Insect, spiders, crustaceans, centipedes, and millipedes are members of Phylum arthropods. The word Arthropods means "joint foot" Many arthropods are fully adapted to life on land; many others can fly, while number live in the water.
The presence of an outside skeleton distinguishes arthropods from many animal groups. The exoskeleton is made up mainly of chitin and proteins. The exoskeleton of flies and spiders is thin and flexible. The shell of a crab is actually a part of the exoskeleton, it hard and thick. The same kind of exoskeleton is found in lobster and other crustaceans, beetles, ticks, horseshoe crabs, and other arthropods. The exoskeleton protects an arthropods body from injury and from drying up.
How can arthropod such as crab grow if its body is covered with a hard, unstretchable exoskeleton? Many arthropods get rid of their exoskeleton and replace it with a new one in one process called molting. Have you seen a crab with a soft shell? Such as crab has just undergone molting. Before an arthropod undergoes molting, it produces a new exoskeleton. Then, the hard, old skeleton splits. The arthropod, then crawls out of it. The new exoskeleton hardens. Before it reaches maturity, an arthropod undergoes four to seven molting. After reach molting, the arthropods grow.
The classes Phylum Arthropods share many characteristics. Member of class Crustacea have two pairs of antennae in front of the body and two body regions, abdomen and cephalothorax. All crustaceans, except the pill bug, are aquatic. They breath by means of gills. They range in size from tiny waterfleas to crabs and lobsters weighing several kilograms.
Crustaceans have gills- i knew shrimps and lobsters had gills but never thought crabs did too. This is cool for a first article