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.