Collier's New Encyclopedia (1921)/Turbellaria

TURBELLARIA, in zoölogy, an order of Platelminthes; flat worms of low organization, ribbon-shaped, leaf-shaped, oval, broad, or long, inhabiting fresh or salt water, or damp localities on land. The smallest are not larger than some of the Infusoria, which they approach in appearance, while the largest are many feet long. Only one genus, Alaurina is divided into distinct segments, and the outer surface of the body is everywhere beset with vibratile cilia. The aperture of the mouth is sometimes situated at the anterior end of the body, sometimes in the middle, or toward the posterior end of its ventral face. In many the oral aperture is surrounded by a flexible muscular lip, which sometimes takes the form of a protrusile proboscis. All have water vessels, opening externally by ciliated pores, and pseudhæmal vessels; most possess eyes, and some have auditory sacs. Some are monœcious, and others dioecious; in most the embryo passes by insensible gradations into the form of the adult, but some undergo a remarkable metamorphosis. The Turbellaria are variously divided by different authors. Huxley divides them into Aprocta (having no anal aperture) and Proctucha (having an anal aperture). The first group contains the Rhabdocæla and Dendrocæla of other authors; the second is equivalent to the Rhychocæla or Nemertea.