Phylum Arthropoda: Joint-legged animals

This is a huge assemblage of animals, and the largest phylum. Among its many members are the half-million named insects, plus the numerous spiders, horseshoe crabs, millipedes, centipedes, crabs, and lobsters. Though insects are conspicuously absent from the oceans, some species of arthropod is likely to be encountered anywhere in the biosphere.

All arthropods share certain features, including a segmented bilaterally symmetrical body, jointed legs in 3, 4, or 5 pairs, a separate mouth and anus, an external hard skeleton of chitin, and protostomal embryological development (the mouth forms before the anus).