Phylum Porifera (from the Greek poros, meaning 'channel', and the Latin ferre, meaning 'to bear' or 'to carry').
Most sponges are marine, but a few occur in fresh water. A sponge consists of many cells, which are specialised to form different functions, but which are not organised into tissues and organs. They are supported on a skeletal framework of protein fibres or on needles of calcium carbonate or silica. Bath sponges (not synthetic ones) are the protein fibre skeletons with the living parts removed.
Sponges grow on rocks or on the surface of other animals or plants. 'they do not move around and so they cannot go after food. Instead, they create currents to bring food particles in through holes in the surface. The holes lead to canals and then to internal chambers lined with cells that are like flagellate protozoans. The flagella create the currents and the individual cells take in the food. The water then leaves by exit canals and larger openings in the surface.
Cnidarians: cup-shaped stinging animals
Phylum Cnidaria (pronounced 'nye- daria'; from the Greek knide, meaning 'nettle'). This phylum is sometimes referred to as Phylum Coelenterate.
Sea anemones, corals and jellyfish have a simple cup-shaped body with a single layer of cells on the outside and a second layer on the inside. Between these two layers of tissue is a jelly- like material that can be thick (as in jellyfish) or thin (as in sea anemones). Food enters and undigested wastes leave through the one opening, the mouth. Surrounding the mouth is a ring of tentacles with stinging cells that can attach themselves to and paralyse small organisms. The powerful toxin of some species can inflict a painful sting to humans (bluebottle) or can even be fatal (box jellyfish).
A cnidarian can be a polyp attached by its base and with the tentacles and mouth facing upwards (sea anemone, coral), or a medusa, which is free-swimming and has tentacles and mouth pointing downwards (jellyfish). Some live as colonies of many individuals (bluebottle).
Many cnidarians can build skeletons. The Great Barrier Reef and other tropical reefs and coral islands have been built up largely from the secretions of tiny coral polyps.
Flatworms
Phylum Platyhelminthes (from the Greek platys, meaning 'flat', and helming, meaning 'worm').
Flatworms are distinguished from other worms by their flattened shape. They creep over surfaces in the sea, fresh water, or in damp places on the land, or live as parasites in the bodies of other animals (eg, flukes and tapeworms). Their internal structure is more complicated than that of the cnidarians, as their tissues form organ systems. They have a definite head and move head first, but the gut has only one opening, the mouth.
Roundworms
Phylum Nematoda (from the Greek nema, meaning 'thread').
Roundworms are cylindrical with tapering ends and they move by a characteristic lashing motion. Most are small (some are even microscopic) and live in vast numbers in the sea, fresh water and the soil. Although there are many species, and they tend to look very similar, the different species are adapted to living in very different environments - from the polar regions and high altitudes, to ocean depths, hot springs and deserts, or as parasites in plants or animals. The body organs lie in a fluid-filled body cavity, and the gut has an opening at both ends, with a mouth for ingesting food and an anus for releasing undigested wastes.
Segmented worms
Phylum Annelida (from the Latin anulus, meaning 'ring').
An annelid worm is made up of a large number of similar sections or segments. Grooves or rings around the body wall mark the partitions that divide the body cavity into segments. The gut passes undivided from the mouth at the front to the anus at the rear. Segmented worms include many marine worms, earthworms and leeches . Marine worms live in mudflats and sandy beaches, and on rocky shores and the ocean floor. Some live freely on the surface or in burrows, while others live permanently in tubes they have built, only extending the head outside to feed.
The best-known annelids are the earthworms, which live in damp soil. Earthworms are extremely important in the soil because they break down and recycle dead plant material. They are very sensitive to herbicides and pesticides. The largest known earthworm, Megascoloides, grows to over 3 m long and lives in the Gippsland region of Victoria, Australia. Leeches are abundant in fresh water and wet gullies. Some feed by eating other small invertebrates, but the best-known feed on the blood of vertebrates.
Arthropods
Phylum Arthropoda (from the Greek arthron, meaning 'joint', and podos, meaning 'foot').
Three-quarters of all animal species are arthropods, and most of them are insects . Like annelids, arthropods have a segmented body, but the segments can be fused to form regions such as the head, thorax and abdomen of insects. Appendages are paired, jointed and adapted for a variety of functions such as sensing, feeding, swimming, walking and reproduction. The skeleton forms an external hard cover, or exoskeleton, to the body, and has joints in it to allow movement. The muscles are attached internally. Because the exoskeleton does not grow, it has to be shed (moulted) at intervals to allow the animal itself to grow.
Arthropods can be classified into three groups according to the structure of the appendages and the way the body is divided into regions.
Insects, centipedes and millipedes: arthropods with one pair of antennae
There are more species of insects than all the other animals put together (see Insects and human welfare). Together with the vertebrates insects comprise the dominant group of land-dwelling animals. Some are adapted to live in fresh water for at least part of their life cycle; very few live in the sea. Most adult insects have wings, which are membrane-like outgrowths of the upper part of the thorax. In fact most have two pairs o wings, but some (eg, flies) have only one pair. The first pair of wings o beetles, cockroaches and some grasshoppers is formed into a hard protective covering over the delicate flight wings. Not all insects have wings. Some, for example silverfish, are a continuation of a line o primitive flightless insects, but others, including fleas, stick insects and some bush cockroaches, have evolved from winged ancestors.
The young of many insects (beetles, flies, butterflies, bees, wasps and ants) are very different from the adults in appearance and feeding habits. The egg hatches into a larva (called a caterpillar in the case of butterflies and moths) that feeds voraciously. The larva changes into a pupa and, within the pupal covering, the juvenile structures are resorbed and the adult ones grow. This process of change in the body form during life is called metamorphosis (from the Greek meta, meaning 'change', and morphs, meaning 'form'). The young of other insects (cockroaches, dragonflies and grasshoppers) are much more like the adults, and their incomplete metamorphosis involves a series of gradual changes. The last change is when they develop wings and become sexually mature adults.
Centipedes and millipedes are long animals with a head, and a body divided into many similar segments, each with jointed legs. Both are commonly found under logs or stones, in rotting logs or leaf litter. Centipedes are flat and are carnivores. They kill their prey with venom from a pair of fangs located just behind the head. Millipedes are round in cross-section, have two pairs of legs per segment (except for the first four segments) and feed on decaying plant matter. They burrow through the leaf litter with great force.
Chelicerates: spiders, scorpions, ticks and mites This group of arthropods is named after their feeding appendages, called chelicerae, which are at the front of the head.
Spiders are carnivores that paralyse their prey by injecting venom through their chelicerae or fangs. Although almost all are venomous, only a few are capable of killing large animals and people. (In Australia these include funnel-webs and red-backs.) All spiders spin a silk-like material from glands near the rear of the body. Many use this silk to build webs of various shapes to ensnare insects, but some hunt their prey.
Scorpions hold their prey with a pair of large pincers, and tear it apart with a smaller pair, the chelicerae, near the mouth. A sting on the end of the long abdomen can inject venom into the prey or be used in defence. Both spiders and scorpions secrete digestive juices to liquefy their food outside the body. They then suck in the liquefied food. All ticks are external parasites that feed on the blood of vertebrates. Mites are similar to ticks in appearance and some are parasitic, feeding on the skin itself, but most are free- living and feed on live or dead animals and plants. Many mites are significant crop pests.
Insects and human welfare
Insects are by far the largest group of animals on Earth today, and they have very important effects on human welfare some to our advantage and some to our disadvantage. As insects have a very high reproductive rate and most are very mobile, enormous numbers of individuals can build up quickly under favourable conditions.
Some insects, such as bush flies, are a nuisance. Others, including mosquitoes, fleas, lice and biting flies, cause irritation through bites that become inflamed. It is the biting and blood-sucking of many insects that produce one of the most important impacts on humans - the transmission of disease, from other animals to humans or from one person to another. They do this by acting as vectors for disease viruses, bacteria and protozoa. eases transmitted by these animals to humans bubonic plague (transmitted by fleas from rats to malaria (transmitted by Anopheles mosquitoes), transmitted by the human body louse) and sleeping transmitted by the tsetse fly). In Australia, it is fortunate that at present none of these diseases pose a serious health problem. However, outbreaks of malaria occur occasionally in northern Australia. They have been traced to people who have contracted the disease overseas. Although potential mosquito vectors do occur in the north, the small size of the human population has so far prevented the disease from taking hold.
The most important insect-borne human disease in Australia is Ross River arthritis. It occurs throughout Australia but mostly in rural areas. This virus causes a debilitating arthritis of the small joints (eg, hands, wrists, knees, ankles). It is thought that the disease is transmitted by mosquitoes from wallabies and kangaroos to humans.
Many insects, particularly flies and cockroaches, act as indirect vectors of disease by contaminating food. Various forms of dysentery, gastroenteritis and hepatitis A are transmitted in this way.
Insects are also vectors of many diseases that affect crops and domestic animals. Equally important to human welfare are insects that destroy crops (eg, plague locusts, aphids, scale insects and caterpillars), domestic live stock (eg, the sheep blowfly), stored food (eg, moths, weevils and other beetles) and manufactured materials (eg, termites, wood-boring beetles, silverfish and clothes moths). All of them have an enormous economic effect on humans, and vast amounts of money are spent each year on research and on combating the problem of insect pests. Means of control include:
changing the environment so that it is not suitable for the insect pest (eg, draining swamps to control mosquitoes);
rotating crops so that a particular pest does not build up large numbers;
producing strains of domestic animals and plants that are resistant to particular insect-borne diseases;
developing vaccines;
producing insecticides against particular insect pests;
introducing predators as biological controls or, better still, encouraging native insect predators as part of agricultural practice; and
establishing strict quarantine regulations to prevent the introduction of a particular pest into a country where it does not occur.
There are far-reaching consequences of using insecticides. Many accumulate throughout the ecosystem and affect non-target organisms, and, in some cases, strains of insects have become resistant to a particular insecticide so that it is no longer effective.
However, there is another side to this rather gloomy picture. Many insects have contributed in a positive way to human welfare. Insects play a vital role in the balance of nature. They are important in the food chains on which other animals depend, including humans. They are also essential as pollinators of flowering plants. We use the honey bee for food production and the silk moth as a source of a very desirable fabric. Insects have also been used extensively in biological research. The use of the vinegar fly, Drosophila, as an experimental animal has contributed much to our understanding of genetics and stages of development.
It is a human interpretation to consider insects as pests, and indeed it has been humans, with their population explosion and consequent changing of the environment, that have to a large extent created the conditions that have enabled many insect pests to flourish. From a non-human point of view we could be considered to be the most notorious pest species on Earth.
Crustaceans: arthropods with two pairs of antennae
Most people are familiar with freshwater crayfish and marine crabs, prawns and lobsters as food, but there are many other crustaceans. Many are small or even microscopic and occur in the sea and in fresh water in countless numbers . They form an important food supply for larger animals, even-the largest of all, the blue whale. The two pairs of antennae distinguish crustaceans from the other arthropods.
Most crustaceans are aquatic and have appendages modified to form paddles for swimming. Those that live on the sea-floor also have legs. Barnacles have free-swimming young like those of other crustaceans, but they are unique in the way that they settle down, become permanently attached to a surface, and develop a hard shell from which they extend their legs to catch small organisms that drift by. Only a few groups of crustaceans live on land; the best-known, the slaters, live on decaying vegetation. (See "The case for splitting Phylum Arthropoda into three, and Peripatus puts a piece into the puzzle of arthropod evolution and classification.")
The case for splitting Phylum Arthropoda into three
Traditionally, all animals with a segmented body, a hardened exoskeleton and jointed limbs were placed into the Phylum Arthropods. Now some biologists are questioning whether animals as different as the insects, the spiders and the crabs should be lumped together. The idea is that the distinctive features of the arthropods have been acquired separately by the above three groups from different soft-bodied, worm-like ancestors. The groups differ in the detailed structure and functioning of the appendages and in the way the body of the early embryo develops. On this basis, the insects, centipedes and millipedes form one evolutionary lineage - the Uniramia. Their early development resembles that of annelid worms such as the earthworm. The pattern of development of lobsters, crabs and shrimps - the Crustacea - is different from both the uniramians and the Chelicerata (spiders, mites, ticks and scorpions). The segmented ancestors of the crustaceans and the chelicerates are unknown.
If the evolutionary lineage of the three groups is different, should each be elevated to phylum rank and the Phylum Arthropoda disbanded? Opinions differ. Many biologists favour the single Phylum Arthropoda but many others would prefer to have three separate phyla.
Peripatus puts a piece into the puzzle of arthropod evolution and classification
Peripatus live in the mouldy leaf litter of wet forests in the tropics and the southern land masses, including coastal Australia (from Cape York to Tasmania, around the Adelaide hills and in south-western WA). Peripatus are savage predators that snare small invertebrates in sticky threads they shoot, with great accuracy, from two turrets on either side of their heads, before ripping them apart with slashing jaws.
To biologists, peripatus, or velvet worms (Phylum Onychophora), are quite famous. They are believed to represent a missing link between the segmented worms and the arthropods. Like annelids, peripatus have soft bodies that they contract and expand by muscles acting on a fluid-filled body cavity. Otherwise, peripatus are more like arthropods. Their soft limbs contract and expand, moving them along much like a centipede, and their early development is similar to that of all the millipedes, centipedes and insects. Although some Australian peripatus lay thick-shelled eggs, most give birth to live young, and some South American species have developed a placenta through which nutrients are passed to the developing embryos. So these primitive creatures display reproductive strategies similar to those of mammals.
Peripatus may represent the group of soft-bodied worms that developed limbs and moved onto land to diversify as the centipedes, millipedes and insects (the uniramians). In the classification of arthropods as three phyla (see box, The case for splitting Phylum Arthropoda into three) the onychophorans are believed to represent a link between only the segmented worms and the uniramians, whereas in the classification as a single phylum they are believed to represent a link between the segmented worms and all arthropods.
Molluscs: soft bodies and hard shells
Phylum Mollusca (from the Latin mollis, meaning 'soft').
No other group of invertebrates is as diverse as the molluscs, which range from the immobile oyster to the slow-moving limpet and the very active squid, cuttlefish and octopus. The Mollusca is the animal phylum with the second largest number of species. Most molluscs are aquatic, living in the sea or fresh water, but some (snails and slugs) live in damp places on land. A few snails are even found in deserts. The body of a mollusc is soft and slimy, and most have a hard external shell to protect and support them. Others have no shell or one that is reduced to an internal structure that supports the body but is less cumbersome than an external shell.
The gastropods, such as marine and land snails, have a single shell, which is often coiled and into which they can retreat for protection. Bivalves such as oysters, mussels and pipis have a shell in two pieces. The chitons have shells of eight interlocking plates . The cephalopods - the squid, cuttlefish and octopus group - are descended from a group with a large external shell (represented today by the pearly nautilus) but have a small internal shell (the cuttlebone of cuttlefish and the pen of squid) or no shell at all (the octopus).
The land and marine snails and chitons feed by rasping with a file-like (tongue' called a radula. Bivalves draw water in between the two halves of their shells and filter out small particles of food. Octopus, squid and cuttlefish are active predators that catch and hold their prey in their tentacles, crush it with a sharp horny beak, and tear the flesh with their radulae.
Echinoderms: spiny-skinned animals
Phylum Echinodermata (from the Greek echinos, meaning 'spine', and derma, meaning 'skin').
Echinoderms are slow-moving and do not have a distinct head. The sea-urchin has a rounded body; sea-stars, brittle-stars and feather-stars have arms radiating from a central body; and the sea-cucumber has a soft, sausage-shaped body. The skin has an internal skeleton of calcium carbonate plates, which can be separate, giving a leathery texture to the skin (sea-stars), or fused to form a sphere (sea-urchins). The sea urchin's skeleton also has moveable sharp spines, which it uses to protect itself and to move around.
The most distinctive feature of echinoderms is their tube feet, which are thin and tentacle-like and can be extended from the body. Suckers at the end are used to collect food and move around.
Mature echinoderms are very different from mature chordates (the next phylum we shall look at). There are, however, many similar features in the developmental stages of these two phyla. These features are quite different from those of the molluscs, annelids and arthropods.
Chordates: an apparently diverse group of animals
Phylum Chordata (from the Greek chortle, meaning 'string').
This phylum includes some invertebrates and all of the vertebrates. The chordates have a flexible supporting rod, called the notochord, running along their back (dorsal side) at some stage of their development. Adjacent to the notochord and towards the outside of the animal is a hollow nerve cord. Vertebrates have a notochord only in their embryonic stage and, during development, vertebrae grow around the notochord to form the vertebral column. However in some relatively minor sub-phyla of chordates the notochord is not replaced by a vertebral column. The best-known examples are the cunjevoi of marine rock platforms, which are sea-squirts (sub-phylum Urochordate, and small, marine, fish-like animals, the lancelets (sub-phylum Cephalochordata).
The vertebrate sub-phylum is divided into a number of classes, and we will consider the major ones individually.
Fish
Because of their very distinct differences, fish are placed in three classes. Over 20 000 species have been described.
Most of the fish we catch and eat have skeletons made largely of bone. Bony fish (Class Osteichthyes, from the Greek osteon, meaning 'bone', and ichthyes, meaning 'fish') range in size from the little guppies often kept in aquariums to swordfish over 5 m long. Flying fish can glide through the air on expanded fins; mud skippers use their fins and tail to hop over mudflats; long, snake-like eels hide in crevices; flounder are adapted to lie on one side on the sea-floor; and many fish of the coral reefs are brightly coloured. Bony fish live in nearly all the waters on the Earth, from the ocean depths to alpine and underground streams. Some prey on other active animals, some on attached animals like corals, and others browse on seaweeds.
Not as common, but more feared, are the sharks and rays, which have Skeletons of cartilage (Class Chondrichthyes, from the Greek chondros, meaning 'cartilage', and ichthyes, meaning 'fish'). Many chordates have cartilage in their skeleton (eg, in noses, ears and joints) but in the cartilaginous fish, the sharks and rays, the whole skeleton is made of cartilage. It is firm enough to give support to an animal living in water and is more flexible than bone.
Nearly all sharks and rays live in the sea. Most sharks are streamlined, capable of bursts of speed, and have numerous sharply pointed teeth that are continually replaced - all good qualities for catching active prey. But some feed on shellfish, and the large, sluggish basking sharks and whale sharks (which grow up to 14 m long) strain shrimps and similar organisms from near the sea's surface. Manta rays also strain food from the sea or feed on shellfish.
The third class of fish, the Agnatha (from the Greek agnathos, meaning without a jaw'), is small. Its members, such as the lampreys, have skeletons of cartilage, no jaws and no paired fins.
Amphibians
Class Amphibia (from the Greek amphibios, meaning 'living a double life'). Members of this class, the frogs, toads and salamanders (which look like scaleless lizards and live in the northern hemisphere) have a soft skin without scales and no claws. Many live a 'double life', first as tadpoles, 'breathing' by gills in water, and later as air-breathing adults.
Amphibian eggs have no waterproof coat and are laid in ponds, moist pockets in soil, burrows, or even in pockets in the skin of the adult. Some kinds of frogs wander away from water and burrow for protection during dry weather.
Frogs are the only amphibians to occur naturally in Australia. Two main families are the tree frogs, which have large finger and toe pads for climbing (about 70 species), and the southern frogs of streams and leaf litter (about 100 species). Their nearest relatives live in South America, suggesting that they arose in Gondwana, the ancient supercontinent that gradually separated into South America, South Africa, southern India, Antarctica and Australia.
Reptiles
Class Reptilia (from the Latin repere, meaning 'to crawl' or 'to creep'). Members of this class, the snakes, lizards, crocodiles and turtles, are covered in scales. They are not warm-blooded and so they are more active when they obtain warmth from their surroundings. They cannot maintain their body temperature and they are inactive in cold weather. Reptile eggs usually have a soft waterproof shell, which protects them from drying out when laid in dry soil. The hatching young fend for themselves.
Snakes are probably the most feared of Australian animals, and with good reason - a bite from a taipan, death adder or tiger snake can kill an adult human. Venomous snakes use their venom to kill or paralyse their prey by injecting it through channels in the fangs of their upper jaw. Other snakes, such as the very large pythons, do not have fangs or poison glands. They kill their prey by squeezing it between coils of their body.
There are more than 500 Australian reptiles. Most of them are lizards. Lizards range in size from the perentie, which may be over 2.5 m long, to the small skinks of the bush and gardens and the nocturnal geckos, which are only a few centimetres long. Lizards are predators; they grasp and crush their prey in their mouth.
Two species of crocodile - a saltwater and a smaller freshwater crocodile - occur in waters in northern Australia. The saltwater crocodile can grow to 7 m long and may take large prey, including people.
Australia has many different tortoises and turtles. Freshwater tortoises live in rivers and lakes. They are small, long-necked, and have webbed feet. Nearly all turtles are marine. They are much larger and have paddle-shaped feet. Marine turtles come ashore to lay, burying their eggs on the beaches of offshore islands, including some in the Great Barrier Reef.
Birds
Class Aves (from the Latin avis, meaning 'bird').
Feathers and the power of flight distinguish birds from other animals. Feathers enable birds to fly and also insulate their bodies. Structurally, birds are like warm-blooded, feathered reptiles - they have scales on their feet and a four-chambered heart. Like most reptiles birds lay eggs, but their eggs have a hard shell and they must be kept warm until they hatch. Most birds build a nest to hold the eggs and young until they can fly. Birds have a beak for feeding, but no teeth, hollow bones to reduce body weight for flight, and massive chest muscles attached to a keel on the breast bone to provide power for flight. The keel and chest muscles of flightless birds such as emus, ostriches and kiwis are too small to enable them to fly.
Birds are easier to see than native mammals because most of them are active during the day (ie, diurnal) whereas most native mammals are active by night (nocturnal), and there are more species of Australian birds than there are of mammals. There are about 700 species of native birds, but only about 250 species of native mammals.
The birds are classed in two main groups - the song birds or passerines (about 300 species) and all the rest, the non-passerines (eg, parrots, waders, birds of prey, quail, cuckoos, kingfishers). The song-birds (including lyrebird, warblers and magpie) have a special voice box and use their song to defend their territories. Most Australian song-birds resemble Eurasian flycatchers, robins and warblers, but recent biochemical and molecular evidence shows that they are not closely related, and so they could have come from Gondwana, like the marsupials. The oldest known song-bird fossils have been found in Australia.
Mammals
Class Mammalia (from the Latin mamma, meaning 'breast').
Mammals have hair (or fur) and are warm-blooded. The hair traps air which keeps in the body heat. This enables faster circulation of the blood so that, like birds, mammals can be active all the time, not just when they are in a warm environment. The hair can be short and coarse, long and fine, or long and tough (eg, the spines of echidnas). Some aquatic mammals (eg, whales) have a layer of fat, the blubber, that forms the insulating layer. Females have mammary glands, which produce milk to feed the young. Hair and mammary glands are the features that separate mammals from all other animals.
The young of most mammals are nourished inside the body of the female during early development. The fetus develops a special structure, the placenta, through which it receives food and oxygen from the mother and passes wastes into the mother's bloodstream. Mammals with a well-developed placenta are called placental mammals. They include most of the world's mammals. In Australia there are many placental mammals, including rats, mice, seals, bats and the dingo, but we also have two other types of mammal - the marsupials and the monotremes.
The young of marsupials (from the Latin marsupium, meaning 'pouch') are nourished by a placenta for only a short time, are born at an early stage Of development, and are then nurtured in a pouch, which contains the mammary glands. Wombats, kangaroos, possums and the koala are all marsupials, and so is the opossum of America. Apart from the opossum, marsupials are native only to Australia, New Guinea and South and Central America. Some Australian marsupials are in danger of extinction .
Monotreme mammals lay eggs and development begins in the eggs before they are laid. The pores that release the milk from the mammary glands are not grouped in raised teats. The only monotremes are the platypus and echidnas, and they occur only in Australia and New Guinea.