There are five main groups of Arthropods: horseshoe crabs, arachnids, crustaceans, centipedes and millipedes,.
and insects. Arachnids consists of scorpions, spiders, and mites. Crustaceans are nearly all aquatic and consist.
of lobsters, crabs, shrimps, barnacles, etc. Phylum Arthropoda make up about two-thirds of the earth's.
organisms, and includes 1 million named species (estimated total species number rising to 30.
million).Arthropods are currently thought to have evolved from Annelids.
All Arthropods are bilaterally symmetrical, and their bodies are covered with a tough exoskeleton. The.
exoskeleton consists of layers of chitin (type of polysaccharide) and proteins. This hard covering protects the.
animal, and provides points of attachment for muscle cells; but also presents it with a unique problem during.
growth: because, unlike human epidermis, it is not living, and cannot grow with the organism. Once the.
Arthropod has grown to large, it sheds the exoskeleton, and a new one is grown. This process is called.
molting. Arthropods bodies are also segmented, and divided into distinct sections (usually the head, thorax,.
and abdomen; although the head and thorax can be joined by a cephalothorax).
Arthropods live in every possible environments: fresh water, salt water, soil, and even in the most.
forbidding regions of Antarctica and high mountains. They also live in the tropics, the poles, bottom on the.
oceans, and both underground and inside other animals and plants.
The Arthropod coelom is reduced and in many adult Arthropods is absent altogether. The Arthropod.
body cavity is a hemocoel. The coelom is much reduced and usually restricted to portions of the reproductive.
and excretory, and is filled loosely with tissue, sinuses, and blood. Although developing Arthropod embryos.
show signs of a true coelom, it is lost during the course of later development. The loss of the coelom is.
probably related to its important role as a hydrostatic skeleton in other forms.