Lophotrochozoans include animals with diverse body types. Wormlike forms include some flatworms, ribbon worms, phoronids, and annelids. There has been convergent evolution of lophophores (in bryozoans and entoprocts versus brachiopods and phoronids) and of external two-
learning outcomes
You should be able to:
Associate major morphological features of protostomes with the environment and life history of the respective organisms.
Provide examples demonstrating how modifications of existing structures can lead to increases in species diversity.
How do flatworms survive without an internal transport system?
The dorsoventrally flattened body of flatworms ensures that each cell is near a body surface to allow gas exchange directly with the surface. In addition, the highly branched gut increases the surface area for transfer of nutrients to the nearby cells.
Why are most annelids restricted to moist environments?
Most annelids have a thin, permeable body wall that serves as a surface for gas exchange. Annelids rapidly lose water across this body wall if they are removed from moist environments.
Mollusks have evolved a great diversity of body plans. Provide some examples that demonstrate how the basic body organization of mollusks has been modified to yield this diversity.
The basic body organization of a mollusk includes a muscular foot, a visceral mass, and a mantle that typically secretes a hard, calcareous shell. The primary modifications of the major groups of mollusks have involved the foot and the shell. Over the course of evolution, the foot has been modified as a crawling structure (as in gastropods), a burrowing structure (as in bivalves), a clinging structure (as in chitons), or a sensory and feeding structure (as in the arms and tentacles of cephalopods). The shell has been modified to form a series of flexible but protective plates (as in chitons), a hinged pair of valves (as in the burrowing bivalves), a greatly reduced structure for internal support (as in many cephalopods), or a mobile, spiraling chamber to protect the visceral mass (as in gastropods).
The second of the two major protostome clades, the ecdysozoans, contains the vast majority of Earth’s animal species. What evolutionary innovations led to this massive diversity?