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.