Tropomyosin is a protein found in crustaceans.  It’s common in almost every crustacean and in insects, arachnids and mollusks.  It’s responsible for contraction of skeletal muscle and makes up the exoskeleton.  The fact that this is found in all of these organisms proves that they have evolved from a common ancestor, considering not all these organisms aren’t crustaceans.

Chitin is a polysaccharide that makes up the exoskeleton of crustaceans.  It protects delicate tissues.  It is not only found in crustaceans but also many arthropods like insects and spiders. This proves that they may have come from a common ancestor because they all use the same substance for protection.

Annelida and Protoannelida are marine worms that have diverged from a common ancestor.  They’re both arthropods that lived in ocean sediment during the Cambrian period.  They we’re very similar and we’re both symmetrical, had soft bodies and we’re multi segmented. But, structural difference showed that they most likely evolved into two different species from an annelid ancestor 600 million years ago.  

 Coevolution

An example is rhizocephalan, which is a barnacle parasite on decapods like crabs and shrimp.  The rhizocephalan enters the host and grows throughout the digestive system.  It then plants a sac like externa for reproduction.  Once fused it produces male and female eggs.  The host thinks the externa is its own egg sac and cares for it.  Over time, these organism evolve to fit to others needs together.  The rhizocephalan has evolved so much that it's barely recognizable as a member of the cerripedia group and is hard to tell it’s not part of the host.

The complex eyes of crustaceans are very similar and perform the same function as the eyes of arthropods and cephalopods.  But, they evolved separately.