Platyhelminthes: Class Turbellaria
Class Turbellaria
The mostly free-living, primarily carnivorous, flatworms of class Turbellaria are characterized by a soft epidermis that is ciliated, at least on the ventral surface. The movement of the cilia propels the smaller forms. Larger species glide along by muscular waves, usually over mucous beds secreted by special cells.
Turbellarians are generally divided into five groups based primarily on differences in the form of the digestive cavity, a structure that is readily observable through the transparent body wall. The most primitive turbellarians, the acoels, have no digestive cavity. The ventral mouth, and sometimes a simple pharynx, lead to an inner mass of nutritive cells. Most species measure less than
The rhabdocoels have straight, unbranched digestive cavities. Some authorities believe that the rhabdocoels gave rise to both the trematodes and cestodes because several rhabdocoel species exhibit commensal relationships, which presumably could have given rise to parasitism. The allocoels were formerly classified together with the rhabdocoels; the gut can be either saclike or branched.
The triclads, also known as planarians, are relatively large flatworms named for their three-branched gut. Most species range from
The group of turbellarians known as polyclads tend to be larger (1–2 in./2.5–5 cm) and more oval-shaped than the triclads. Their bodies are extremely flat and leaflike, and the gut is subdivided into numerous branches. Many are brightly colored and some have ruffled edges. Some species have numerous eyes scattered over the front end of the body.
Sections in this article:
- Introduction
- Class Monogenea
- Class Cestoda
- Class Trematoda
- Class Turbellaria
- Anatomy
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