Crustaceans are a subphylum of the phylum with the most members and species -- the Arthropods.  Arthropods are segmented animals with exoskeletons.  This is a very diverse phylum, with many of the animals being land dwellers.

    Crustaceans are animals that have solved the predator / prey problem by growing exoskeletons.  An exoskeleton is one that is outside the body.  Inside the exoskeleton all the muscles and organs are protected.  The muscles are attached to the exoskeleton so that the limbs can be moved.  The legs are jointed, and crustaceans can move very quickly.

     Crabs and lobsters have exoskeletons. The skeletons provide protection and support for the animal's soft tissues. They give form to its body. However, the exoskeleton hardens and does not grow.  If the animal is to grow, it has to be able to get out of its exoskeleton somehow. It may grow a soft new exoskeleton underneath the hard protective one. Then it sheds the old one, and hides for a few hours to let the soft new skeleton stretch out into a larger size and harden.  The crab is vulnerable when the shell is soft..  The hard shell that covers a crab's back is called the carapace.  Related crustaceans, such as shrimps and lobsters, also have carapaces, which protect their bodies from predators and injury.

     There are many different kinds of crabs that have similar structures   The proportions of their bodies and legs differ, but they all follow the same general body plan.  They are hardy and adaptable, and are found at all depths in the oceans and in fresh water as well.

     Scientists have been studying crab DNA, and they think that these crabs evolved, not once, but as many as five times from lobsters and / or shrimps.

     A crab is built a lot like a flattened lobster with its tail tucked under its body.  The flattened under body has more area than a lobster's body of about the same size,  and is a better place for the females to attach their spongy egg mass.  Crabs are good mothers, and they carry the developing eggs around with them until the eggs are ready to hatch.  They also clean the eggs, and keep algae from growing on them.  When the crab larvae are ready to hatch, the mother finds a place with a good current to carry the babies away.

     This little creature is a larval crab.  It doesn't look very much like its parents yet.  It is a very small creature, and it will spend time  floating in the phytonplankton, eating organisms smaller than itself and molting out of its shell.  When it is large enough it will settle down in the environment where it will spend its adult life.

     Other crustaceans include the shrimp and lobsters.

     As for lobsters and crabs, it is very difficult to say which is more derived. They are VERY closely related, both being in the Phylum Arthropoda, Subphylum Crustacea and Order Decapoda (which also includes shrimps).

Shrimp