PHYLUM PLATYHELMINTHES 91 



Planaria has no lens it cannot perceive objects but can only distinguish different 

 degrees of intensity of light. The general body surface of Planaria is also 

 sensitive to light but less so than the eyes. The pointed tip of the head and the 

 auricles appear lighter in color than the rest of the body. They are important 

 sensory areas, containing a variety of sensory cells, principally for contact and 

 chemical sense, the latter essential to the detection of food. 



c) The digestive system: In the center of the body is an oval clear region, the 

 pharyngeal chamber, in which is located a cylindrical contractile tube, the 

 pharynx. The pharynx is attached at its anterior end to the wall of the pharyngeal 

 chamber, and its free posterior end has a wide opening. The pharyngeal cham- 

 ber opens on the ventral surface by the mouth, which appears as a small circular 

 clear area, devoid of granules, situated in the median line anterior to the posterior 

 end of the pharynx. When the animal feeds, the free end of the elastic pharynx 

 is protruded as a proboscis through the mouth. The student should understand 

 clearly the relations of mouth, pharynx, and pharyngeal chamber and have the 

 assistant make further explanations, if necessary. 



At its anterior end the pharynx opens into the intestine or digestive tract 

 proper (the pharynx being really an outgrowth of the body wall), which divides 

 at once into three main branches, a median anterior branch extending forward 

 as far as or between the eyes, and two posterior branches which pass backward, 

 one on each side of the pharyngeal chamber, to the posterior end. Each main 

 branch gives off numerous side branches or diverticula, often very irregular in 

 form, and these may make secondary fusions to form further longitudinal 

 branches. The digestive tract generally appears gray or brown and is easily 

 seen if the animal is well pressed out and more than the usual amount of light 

 admitted through the diaphragm. 



Add to your outline drawing these details of structure. 



The digestive canal of Planaria, although commonly called intestine, differs 

 from the true intestines of the higher animals in that it consists of a single layer 

 of cells, the entoderm, and is not separated from the other structures of the body 

 as a definite tube. It is in reality much more comparable to the wall of the 

 gastrovascular cavity of Hydra and its cavity corresponds completely to this 

 cavity in Hydra, except that it is more branched and hence more effective as a 

 food-distributing system. 



d) Other systems of Planaria: The animal has well-differentiated nervous, 

 excretory, and reproductive systems, but unfortunately these are difficult or 

 impossible to study. Read carefully Hegner's description of them (pp. 155-57). 

 In the nervous system note especially the concentration of nervous tissue in the 

 head between the eyes, constituting the beginning of a brain. The structure 

 of the excretory system should also be carefully studied, as it is constructed upon 

 a primitive plan from which the excretory systems of nearly all animals, including 

 the vertebrates, can be derived. 



