What are the more common body plans in protostomes? How have they been modified in different groups?
Protostomes are bilaterally symmetric (one plane of symmetry) and triploblastic (three tissue layers). Most also follow the tube-within-a-tube design, where the outer tube is the animal’s skin and the inner tube is the lining of the gut. Within the inner tube are the food particles, enzymes and waste products of the digestive system. All other muscles and organs are located between the two tubes.
This body plan is modified in many ways. Animals differ in the way muscles and organs are organized between the two tubes. In coelomates, such as annelids, this space is a body cavity fully lined with muscle and filled with fluid. Organs can move throughout this cavity more freely. In acoelomates, like platyhelminths, there is no body cavity and organs are relatively rigidly set in place. Other phyla, like the arthropods and mollusks, don’t have fully functioning coeloms because they have other structures to provide fluid circulation and structure. Arthropods have a body cavity called a hemocoel which uses an exoskeleton instead of muscles as lining. Mollusks use a region called the visceral mass to contain most internal organs. This cavity is supported by a mantle, not muscle.