Arthropods typeArthropoda;

Type invertebrate. The largest group of animals in the world. Arthropods are divided into nine classes (not counting the extinct trilobites) the most important being crustaceans (class Crustacea), arachnids (class Arachnida), myriapods, and insects (class Insecta). Despite their common body plan arthropods exhibit tremendous diversity. Their segmented body is bilaterally symmetrical.

The segments often join to form two or three parts. Myriapods have a head and a thorax, crustaceans and arachnids have a cephalothorax and an abdomen, and insects have a head, a thorax and an abdomen. All arthropods have jointed appendages (hence the name Arthopoda, meaning jointed legs). They develop an outer exoskeleton providing attachment surfaces for muscles. They have well developed senses, in particular smell and vision. All arthropods have digestive, nervous, respiratory, circulatory, and excretory systems. Arthropods reproduce sexually and the sexess are separate with very few exceptions.

From among arthropods, reefs are inhabited mostly by crustaceans: shrimp (cleaning parasites from other animals by shrimp is a very interesting phenomenon), crabs and lobsters.