XI. PHYLUM PLATYHELMINTHES 



A. PLANARIA 



1. General appearance and behavior. Obtain one of the living animals in 

 a watch glass and examine with a hand lens (Hegner, pp. 153-58). Compare 

 form, symmetry, and arrangement of parts with that of Hydra and the frog. 

 Which of these two animals does it most resemble? What type of symmetry 

 does it possess? Does it have anterior and posterior ends? dorsal and ventral 

 surfaces? Is the head better differentiated than in Hydra? On the dorsal 

 surface of the head is placed a pair of eyes, and each side of the head is extended 

 into a blunt contractile lobe, or auricle. 



Observe the peculiar gliding form of locomotion. To what is it due (-4)? 

 Does the animal exhibit muscular movements also? Do these suggest a better 

 developed muscular system than is present in Hydra? How do the anterior 

 end and auricles behave during locomotion? What does this suggest as to their 

 function? Touch various parts of the animal with a needle and note its behavior 

 and relative sensitivity of different regions. Do you think that Planaria is able to 

 respond more effectively to stimuli and in a greater variety of ways than Hydra ? 



Make an accurate outline drawing several inches long of the animal. 



2. Detailed structure. Place a small Planaria on a slide and anaesthetize 

 with a few grains of chloretone (A}. After it has become motionless, turn it 

 ventral surface up and cover with a cover glass, pressing gently so as to flatten 

 the animal. If a cover glass is not heavy enough, use a piece of thin slide. The 

 animal must not be crushed. Examine under the low power of the microscope. 



a) General regions and surface of the body: The narrow clear border of the 

 animal is the ectoderm, the central branching dark brown or gray material is the 

 entoderm, and the lighter brown material between these two is the mesoderm. 

 Focus on the surface of the body and note the numerous minute pigment granules 

 which give the animal its dark color. Focus on the ectoderm with the high power, 

 and observe the numerous rodlike bodies, placed at right angles to the surface, 

 which it contains. These are called rhabdites, and when discharged on the 

 surface they soften to produce mucus. The ectoderm is clothed with cilia, 

 particularly on the ventral surface, and these may be seen by focusing on the 

 edge of the ectoderm. 



b) Sense organs of the head: Examine an eye. It consists of a clear area, 

 where the sensory cells are located, and a crescent-shaped mass of very black, 

 closely packed granules, whose function apparently is to concentrate the light 

 upon the sensory cells by preventing reflection and diffusion. As the eye of 



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