The Phyla Arthropoda has spiders, insects, and crustaceans in it.  Arthropods like most other animals are bilaterally symmetric.  Sexual reproduction is the only reproduction used by arthropods. Arthropods have different segments each performing their own special function to contribute to the organism.  The exoskeleton of Arthropods is made up of Chitin, a tough semi-transparent substance used in exoskeletons. Arthropods have an open circulatory system making it easier for the heart to transport blood.  In Arthropods some species have gills for gas exchange and other employ Tracheae(book lungs) for gas exchange.  Metamorphis is the stages of which an insect grows. Incomplete has 3 stages, and complete has 4 stages. I think Arthropods are most represented because of their organism diversity.

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Scorpion

Scorpions have adapted to many environmental conditions living in many places around the world. 

Barnacles

Barnacles are encrusters, meaning they attach themselves permanently to a solid substrate. 

Horseshoe Crabs

Horseshoe Crabs live primarily in shallow waters with soft sandy/muddy bottoms.