Describe the Phylum Arthropoda and Metamorphosis?

More than one million different species of arthropods are estimated to exist on the earth today-more than all other animal species combined! In terms of numbers of individuals, some scientists estimate there are more than one quintillion (one billion billion) arthropods of all kinds living on earth. Because of these statistics, and the fact that they have been around for over 600 million years, arthropods deserve to be characterized as a very successful group.

What is it about arthropods that makes them different? Three features are thought to have contributed to the success of the arthropods. 1.) Their bodies are segmented. 2.) They all have a hard exoskeleton made of the carbohydrate chitin. 3.) They all have jointed appendages. By the way, the name "arthropod" reflects this feature, as "arthro" is derived from the Greek word meaning "joint," and "pod" is the Greek word for "foot."

Segmentation among the arthropods evolved to a level beyond the Annelids, in that several individual segments became fused into larger functional units called tagmata. Examples of "tagmatization" are the evolution of distinct units such as the head, thorax, and abdomen. In some cases, such as a lobster, a head unit was fused to a thorax and formed a cephalothorax.

The exoskeleton, or exterior skeleton, provides for physical protection from damage, gives support for the body, and, connected to muscles, enables very efficient locomotion. A cuticle covering the exoskeleton helps the body retain water that would otherwise be lost to evaporation. Having an exoskeleton necessitates several interesting processes that are peculiar to the arthropods. Since the exoskeleton is a rigid structure, it must be periodically molted, or shed, in order to allow the organism to grow.