There are about 5000 species of bryozoans.
- the Chaetognatha (meaning “Longhair jaw”) contains the 100 or so species of Arrow worms. Although there are few types of these tiny plankton, the population of some species is enormous!
- Chordata means “Cord”. Animals in this phylum have a hollow backbone (or similar) containing an important nerve. Yes, just like us! And all the animals we are most familiar with – cats, dogs, birds, fish, frogs, turtles, and so on – including the world’s largest. There are about 63000 species of Chordata.
- Cnidaria means “Stinging Nettle”. This is a very appropriate name for this phylum, which includes the 10000 species of jellyfish, sea anenomes, sea wasps and various other soft marine creatures with stinging tentacles.
- Ctenophora, meaning “Comb bearer”, contains only about 100 species of animals. Often confused with jellyfish, the ‘comb jellies’ are actually a different phylum altogether.
- Cycliophora means “Wheel carrying”. This phylum has only 3 known species of very unusual animals, which share a symbiotic relationship with certain species of lobster.
- Echinodermata means “Spiny skin”. These include about 7000 living species of starfish, urchins, sea cucumbers and similar creatures. All have, more or less obviously, the same kind of symmetry as the starfish.
- Echiura means “Spine tail”. These worms (140 different types) are not segmented like the Annelids, but have spikes on their tails. Most live in water.
- The Entoprocta have mouths that look similar to the Brachiopods and the Bryozoans. However, they also expel their waste near their mouths. Yuk!
- Gastrotricha means “Hairy Stomach”. These microscopic creatures (about 450 different species) live for a very short time (about 3 days).
- The Gnathostomulida are another small phylum of microscopic animals. Gnathostumulida are also known as “Jaw worms”.
- Hemichordata means “Half cord”. These are the 70 or so types of ‘Acorn worm’, and certain other similar creatures. Their distinguishing characteristic is that the part of their mouth cavity sticks out of their body like a big collar.
- Kinorhyncha means “Motion snout”. These are the whimsically named ‘Mud dragons’. They are very small creatures, with segmented spiny bodies, and a bunch of spikes around their head that they use to pull themselves through the mud. There are about 150 species known.
- Loricifera means “Corset bearer”. There are only about 20 species of these tiny animals. They live in sea mud, and move around using scales that look like umbrellas.
- Micrognathozoa means “Tiny Jaw Animal”. In 2000, the only known species of micrognathozoa was discovered. It was so different from other animals, it was given its own phylum. It’s jaw has 32 moving parts, making it one of the most complex in the animal kingdom.
- Mollusca means “Thin shell”. These are the molluscs. With 112000 species, including snails, barnacles, bivalves (clams and so on), and even squids and octopuses, it is the second or third most diverse phylum.
- Myxozoa means “Slime animal”. Earlier these were thought to be not animals at all. They are microscopic parasites affecting fish and worms.
- Nematoda means “Threadlike”. There are many species of nematodes, which look like microscopic worms. Many are parasites, and some of these are far from microscopic, but can grow up to a few metres in length! The nematodes are commonly called ’round worms’.
- Nematomorpha means “Thread form”. These long thin worms (called horsehair worms) are parasites attacking certain insects and crabs. There are about 320 species.
- Nemertea, meaning “Sea nymph”, is the phylum of the 1200 species of Ribbon worm. The can turn their mouths inside-out, and eat by swallowing their prey whole. They can expand their bodies to swallow prey several times wider than themselves. One species is suspected to grow even longer than the blue whale!
- Onychophora, meaning “Claw bearer”, is a small phylum of caterpiller-like creatures called Velvet Worms. Some scientists say the Velvet Worms don’t belong in their own phylum, but are actually arthropods.
- Orthonectida means “Straight swim”. These simple creatures (there are only about 20 species known) act as parasites in some worms, molluscs and echinoderms.
- Phoronida is a small phylum of sea creatures that make a tube-shaped shell for themselves. They have a bunch of tentacles around their mouths. They are commonly called the ‘Horseshoe worms’, after the shape of their gut.
- Placozoa means “Tubular animals”. There’s only one known species, and it basically looks like a blob.
- Platyhelminthes means “Flat worms”, and is the phylum of the, well, Flat Worms. Most of the 25000 species are microscopic and free-living, and harmless. However, there are a few species which are parasites, including the infamous liver fluke, which affects about 30 million humans worldwide.
- Porifera means “Pore bearer”. These are the sea sponges – about 5000 species alove today. Sponges are unique amongst animals that they don’t have organs as such, but their bodies are just a bunch of cells working together.
- Priapulida are a small phylum (about 17 species) of sea-dwelling worms. They weer discovered a century ago, but are still not well-known.
- Rhombozoa means “Lozenge-shaped animal” is a phylum of tiny parasites affecting octopuses and squids. Every adult (of a given species) has exactly the same number of cells!
- Rotifera means “Wheel bearer”. These microscopic animals have tufts of hair around its mouth, which they use to wash food in. As they move the hair, it looks like the motion of a wheel. There are about 2000 species.
- Sipuncula means “Small tube”. These are called ‘peanut worms’, because of the shape they take when frightened. The mouth has a mass of tentacles which can be inverted into its body. They are typically less than 10cm long.
- Tardigrada, meaning “Slow step”, is the phylum of the ‘water bears’. These are very small animals, that look a bit like a six-legged mole. They are found in extreme conditions, from the top of the Himalayas to the very depths of the sea. There are about 750 species.
- Xenoturbellida means “Strange flatworm”. Only 2 species are known. This animal has no well-defined organs except a nervous system. It eats molluscs.
What is the world’s largest animal?
The world’s largest (although maybe not the world’s longest) animal is the Blue Whale. It can grow up to 30 metres (100 feet) long or more, and weigh around 140 tonnes – that’s 140000 kg, or 300000 pounds! It is even bigger than the largest dinosaurs ever were. Sadly, it almost went the way of the dinosaurs in the 20th century, due to whaling. Hunting of Blue Whales was banned in the 1960s, and finally ended in the 1970s. Even now, the population of blue whales is still less than 1% of what it was before whaling began.