Scientists classify or group all invertebrates into several phyla. The animals in the same phylum share the same characteristics, indicating a common ancestor. One of the features most apparent in invertebrates is symmetry, which is the arrangement of the individual parts of an object. Most of the animals have radial or bilateral symmetry. In radial symmetry, body parts are arranged in a circle around a central point. Eg., jelly fish, sea urchins. Animals with bilateral symmetry have parts that are mirror images of each other. Grasshoppers and lobsters are examples of bilateral symmetry. Some animals have no symmetry at all, they are asymmetrical. Their bodies cannot be divided into matching halves. Eg., sponges

Sponges are the simplest of animals. They bridge the gap between single celled organisms and more complex animals. Their body structure is made of two layers of cells. The sponges are sessile,(they remain attached to one place during their lifetimes )Sponges live in water, they are called filter feeders since they filter food out of the water that flows thro’ their bodies. Sponges belong to the phyla Porifera, named after the pores. The soft bodies of many sponges are supported by sharp, glasslike structures called spicules. Many other sponges have spongin a material that makes them soft and elastic. Sponges reproduce both sexually and asexually. Asexual reproduction occurs when a bud on the side of the parent sponge develops into a small sponge. The small bud breaks off, floats away, and attaches itself to a new surface. Most sponges reproduce sexually producing both eggs and sperm.

Cnidarians are a phylum of hollow bodied animals that have stinging cells. They have radial symmetry that allows them to locate food that floats by from any direction. Their bodies have two cell layers, the inner layer forms a digestive cavity where food is broken down and tentacles surround the mouth. Stinging cells shoot out to stun or grasp prey. Cnidarians can have two different body plans, a polyp and a medusa. Cnidarians reproduce both asexually and sexually. Asexual reproduction is by budding. Sexual reproduction occurs when eggs are fertilized by sperm and develop into a new polyp. Medusa forms of cnidarians such as jellyfish have both an asexual stage and a sexual stage.

Flatworms, unlike sponges and cnidarians, actively search for food. Worms are invertebrates with soft bodies and bilateral symmetry. Flatworms are members of phyla Platyhelminthes. They have long, soft flattened bodies. They have three distinct layers of tissue organized into organs and organ systems. Some flatworms are free living, which means they do not depend on one particular organism for food or a place to live. Most flatworms are parasites. One parasitic flatworm that lives in humans is called tapeworm. It lacks a digestive system, to survive it lives in the intestines of its hosts. The tapeworm absorbs nutrients directly into its body from digest material in the host’s intestines. The hooks and suckers on the head attach the tapeworm to the host’s intestines. A tapeworm grows by adding sections directly behind its head. Each body segment produces both eggs and sperm from separate male and female reproductive organs.

Roundworms belong to the phyla Nematoda. Roundworms are the most widespread animals on earth, billions can live in one acre of soil. A roundworm’s body is described as a tube within a tube with fluid in between. The cavity separates the digestive tract from the body wall. Roundworms are more complex that flatworms because their digestive tract is complete with two openings. Food enters through the mouth and wastes exit through an anus. Roundworms are a diverse group. Some are decomposers, some are predators, some are parasites of animals and some of plants.

The snail, slug, mussel and octopus belong to the phylum Mollusca. Mollusks are soft-bodied invertebrates that usually have a shell. Characteristics shared by mollusks include a mantle and a large, muscular foot. The mantle is a thin layer of tissue covering the mollusk’s soft body. It secretes the protective shell of those mollusks that have a shell. The foot is used for moving the animal or for attaching it to an object.Between the soft body and the mantle is a space called the mantle cavity, gills are organs that exchange oxygen and carbon dioxide with the water. Gills are present in water dwelling mollusks. Land dwelling mollusks have lungs for respiration. Mollusks have a complete digestive system with two openings. Many have a scratchy tonguelike organ called the radula, which acts file a file with rows of teeth to break up food into smaller pieces.Some mollusks have an open circulatory system; the blood does not flow in vessels but surrounds the organs. The organs are grouped together in a fluid filled body cavity. Scientists classify mollusks depending on whether they have a shell. Then they look at the kind of shell, they also look at the kind of foot.

Gastropods are the largest class of mollusks. Snails and conches have a single shell. Slugs are also gastropods but they do not have a shell. All move about on a large muscular foot, and secrete mucus to help glide over objects.

Bivalves are another class of mollusks. A clam is a bivalve or an organism with two shell halves joined by a hinge. Powerful large muscles open and close the shells. Bivalves are water animals that are also filter feeders.

Cephalopods are the most complex type of mollusk. Squid, octopuses and the chambered nautilus are all cephalopods. Most cephalopods have no shell but they do have well developed head. The foot is divided into tentacles with strong suckers. These animals also have a closed circulatory system in which blood is carried through blood vessels. Cephalopods move by jet propulsion.

Segmented worms belong to the phylum Annelida. Earthworms, leeches and marine worms belong to this phylum. An annelid’s body is made of repeating segments or rings. Segmentation allows an annelid to be flexible, each segment has nerve cells, blood vessels, part of the digestive tract and coelum. The coelum is the internal body cavity that separates the internal organs from the body wall. Annelids also have closed circulatory systems and a complete digestive system with two body openings.

Earthworms have more than 100 body segments. Setae are bristle like structures which help them move or hold onto soil. Earthworms get the energy from the bits of leaves and other living matter found in the soil. Earthworm lives in a thin film of water; it exchanges CO2 and O2 by diffusion through its skin.

Leeches are parasites that have a lifestyle different from earthworms. These worms have flat bodies and sucking disks on both ends of their bodies. Some leeches can store as much as ten times their own weight in blood.

Marine worms belong to a group of annelids called polychaetes, meaning many spines. While earthworms find nutrients in the soil, leeches are parasites, polychaetes are predators. Some use powerful jaws or tentacles to catch prey. Some of these strange looking annelids can even produce their own light.


The largest group of animals belongs in the phylum Arthropoda. The term comes from arthros meaning jointed and poda meaning foot. Arthropods are animals that have jointed appendages. They are similar to annelids but have fewer, more specialized segments. Instead of setae they have different kinds of appendages like claws, legs, and even antennae. Every arthropod has an exoskeleton that protects its body. The exoskeleton is shed in a process called molting. There are five different types of arthropods; they are spiders, insects, centipedes, millipedes and crustaceans.

Insects are the largest group of invertebrates. There are more than 700,000 classified species of insects. The insects have three distinct body regions: the head, the thorax and the abdomen. The head has well developed sensory organs, including the eyes and antennae. The thorax has three pairs of jointed legs and in many species, one or two pairs of wings. The wings and legs of insects are highly specialized.

The abdomen is divided into segments and has neither wings nor legs attached to it. Reproductive organs are located in this region. Insects produce many more young than can survive. Insects have an open circulatory system. O2 is not transported by blood, but is directly brought to the tissues through small holes called spiracles located along the thorax and abdomen.

Metamorphosis is the change in the body form of an insect as it matures. There are two types of metamorphosis. Complete metamorphosis has four stages: egg, larva, pupa and adult. Butterflies, ants, bees, and moths undergo complete metamorphosis. Incomplete metamorphosis has only three stage egg, nymph and adult. Grasshoppers undergo incomplete metamorphosis.

Spiders, ticks, mites and scorpions are often confused with insects but they actually belong to a group of arthropods called arachnids. Arachnids have two body regions. The first, called the cephalothorax, is made of the fused head and thorax region. The abdomen is the second body region. All arachnids have four pairs of legs attached to the cephalothorax. Spiders are predators, they cannot chew and eat prey the way the insects do. Instead the spider uses a pair of fanglike appendages in its mouth to inject venom into the pray and paralyze it. The spider then drinks its food.

Centipedes and millipedes are long, thin, segmented arthropods that look like worms. Instead of setae, these arthropods have pairs of jointed legs. Centipedes have one pair of joined legs attached to each body segment, millipedes have two pairs. Centipedes are predators that use poisonous venom to capture their prey. Millipedes eat plants.

Crabs, crayfish, lobsters, shrimp, barnacles and water fleas are crustaceans. These arthropods live in the water. They have five pairs of jointed legs. The first pair is usually large and thicker and is used as claws to hold food. The other four pairs are walking legs. The five pairs of appendages on the abdomen are swimmerets. These are used to help move the animal through water and for reproduction. The swimmerets also force water over the feathery gills. If a crustacean loses an appendage, it can regenerate the lost part.

The name echinoderm means spiny skin. The echinoderms have spines of various lengths that cover the outside of their bodies. Most echinoderms, such as sea stars, are supported and protected by an internal skeleton made up of calcium carbonate plates. These plates are covered by thin spiny skin. Sea stars have a water vascular system. This system is a network of water filled canals. Thousands of tube feet are connected to this system. As water moves into and out of the water vascular system, the tube feet act as suction cups and help the sea star move and eat. The sea stars also have a unique way of eating. The sea star actually pushes its stomach out of its mouth and into the opened shell of the oyster. It then digests the oyster’s body while it is still inside the shell. Sea cucumbers are nothing like the other members of the echinoderm class. They have soft bodies with a leathery covering. They have few calcium carbonate plates. Sea cucumbers have tentacles around their mouth that are used to capture food. These animals are also important in keeping saltwater environments free of pollution; they feed on dead organisms and help recycle materials within the environment.