Arthropods are considered the most successful animals on the planet. They live on land and mountains, in sea and air. Nearly 3/4 of all known living and fossil organisms are arthropods. The true number of arthropods is not known, as there are many undiscovered species, especially in tropical rain forests. The phylum includes crustaceans such as crayfish, lobsters, crabs and shrimps, insects, spiders and other arachnids. Arthropods vary in size, from being microscopic to having a 12-foot arm span like the king crab.
All arthropods have a hard exoskeleton made up of a compound known as chitin. The exoskeleton prevents loss of water and protects the body. To facilitate movement within the exoskeleton, the chitin is present in the form of plates with joints in between giving the phylum its name, which means "jointed feet". The chitin is covered with a hard waxy cuticle, which forms a connection between the body segments. As the animal grows, the exoskeleton is shed through a process called molting.
Division of Arthropods
The phylum is divided into chelicerates and mandibulates. The chelicerates have a fused head and thorax and abdomen. They lack antennae and have four pairs of jointed legs. The first pair of legs is modified into claws. This group consists of arachnids, sea spiders and marine horseshoe crabs. The mandibulates have a head, thorax and segmented abdomen. They have one or two pairs of modified appendages that work as antennae and the next pair of appendages is modified into jaws. The group includes crustaceans, millipedes, centipedes and insects.
The breathing technique varies based on where the arthropod lives. Land-based arthropods breathe with the help of air tubes known as tracheae. Arachnids, such as spiders, use book lungs, which are thin tissue flaps arranged similarly to pages of a book, for breathing. Water-dwelling arthropods breathe with the help of gills.
Arthropods have an open body cavity known as hemocoel. The blood flows through this cavity and in the process covers the tissues and organs. Arthropods usually do not have blood vessels.
Most arthropods start their lives as eggs and then have different lifecycles based on the group they belong to. Some insects hatch into miniature adults and others hatch as nymphs and then go through several developmental stages before becoming adults. There are other arthropods that hatch into larvae and overwinter in cocoons as pupae before coming out as adults.
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