Invertebrates

An invertebrate is a multicelled animal that does not have a backbone. Within this seemingly simple grouping there is an amazing variety of complex life forms: from sponges and starfish to earthworms, clams, spiders, and butterflies. Of the roughly 1,500,000 different species of animals in the world, more than 95 percent are invertebrates. They inhabit nearly every type of environment on Earth and vary greatly in the way they live and reproduce. An invertebrate may be as soft as a jellyfish or as hard as a lobster but they have one distinction in common—they have no bony vertebral column or backbone.

In nature there is no actual dividing line that separates animals with backbones from those without one, but grouping the members of the kingdom Animalia in this manner allows biologists to sort them into very broad groupings. The animal kingdom is divided into major groups called "phyla," (singular, phylum), and of all the animal phyla identified (some say there are as many as thirty-eight), only one includes vertebrates. The rest are invertebrates. This gives some sense of how successful these "lower" animals have been in the race for survival. Invertebrates not only live almost everywhere on Earth, but range in size from an organism too small to be seen without a microscope to a giant squid measuring 60-feet (18.29 meters). Invertebrates are often considered to be pests, yet despite our best efforts to exterminate them, they seem to adapt and thrive.

SPONGES

The sponge is the simplest of all invertebrates (and therefore the simplest of all animals) and lives at the bottom of the sea. Early naturalists considered sponges to be plants since they looked like a plant and did not move. Later, as they were studied more, sponges came to be considered "zoophytes" or plant-animals. Today they are considered to be the simplest of animals and are placed in the phylum Porifera. The light brown, oddly shaped sponge we sometimes use to bathe ourselves or wash the car began its life attached to the bottom of one of Earth's seas, where it used its many holes or pores to draw in water and filter it for food. Sponges have bodies resembling a sack with an opening at the top. They have no organs or nervous system, and usually reproduce sexually (with sperm fertilizing an egg). However, some sponges can also reproduce asexually. For example, when a sponge piece breaks off, floats away, and happens to settle in a proper place it begins to grow. Sponge "farmers" cut up living sponges and place their pieces on a rock underwater where a full-sized sponge will grow in a few years. The sponge we use in the bath is the dried (but very absorbent) skeleton of an invertebrate that was once alive. Sponges are so different from all other animals that some biologists believe that they should have their own animal subphylum.

CNIDARIANS

One step up the invertebrate ladder of complexity from sponges are the members of the phylum Cnidarian (the "C" is silent). Also called coelenterates (Latin for "hollow gut"), cnidarians include jellyfish, sea anemones, corals, hydra, and their relatives, all who live in the water. Although they are not related to sponges, cnidarians are usually listed after sponges up the invertebrate ladder because they started with the sponge's simple multicelled form and added many features not found in sponges. One of the things cnidarians added was a "hollow gut" or specialized digestive cavity, which is attached to a type of mouth (that also serves as an opening to expel waste).

A cnidarian has only one opening that serves two purposes: to take in food and expel waste. All cnidarians have armlike projections called tentacles that hang down around their mouth. When a small fish bumps into them, the tentacles react and sting or grab the fish, reeling it into its mouth. Since cnidarians are designed to be organized around a single food-gathering mouth, their body form is described as having "radial symmetry." This means that the cnidarian body has no definite right or left side but resembles spokes radiating from the hub of a wheel. Some cnidarians, like coral, are filter feeders and stay in one place, while others, like the jellyfishes, can swim around.

Cnidarians reproduce sexually, but they can also duplicate themselves asexually by budding or producing new cells that separate from the parent and become independent organisms. Cnidarians are named because of specialized nerve cells called "cnidoblasts," which makes their stingers work. Stinging tentacles are used to get food and for defense. These tentacles are not linked by a central nervous system but operate independently and almost automatically in a stimulus/response manner. This is why a dead jellyfish can continue to inflict a bad sting to someone touching its tentacle.

WORMS

Worms are more complex than sponges or cnidarians, and although the word worm refers to any animal that has a long, soft body without legs, a worm is far from a simple animal. The most important difference between worms and the other two invertebrate types is not obvious, however. Instead of having two layers of cells in their bodies, like sponges and cnidarians, worms and other "higher" animals have three layers of cells. Because of this middle layer between its external and internal layers, the worm has specialized tissues and organs that sponges and cnidarians do not have. The many different kinds of worms are gathered into three groups: flatworms, roundworms, and segmented worms. All members of the last two groups show bilateral symmetry, meaning that if they were cut down the middle, there would be two matching halves.

Flatworms. Flatworms, which belong to the phylum Platyhelminthes, move about for their food and therefore have a definite front and back end. This means that they have developed a head or at least a forward part in which nerves and senses are concentrated. Along with a nervous system, flatworms also have a separate excretory system and a reproductive system. The simplest flatworm is flat and always found in water. The most common is the planarian, which has only one opening to take in and

expel waste. A tapeworm is a parasitic flatworm that lives in the digestive system of its host where it attaches to an intestine wall and absorbs already-digested food. Flatworms reproduce sexually and asexually, and if cut in half, both parts will grow into a complete animal.

Roundworms. Roundworms have a cylindrical body, a tough outer cuticle, and are found in both water and soil. Also called Nematoda, many are parasitic. Pinworms are a well-known roundworm, as are the trichina that causes trichinosis (a disease caused by eating infected pork or meat). Roundworms reproduce sexually and have a separate digestive and circulatory system.

Segmented worms. Segmented worms or true worms belong to the phylum, Annelida which means "little rings." An annelid's body is therefore made up of segments or little rings attached together. The earthworm is a good example of a typical annelid in that its body is more complicated than that of other types of worms. Their digestive system contains organs with special jobs, and their nervous system has a distinct brain in the front end or head. Since each body segment has a set of muscles, annelids can slowly move about by changing their segment shapes. A very important annelid advance is the development of a "coelum," a lined body cavity that not only provides support but allows organs to be suspended inside the body. The coelum is found in all the more complex animals, including humans. Most annelids live in the soil, and some, like leeches, are parasitic. Reproduction in annelids is usually sexual. Although an earthworm is both male and female, it must mate with another earthworm before each can lay eggs.

MOLLUSKS

Although the next invertebrate phylum Mollusca means "soft," most of these invertebrate have a hard shell. Clams, oysters, and scallops are all mollusks as are squid, octopus, and snails. Despite the shell of some, they all are soft-bodied with some form of covering or mantle. In some, it is hard tissue and in others it is a very hard shell. Most mollusks have some sort of foot or appendage for feeding and moving about. Besides a digestive and circulatory system, they also have a well-developed nervous system, and some even have eyes. Finally, mollusks that live under water have specialized gills that take oxygen out of the water and put it into their blood. Mollusks also reproduce sexually.

ECHINODERMS

The phylum Echinoderm means "spiny skinned" and is made up of invertebrates like starfish and sand dollars that have hard outer plates. These plates usually cover a body that has five separate parts like the spokes of a wheel. Echinoderms live in the sea and have a specialized system of canals in their bodies that connect to their many tube feet. These feet suck in water and allow the echinoderm to attach itself to something solid (like a clam shell which it can then pry open). The echinoderm has a complete digestive system although it has no excretory or respiratory system. All echinoderms reproduce sexually. They also are able to grow back any lost parts through regeneration.

ARTHROPODS

The phylum Arthropoda is considered the largest and most successful phylum in the kingdom Animalia. Arthropods live nearly everywhere on Earth. All have at least three pairs of jointed legs and a body divided into jointed segments that is covered by a hard, outer case called an exoskeleton. Arthropods have internal body systems that break down food, take in oxygen, circulate blood, and carry away wastes. Most reproduce sexually. Arthropods are so varied that there are five major types: crustaceans (lobsters, crabs); arachnids (spiders, ticks, mites); insects (bees, ants, beetles); centipedes; and millipedes.

As the largest animal group, invertebrates are an essential part of every ecosystem. Humans could not function without them since they are responsible for the decomposition of organic waste, which allows the recycling of essential chemicals. Invertebrates are also involved with the pollination of plants and are a crucial link in the food chain where herbivores (plant-eating animals) convert the energy in plants into a form useful to meat-eating animals.

[See alsoArachnids; Arthropods; Crustaceans; Insects; Mollusks; Protozoa ]