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.
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.
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.
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