Of the million or so existing animal species, more than 95% have no backbone and are classed as invertebrates. They have a wide variety of shapes and sizes, from microscopic single-celled (acellular) animals only a fraction of a millimeter in length, to a giant squid (Architeuthis sp.), which may measure up to 65 feet (20 meters) long and is strong enough to fight with whales that prey upon. Invertebrates have an extraordinary range of different life styles and occur at every level of the complex food web that links all forms of life.
There are about 33 invertebrate groups (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% of all living invertebrates. From the simplest form to the most complex these nine groups are:
- Acellular animals (Protozoa)
- Sponges (Porifera)
- Jellyfish and their allies (Cnidaria)
- Flatworms (Platyhelminthes)
- Roundworms (Nematoda)
- Snails, squids and clams (mollusks)
- Earthworms, raworms and leeches (Annelida)
- Insects, spiders and decapods (arthropods)
- Starfish and sea urchins (Echinodermata)
The protozoa are very small, ranging from 3 millimeters in length to a microscopic size consisting of a single cell within which all the functions for life are carried out. For this reason many scientists classify Protozoa in their own subkingdom to distinguish them from the multicellular animals, which are classed as Metatzoa. The simplest multicellular group is the sponges, with some 10,000 species.
The cnidarians include the jellyfish, sea anemones, corals and hydroids. They are one of the most simply organized of the multicellular animal groups, having 2 layers of cells which surround a tubular body cavity, with an opening on one end forming a mouth, and no specialized tissue for respiration or excretion. Most cnidarians are marine animals, although there are a few freshwater species.
The platyhelminths fall into 3 classes: the free-living turbellarians, the parasitic flukes (Trematoda) and the parasitic tapeworms (Cestoda). They have developed a little further than the cnidarians, having 3 layers of body cells, but most of them lack a body cavity, circulatory and excretory system, these functions being carried out by cells on the surface of the body. The turbellarians and flukes have a digestive tract which opens from the mouth. The Cestoda absorb food directly from their hosts and have evolved with no digestive tract at all.
The roundworms, or Nematoda, are unsegmented and usually have a digestive tube with a mouth at one end and an anus at the other. The gut is surrounded by a fluid-filled cavity, the pseudocoel, which acts as a hydrostatic skeleton. They are an extremely successful group and are found free-living in all environments, although most of the 10,000 known species are parasitic. Nearly all vertebrate animals and many invertebrates can be hosts to parasitic roundworms.
The annelids, or segmented worms, are the most highly developed worms. They are divided into three major groups: bristle worms (Polychaeta), most of which are marine; earthworms (Oligochaeta), which are mainly freshwater and terrestrial, and leeches (Hirudinea), which are found in the sea, freshwater and on land.)
The phylum Echinodermata includes starfish, sea urchins, sea lilies, sea cucumbers and brittle starts. These five distinct groups are based on the same radial structure, usually consisting of five or ten arms radiating from a single mouth. They are exclusively marine and are all slow-moving or sessile.
The phylum Mollusca is the largest after the arthropods, containing more than 100,000 species, and probably the most sophisticated 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 mollusks contain 3 main classes&, Gastropoda (such as snails); Bivalvia (which contains oysters, mussels and clams); and Cephalopoda, which includes octopuses, cuttlefish and squids.
The arthropods are the most successful invertebrates in terms of numbers of species (about 900,000). All have jointed limbs and segmented bodies, covered with 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 are the insects (Insecta), which are primarily land-dwelling. Crustaceans, which are found mainly in salt and freshwater, are the second largest class of arthropods, and show a greater diversity of structure and physiology than the insects. They range from 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 their allies) which are mostly terrestrials and usually have four pairs of legs, no wings or antennae, and pincers rather than simple mouths. Of the six remaining classes of invertebrates, only the centipedes and the millipedes are represented in great numbers.
The remaining phyla include the smallest phylum Placozoa&, with only a single species&, and Priapulida, which contains only 9 known species of tiny, cucumber-shaped, seabed-dwelling worms. Most of the small phyla comprise marine species which 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 (lancelets), which are member of the phylum Chordata and have a simple skeletal rod (or notochord) at some stages of their development.