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.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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