Physical characteristics

The class Turbellaria share the following characteristics with other classes within the Platyhelminthes:

• triblobastic (three tissue layers)

• acoelomate (no fluid filled body cavity or coelom)

• bilaterally symmetrical

• dorsoventrally flattened

• spiral cleavage and mesoderm derived from the 4d cell

• complex, incomplete gut (no anus)

• cephalized, with cerebral ganglion (brain) and longitudinal nerve cords that form a ladder-like nervous system

• numerous sense organs at the anterior end of the body, and tactile receptors distributed over the body, especially around the pharynx

• protonephridia that function in excretion and os-moregulation

• no circulatory system, which restricts the size and shape of these animals

• hermaphroditic with complex reproductive system

Turbellarians also are free-living or commensal (not usually parasitic), usually aquatic, and have a stomodeal pharynx. Their cellular epidermis is usually ciliated and contains mucous secreting cells and structures called rhabdoids that can produce copious mucus to prevent desiccation. Most turbel-larians also have pigment-cup occelli for detecting light; some have an anterior pair where larger species may posses numerous pairs along the body.

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Essentials of Human Physiology

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