key concept 31.2 Many Lophotrochozoans Have Ciliated Feeding Structures or Life Stages

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Lophotrochozoans come in a variety of sizes and shapes, ranging from relatively simple animals with a blind gut (that is, a gut with only one opening) and no internal transport system to animals with a complete gut (having separate entrance and exit openings) and a complex internal transport system. They include some species-rich groups, such as flatworms, annelids, and mollusks. A number of these groups have wormlike bodies, but the lophotrochozoans encompass a wide variety of morphologies, including a few groups with external shells. Some lophotrochozoan groups have only recently been discovered by biologists.

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  • Flatworms have flat bodies because they lack specialized organs for transporting oxygen to their internal tissues.

  • Most annelids lack a rigid external protective covering and instead have a thin, permeable body wall that serves as a surface for gas exchange.

  • Modifications of the shell, mantle, and foot have allowed mollusks to diversify in a wide variety of environments.