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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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.