Chapter 31- Protostome Animals

31.1 What Is a Protostome?

• Adaptive radiation is a period of rapid speciation resulting in a large number of descendant species that have adapted to a variety of habitats. 
• ==Adaptive radiations tend to occur due to a combination of ecological opportunity and morphological, physiological, and/or behavioral innovations==
• Adaptation is a heritable trait that increases the fitness (reproductive success) of individuals in a particular environment.
• ==Land animals exchange gases with the atmosphere readily as long as ey have a large, moist surface area that is exposed to the air==
• Protostomes such as mollusks and arthropods have relatively compartmentalized body plans-their bodies are divided into different regions with different functions. 

31.2 What Is a Lophotrochozoan?

• The name Lophotrochozoa is derived from two distinctive morphological traits that occur in some, but not all, members of this lineage:
  • A  feeding structure called a lophophore, which is found in three phyla 
  • A type of larva called a trochophore, which is common to many of the phyla
• Lophophore is a specialized structure that rings the mouth and functions in suspension feeding.
• A trochophore larva has a ring of cilia around its middle. 
• Flatworms are named for the broad, flattened shape of their bodies.
• Unlike flatworms, most annelids have a coelom, a fully developed digestive tract with a mouth and an anus, and a segmented body.
• The common ancestor of the annelids had a key synapomorphy in addition to segmentation: bristle-like extensions called chaetae that extend from lobe-like appendages called parapodia
• Mollusks are a highly diverse monophyletic group of lophotrochozoans, comprising more than 85,000 species
• One synapomorphy for mollusks is a body plan based on the following three major components:
  • foot, a large muscle located at the base of the animal, used in movement
  • visceral mass, the region containing the main organ systems
  • the mantle, an outgrowth of the body wall that covers the visceral mass, forming an enclosure called the mantle cavity. 
• The organs occupy a different type of body cavity called a hemocoel, where body fluids bathe the organs directly in an open circulatory system
• At the anterior end of the visceral mass, the mouth has a unique molluscan feeding structure called a radula, which functions like a rasp or file.
• The visceral mass often includes one or two external gas exchange structures called gills. 
• In bivalves, the mantle is lined with muscle and forms tubes called siphons. 

31.3 What Is an Ecdysozoan?

• All ecdysozoans grow intermittently by molting-that is, by shedding an exoskeleton or external covering.
• During a molt, an individual sheds its outer layer, or cuticle-called an exoskeleton if it is hard-and slips out of it
• Roundworms are unsegmented worms with a pseudocoelom, a tube-within-a-tube body plan, no appendages, and an elastic cuticle that is molted during growth
• ==In addition to their bilateral symmetry, triploblastic tissue origins, and other protostome traits, arthropods (“joint-foot”) are characterized by the following three key features:==
  • ==A segmented body==
  • ==An exoskeleton==
  • ==Jointed appendages==
• Myriapods have relatively simple bodies-a head region and a long trunk featuring a series of many segments, each bearing either one pair of legs or two pairs of legs 
• A compound eye contains many lenses, each associated with a light-sensing, columnar structure 
• Insects have three tagmata-a head, thorax, and abdomen.
• Crustaceans live primarily in marine and freshwater environments, where they play important ecological roles. 
• Chelicerates are named for a pair of claw like appendages called chelicerae, located near the mouth 
• In incomplete metamorphosis ( a form of direct development) juveniles called nymphs look like smaller versions of the adult. 
• In complete metamorphosis ( a form of indirect development) there is a distinct larval stage.