Crustaceans: Arthropod success in the sea
Another group of armoured animals also evolved from the original segmented worms the crustaceans which exist today in the form of some 35 000 species. They may prowl around rocks and reefs as crabs, shrimps, prawns, lobsters and crayfish, they may become sessile such as barnacles, or congregate and swim in vast shoals such as krill. The size of the crustacean and the form of the exoskeleton varies considerably from the paper-thin exoskeleton of the almost microscopic water flea (Daphnia) to the carapace of giant Japanese spider crab (Macrocheira kaempferi) which measures 3 m from claw to claw.
Microscopic water-flea (daphnia magna) with eggs.
JAPANESE SPIDER CRAB (Macrocheira kaempferi)
In the crustaceans the paired legs have become modified for a variety of purposes. At the anterior end they have become modified into pincers or claws, those in the middle are paddles, or walking legs or tweezers. Some have feather branches acting as gills through which oxygen can be absorbed. All limbs are jointed, tubular and operate by way of muscles. Like the primitive for crustaceans to grow they need to dispose of their calcareous carapace. As trilobites time approaches for moulting the animal absorbs as much calcium carbonate from the carapace into the blood stream, and begins to secrete a new soft wrinkled skin under the carapace. The outgrown armour splits and the crustacean swells its body by absorbing water, and wrinkled new skin stretches and hardens into a new carapace.