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.

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