GENERAL FEATURES 



Planarians or freshwater triclads are elongated, flattened Turbellariaj 

 the native American species measuring in length from about 2 mm (freshly- 

 hatched) to perhaps 40 mm. In the extended, quietly gliding animal one 

 distinguishes an anterior portion or head, often characteristically 

 shaped, bearing important sense organs. The lateral margins of the head 

 may protrude as auricles (which have chemical and mechanical sensory 

 receptors). There are usually two dark eye spots on the head, each 

 located near the medial margin of a white area, often accompanied by 

 smaller accessory or supernumerary eyes. Some species have numerous 

 eyes and subterranean planarians are often eyeless or blind. A median 

 adhesive organ may be differentiated on the frontal margin of the head. 

 The body may or may not be pigmented, often exhibiting various patterns 

 of pigment arrangement. The mouth (m) , which serves also as anus, is 

 situated in the midline on the ventral surface, far removed from the 

 head; the gonopore or genital aperture is found in the space between the 

 mouth and the posterior end. 



The body is covered with a ciliated epidermis containing rhabdites, small 

 rod-like structures which, when discharged, produce abundant mucus. The 



Fig. 2. Digestive system of a 

 planarlan. ai, anterior intes- 

 tinal ramus; au, auricle; e, eye; 

 m, mouth; ph, pharynx; pi, poste- 

 rior intestinal ramus. 



digestive system (Fig. 2) is a gastrovascular cavity with three main 

 divisions (one anterior and two posterior intestinal rami, ai and pi) 

 from which many lateral branches originate. In the prolongation of the 

 anterior ramus is a muscular cylindrical pharynx (ph) lying in a sep- 

 arate chamber, the pharyngeal pouch. When feeding, the pharynx is pro- 



