Planarians-Class Turbellaria

Thursday, September 4, 2014

The best-known turbellarians, which are flatworms, are the planarians, which range in size from microscopic to several feet in length. They have an extensible pharynx in the middle of their underside with which they suck in food to their mouth. Their mouth opens into a gastrovascular cavity. Their body is covered by cilia. They have no anus, and wastes are expelled through their mouth and the pharynx.

A familiar species, Dugesia tigrina, lives in many streams and ponds. The adult is about 2.5 cm long and grey in color. This worm has well-developed reproductive organs, not just single cells that produce eggs and sperm. It also has excretory organs called flame cells and well-developed muscles. However, the most primitive turbellarians are small marine worms called "acoels"; these simple creatures do not even have a gut. The food they ingest is simply engulfed by phagocytic cells in their endoderm.