Of the million or so known animal species, more than 98 per cent have no backbone and are classed as invertebrates. They have a wide variety of shapes and sizes, from tiny animals only a fraction of a millimeter in length to the giant squid, which may measure up to 55 feet (17 meters) long. Invertebrates have an extraor dinary range of life styles, and occur at every level of the complex food web that links all forms of life.

The zebra spider, a jumping spider, is a member of the phylum Arthropoda the largest group of animals, containing about 900,000 species. Jumping spiders can leap a short distance; they hunt primarily by sight and have the best vision of all spiders. Their eight eyes, arranged in two rows of four, consist of a large number of receptors and can perceive a sharp image of considerable size. In addition to their keen eyesight, the spiders have chemosensitive hairs at the tips of their appendages.

Classifying invertebrates

Scientists group invertebrates in a number of ways besides the formal organization into phyla, orders, genera, and species. The first is according to whether the body is bilaterally symmetrical or radially symmetrical. An animal that is bilaterally symmetrical, such as an earthworm, has matching right and left sides and has recognizable front and back ends. An animal that is radially symmetrical, such as a sea star, is wheellike and can be divided into two matching halves by any line that passes through the center. Radially symmetrical animals have no recognizable front or back.

Scientists also distinguish invertebrates according to the number of cell layers they have. A few invertebrate phyla are made up of species whose bodies are composed of only inner and outer cell layers. These are the simplest of all multicellular organisms.

The bodies of most invertebrates, however, consist of three tissue layers. In addition to inner and outer layers, they have a third layer in the middle. These types of invertebrates are further classified according to whether they have a coelom an internal, fluid-filled body cavity.
The coelom provides a space where organ systems typical of higher animals can develop. Animals with a true coelom are called coelo-mates. Animals that have a primitive coelom are called pseudocoelomates. Animals that lack a coelom are classified as acoelomates. The area between the outside of the body and the acoelomate’s gut is solid tissue.

Finally, scientists describe invertebrates by habitat and life style. Unlike vertbrates, which can all move freely from place to place, some invertebrates are sessile, or fixed in one spot. Others are described according to whether they eat meat or plants, whether they filter food from the water, or whether they extract food from sediment. Many invertebrates, such as certain worms, are parasites, which live in or on the body of a plant or another animal. Others live symbiotically with another organism. In a symbiotic relationship, both members benefit.

Types of invertebrates

There are about 35 invertebrate phyla, but because new groups are continually being discovered and the classification of invertebrates is constantly under revision, the exact number and distribution of phyla is debatable.

Nine groups are particularly important because they represent about 90 per cent of all living invertebrates. These nine phyla are: Porifera (sponges), Cnidaria (jellyfish and related species), Ctenophora (comb jellies), Platyhel-minthes (flatworms), Nematoda (roundworms), Mollusca (snails, squids, and clams), Annelida earthworms, ragworms, and leeches), Arthro-poda (insects, spiders, and decapods), and Echinodermata (sea stars and sea urchins).

The simplest of these phyla are the porifer-ans. In fact, scientists once considered them as plants because they lacked so many of the characteristics of higher animals. Cnidaria and Ctenophora are slightly more advanced, with a mouth but no specialized tissues for digestion or excretion. Platyhelminthes are slightly more highly developed than the cnidarians, having three layers of body cells. Most platyhelminthes, however, lack a body cavity, a circulatory system, and an excretory system. The annelids, or segmented worms, which include earthworms as well as numerous species of aquatic worms, are the most highly developed worms.

All the animals in the phylum Echinodermata are based on the same radial structure, usually consisting of five or ten arms radiating from a single mouth. They live only in the ocean and are all slow-moving or fixed to one spot and unable to move around.

The arthropods are the most successful invertebrates in terms of numbers of species: the arthropod phylum contains more than three-fourths of all the different kinds of animals. All arthropods have jointed limbs and segmented bodies covered by a hard exoskeleton. This body covering acts as a protective armor and as a frame onto which muscles can attach.

The largest class of arthropods is Insecta, the insects, which are primarily land-dwelling. Crustaceans, the second largest class of arthropods, range from the relatively advanced lobsters, crayfish, and crabs to the tiny copepods that graze on the phytoplankton in the oceans. The third group comprises the arachnids—spiders, ticks, scorpions, and related species. Of the six remaining classes of arthropods, only the centipedes and the millipedes are represented in great numbers.
Mollusca is the second largest phylum, after the arthropods, containing more than 100,000 species. They are probably the most highly developed of all invertebrates. Most mollusks have a hard shell enclosing a soft body, and although they conform to a general anatomical pattern, their external body forms are extremely varied.

The minor phyla

The remaining phyla include the smallest phylum, Placozoa—with only a single species— and Priapuloidea, which contains only nine known species of tiny, primitive cucumbershaped, seabed-dwelling worms. Most of the small phyla comprise marine species that are sand or seabed dwellers.
Some invertebrates seem to form a transition between the invertebrates and the vertebrates. These are the hemichordates, such as acorn worms; the tunicates, including sea squirts; and the cephalochordates, or lancelets, which are members of the phylum Chordata. Animals of the phylum Chordata have a simple skeletal rod, or notochord, which is considered an earlier stage of the spinal column.

Sea slugs are gastropod members of the phylum Mollusca, which also includes snails, limpets, abalones, and slugs. They are among the most brilliantly colored of all marine invertebrates and, unlike most other mollusks, they have no shell. Some have colored tentacles called cerata along their back.