The Types of Invertebrates

There are well over a million species of animals on the planet Earth. A staggering 98 percent of them are invertebrates. While they are quite varied, one aspect is key. No invertebrate has an internal bone skeleton. Some have tubes filled with fluid while others have outer shells.

  1. Insects

    • About 800,000 species of insects exist. Highly adaptable, they live nearly everywhere in the world. An exoskeleton covers every insect's entire body. It doesn't grow, so many insects have to shed it and develop a new one as they grow in size. The head has a pair of antennae and compound eyes. More than 4,000 lenses provide an insect with very good eyesight. The thorax contains the insect's legs and wings. The abdomen contains the insect's vital organs, the heart and resperatory systems as well as the digestive systems.

    Arachnids

    • Like insects, arachnids have exoskeletons. But they do not have antennae. They usually have four pairs of legs. Sometimes the front two will be used for hunting and feeding. All eight legs are used for walking. Most have eight eyes and poisonous fangs, and many spin webs for catching prey and holding egg sacks. Scorpions are large, some reaching 8 inches in length. They have distinctive pincers for catching and holding prey as well as a poisonous stinger at the end of a tail. Mites and ticks are very small and can be parasitic disease spreaders.

    Crustaceans

    • Crustaceans live mostly in oceans or other bodies of water. They have a hard shell protecting their bodies. The heads have antennae which act as sensory organs. The abdomen includes the digestive system, heart and reproductive system. The abdomen contains the legs, used for walking and swimming. Most crustaceans have claws. Examples included crabs and lobsters.

    Mollusks

    • Some fossils of mollusks have been found to be more than 500 million years old. This makes mollusks one of the oldest life forms on Earth. The body of a mollusk contains a skin-like organ covered by a hard shell. Snails and slugs are mollusks that live on land. They move slowly by a soft sole called a foot. Ocean-dwelling mollusks move by jet propulsion by ejecting water from their bodies. Oysters attach themselves to rock surfaces and feed by filtering nutrients from the water. Mollusks without shells include squid, cuttlefish and octopus. Those with shells include clams and the nautilus.

    Annelids

    • Existing on Earth for more than 150 million years, annelids can be found almost everywhere. They have segmented bodies, extremely well-developed internal organs and a noticeable lack of limbs. Some have long bristles while others, like the earthworm, have bristles so short they aren't seen. Most worms are small, getting no bigger than a few inches. However, the ribbon worm can grow up to 100 feet. Some worms are parasites and can live inside the human body. Leeches are another type of annelid.