Simple Organisms and Invertebrates
|Biology - Animal Classification|
In attempting to classify living things, scientists often describe organisms in general terms, such as lower and higher or simple and more complex. The lowest organisms are single-celled creatures that do not have highly developed internal cellular structures. Higher organisms, such as this spider, are usually made up of many different kinds of cells which have specialised functions. The cells of these higher organisms contain internal structures that are not found in lower-level organisms.
In this presentation, we will examine the very simplest of organisms, those classified as monerans and protists, as well as some of the slightly more complex members of the animal kingdom.
Whether simple or complex, all living things are capable of certain processes such as growth, reproduction, and aging. All living things also utilise food to provide energy for these processes. As the most complex form of life on earth, it is important for us as humans to remember that even the simplest organisms and animals, from algae and bacteria to spiders and butterflies, fill critical niches in the complex web of life.
We must be sure to exercise due care in our dealings with other organisms, so that the delicate balance that sustains life on earth is forever maintained.
Equally adept at surviving in a variety of environments are the members of the phylum Arthropoda. This phylum is divided into twelve different classes. Combined, the twelve classes of the phylum Arthropoda contain over three-quarters of all known animals, including nearly one million species! Members of this hardy group include insects, spiders, crustaceans, centipedes, and millipedes, as well as many more obscure organisms.
One of the primary physical characteristics of arthropods is their tough external skeleton, which is made of a material called chitin. This exoskeleton protects the internal body parts and provides a framework for the body. Arthropods' bodies are composed of a series of segments and are bilaterally symmetrical. That means that the right side is the mirror image of the left. Arthropods also possess jointed legs and other jointed appendages.
The largest class of arthropods is class Insecta. To date, scientists have identified about 750,000 species of insects. All insects share certain body characteristics. All insect bodies are divided into three regions: the head, the thorax, and the abdomen. Insects have compound eyes made up of numerous simple eyes that "look" in many directions at once. These allow them to locate food as well as follow the movements of potential enemies.
The most readily recognized characteristic of insects is their six jointed legs, which are attached to the thorax. Most insects have two pairs of wings that are attached to the thorax as well. Insects have the most efficient muscles of all animals and can easily carry loads in excess of their own body weight. And insects such as bees have wings that move much faster than the limbs of the very fastest vertebrate. Insect wings vary greatly in appearance. Some have colourful scales while others may be thick and brittle, or even transparent.
Most insects breathe through tiny pores called spiracles, located on the thorax and abdomen. Female insects lay large numbers of eggs either in the water or on land. The process by which an insect egg matures into an adult is called metamorphosis. During metamorphosis, extraordinary changes take place, both in the structure of the insect's body and in its behaviour. The most common illustration of complete metamorphosis is demonstrated by the caterpillar, which changes into a butterfly. At first, the caterpillar looks nothing like a butterfly. Its movement is limited by stubby legs, and it lives by consuming large amounts of plant material. But after weaving a silky cocoon around itself, a caterpillar undergoes dramatic changes. When it emerges, the butterfly is a delicate flying insect whose food is the nectar of flowers.
While some insects are commonly viewed as pests, others benefit humankind by pollinating food-producing plants or by destroying other insects that harm crops or spread disease.
The second largest group of arthropods is the class Arachnida. This group includes spiders, ticks, and mites. Spiders are distinguished from insects in two important ways. First, their bodies are divided into only two regions: the abdomen and the cephalothorax. Second, all spiders have eight legs rather than six. Attached to the spider's cephalothorax are eight walking legs, two pedipalpi, which are chiefly sensory organs, and two chelicerae, with fangs at their ends. Located near the mouth, the chelicerae help the spider gather food. Poison travels through the fangs and paralyses a spider's prey, after which it can be seized.
Spiders also have three or four pairs of simple eyes. Most spiders have two or three pairs of spinnerets that spin liquid silk which then hardens to form threads. Spiders weave these threads into webs used to catch insects as well as make protective cases for their eggs.
Unlike insects, spiders do not undergo metamorphosis. Instead, they hatch in the form of a miniature adult. Other arachnids, such as ticks and mites, have bodies consisting of a single fused unit. These creatures live as parasites on both plants and animals and transmit diseases such as tick fever, Lyme disease, and Rocky Mountain spotted fever.
The third largest group of arthropods are the members of the class Crustacea. Consisting of nearly 30,000 species, this group contains crabs and shrimps, barnacles and lobsters, and crayfish, among others. For example, crayfish have two pair of antennae, a hard outer, or exon, skeleton, and gills for obtaining oxygen in an aquatic environment.
Many crustaceans also have special jointed abdominal appendages called swimmerets, such as those shown on this shrimp. One might never recognize these barnacles as joint-legged animals, but in fact, they are. Their legs disappear after the free-swimming, immature larval stage of the barnacle. The last two major types of arthropods may look alike, but are placed in separate classes.
The first group is the centipedes, which are members of the class Chilopoda. Centipedes have long, segmented bodies and have as many as 177 pairs of legs, one pair on each body segment. Similar in appearance to the centipede is the millipede, which is a member of the class Diplopoda. Like centipedes, millipedes live in dark, moist environments and have long, segmented bodies. Millipedes differ from centipedes in that they have two pairs of legs on all body segments except the first four or five parts. Also centipedes are poisonous meat eaters, whereas millipedes eat only decaying plant matter.
As we have seen, the remarkable phylum Arthropoda contains five major classes. These are: class Insecta, which includes nearly three-quarters of a million species, from army ants and dragonflies to beetles and butterflies; class Arachnida, which includes spiders, mites, and ticks; class Crustacea, which includes lobsters, crayfish, barnacles, and crabs; class Chilopoda, which includes centipedes; and class Diplopoda, which includes millipedes. Of all the phyla in the animal kingdom, the arthropods contain the greatest number of species, and these species are found in virtually every ecological region of the world, from rain forests and deserts to the depths of the ocean floor.
A third major group of invertebrates is the one which includes the spiny-skinned members of the phylum Echinodermata. Animals in this phylum include creatures such as sea urchins and starfish. Sea urchins are covered with hundreds of hard, colourful, and brittle spines. Their tiny, tubed feet extend through openings in their skeleton. The sea urchin's spiny feet can be used for walking and burrowing. Amazingly, these tiny powerhouses carve out a rounded pocket from within the solid rocks they inhabit.
Starfish are another familiar member of the phylum Echinodermata. Like sea urchins, starfish come in a rainbow of colours, including red, purple, orange, and yellow. Starfish are often found clinging to rocks. They travel about on rocks by moving the thousands of miniature tube feet that emerge from under their large, radiating arms. The starfish's tube feet are also used to pass food to the centrally located, star-shaped mouth. Oddly, a starfish generally passes its stomach out of its mouth and digests its food and absorbs it before pulling its stomach back into its body.
A final member of the phylum Echinodermata is the soft-bodied sea cucumber, which can occasionally be found in tidal pools. Invertebrates such as worms, molluscs, and echinoderms demonstrate many fascinating methods for surviving and thriving in a wide variety of habitats from the human intestine, to a rock at the seashore, to the leaves of a tree in a temperate rain forest.
A second major group of invertebrate animals are molluscs. Phylum Mollusca contains over 100,000 species, making it the second largest of the animal phyla. Snails, clams, slugs, and octopi are some of the more common species within phylum Mollusca. Molluscs have soft bodies; however, many have protective shells or skeletal structures.
Some, such as snails, have only a single shell so they are called univalves. Other molluscs, such as clams, have two shells and are called bivalves. Clams also have two siphons, which are used to draw in and let out water. Water entering one siphon carries oxygen to the gills and food materials to the digestive system. The other siphon expels water and waste. This technique is known as filter feeding.
Slugs and snails are molluscs known as gastropods because they move about on a slimy muscular foot attached to what we would call the stomach, or gastric, area. Slugs possess two sets of retractable tentacles. The larger set is equipped with eyes. Slugs are hermaphrodites, meaning that they have both male and female sexual organs.
Another common variety of mollusc is the limpet, which attaches itself to rocks in the intertidal zones of marine environments. Mussels, a popular seafood, are yet another variety of mollusc. Clusters of bivalve mussels are found in great abundance in regions with mild temperatures. One of the largest and most unique molluscs is one without a shell, the eight-armed octopus. Do you know how it protects itself?
Monerans and Protists
The simplest types of organisms capable of independent life are those that are classified within the kingdom Monera. Members of this kingdom fall into two categories: bacteria, such as the whitish colonies shown here living in the water of a hot spring; and blue-green algae, such as those found in this pond.
Blue-green algae are among the oldest types of organisms in existence. There are some algae fossil remains that date back over 3 billion years. What distinguishes monerans from more complex organisms is the lack of a nucleus or other organelles. They are thus said to be prokaryotic, which means "before the nucleus. "Unlike the prokaryotic organisms in the kingdom Monera, the organisms within the kingdom Protista possess a definite nucleus. These organisms are thus identified as eukaryotic, or "having a nucleus."
In all there are ten phyla in the kingdom Protista. Among these phyla, a basic distinction can be made between two types: single-celled or simple colonial algae, which can make their own food by photosynthesis; and single-celled or simple colonial organisms called protozoa, which cannot make their own food.
Protozoa do not contain chlorophyll, so they cannot carry out photosynthesis. By definition, proto means "first" and zoa means "animals. " As their name says, protozoa may have been the world's first animal-like organisms. All protozoa are microorganisms, meaning they are too small to be seen without a microscope. The water of any pond is filled with countless numbers of these tiny creatures which provide food for other pond inhabitants.
Phylum Sarcodina is one of the four phyla of protozoa. This phylum contains the amoeba, of which there are over 1,200 species. A major characteristic of phylum Sarcodina is that its members move by means of false feet called pseudopodia. As these finger-like projections of the body are extended, the rest of the body flows into them. In the same manner, food, such as single-celled algae and other protozoa, are surrounded by the pseudopodia. A saclike structure called a food vacuole forms where digestion takes place. The digested food is absorbed through the vacuole membrane into the non-nuclear cell material called cytoplasm. Amoebae reproduce themselves by dividing into two identical new cells, each of which is capable of independent life. This reproductive process is called binary fission.
A second group of protozoa is classified under the phylum Ciliophora. Thisphylum includes organisms such as the paramecium, shown here. In this paramecium's cytoplasm are whole single-celled algae that it has trapped for food. The principle characteristic of protozoa in phylum Ciliophora is the presence of tiny, external, hairlike devices called cilia. Cilia are used for movement and to trap food materials.
Another group of protozoa, called the flagellates, is placed into the phylum Sarcomastigophora. Flagellates possess long, whip-like devices that are primarily used for locomotion. The species pictured here dwells in human blood and is the cause of sleeping sickness. Other flagellates, such as these shown here in greatly magnified detail, contain chlorophyll and are very common in aquatic environments.
The last major phylum of protozoa is Sporozoa. Members of this phylum are incapable of independent movement, and, like spores, are moved around by other forces, usually wind or water. Sporozoa, such as this gregarine, exhibit a well-defined nucleus. Gregarines exist as parasites in earthworms and insects. One type of sporozoa known as Plasmodium is sometimes transmitted by mosquitoes into the human blood stream where it causes malaria, a disease which affects more people worldwide than any other.
Sponges and Coelenterates
As we move from the world of single-celled organisms found in the kingdoms Monera and Protista, to the multi-celled members of the kingdom Animalia, we encounter the first true animals. True animals can be defined as those that possess more complex physical structures and also perform more advanced functions than members of either kingdom Monera or Protista.
The first major phylum of the kingdom Animalia includes sponges, or Porifera. The sponges shown here are reddish due to the presence of a type of algae that inhabits these animals' bodies. Sponges may live in both saltwater and freshwater where they attach themselves to rocks, sticks, and debris. Typically, sponges are really individual animals organized into colonies.
Sponges take the name Porifera from the fact that their bodies are full of pores. Porifera means "pore bearers." Water containing oxygen and food passes through these pores and into the central cavity lined with flagellated collar cells that can trap the food. Water is then expelled through an opening called the osculum. Another characteristic of sponges is that they may produce supporting frameworks made of either glassy silica or an elastic material which makes them useful in their most familiar form: bath sponges.
The next phylum, the coelenterates, contains animals that are one step higher than sponges in the complexity of their physical structure. Members of this phylum include sea anemones, corals, and jellyfish. Like all coelenterates, the body of a sea anemone is hollow and has an opening through which both food and waste pass. The body itself is composed of two cell layers. The outer cell layer enables the sea anemone to expand and contract as well as to feel sensation. The inner cell layer is concerned with the digestion and absorption of food. Like sponges, coelenterates take in oxygen through their body surfaces. Sea anemones and jellyfish both possess stinging cells in their skin which are used for capturing food.
Colonies of coral animals live in conjunction with various algae to create the limestone formations we typically refer to as coral. There are many types of coral. Some resemble mushrooms. Some look like trees, and some, like flowers. In all there are over 700 species of coral in various marine environments around the world.
As we have seen, even the simplest organisms in kingdoms Monera, Protista, and Animalia have the ability to perform many of the functions carried out by more complex organisms. These functions include digestion, reproduction, respiration, and locomotion. In spite of the simplicity of the internal organization of these organisms, each possesses unique and marvellous characteristics, and it is these characteristics that provide the means by which they are able to flourish in many of the world's ecosystems.
Worms, soft-bodied mollusks, and the spiny-skinned creatures known as echinoderms are classified as invertebrate animals, which means they have no backbone. These animals possess more highly specialized cells and tissues than sponges and coelenterates. They also possess certain organs that are not possessed by organisms within the lower animal phyla that we have just discussed.
Worms are distributed over eleven different phyla and reflect a surprising diversity. The three largest worm phyla are the ones containing flatworms, roundworms, and segmented worms.
Flatworms are members of the phylum Platyhelminthes. They can be found either as free-living creatures or as parasites. The parasitic worm shown here inhabits the livers of certain mammals. Another type of parasitic flatworm is the tapeworm, which inhabits the intestines of many animals. This slide shows a microscopic view of the pinkish coloured head of a tapeworm imbedded within the wall of an intestine.
Roundworms are members of the phylum Aschelminthes. These worms possess round, unsegmented bodies covered by a tough skin or cuticle. Many of the over 13,000 species of roundworms are free-living. But there are also many, such as the pinworm shown here, that are parasitic to both plants and animals.
Segmented worms are members of the phylum Annelida. Although there are many complex annelids, the most common of all is the earthworm. Many annelids have well-developed digestive, circulatory, and nervous systems, and are considered the most advanced of all worms.
The earthworm is well adapted to burrowing, which it does by muscular movement aided by four pairs of bristles on each segment. Earthworms breathe through their skin. If excess moisture depletes the soil of oxygen, earthworms must come to the surface to breathe. That's why you can see so many of them after a heavy rainfall.
Ocean-dwelling annelids, such as those shown here wedged within algae, are very common. Some annelids even make tubes for their soft bodies from the minerals in the seawater. Feather-duster worms dwell on the ocean floor and have delicate tentacles for trapping food. They look more like exotic flowers than the segmented worms they are.