PHYLUM PLATYHELMINTHES 



There is a double purpose in the study of this phykim. One 

 is found in the parasitic life which great numbers of these animals 

 lead and the resulting maladies produced in higher animals which 

 serve as hosts. The other purpose is found in their body organi- 

 zation which serves as an index to their relative success as 

 organisms and their elevation to a higher rank than that attained 

 by the forms previously studied. Among other advances there 

 may be noted: (a) the mode of progression, being that of an 

 elongate animal moving with the same end of the body foremost, 

 (b) the regional differentiation of the body as a whole which 

 accompanies this type of locomotion, namely, a true anterior 

 and posterior end, bilateral symmetry (with right and left 

 sides), dorsal and ventral surfaces, and (c) triploblastic body walls 

 which have provided the structural means for these advances in 

 organization. 



Class Turbellaria. Planaria or Procotyla (Dendroccelum). — 

 Either of these genera of flatworms is found commonly in fresh- 

 water ponds throughout the United States. The species of 

 Procotyla are white and somewhat translucent, a condition 

 which greatly facilitates the study of the digestive and other 

 internal organs. These free-living {i.e., non-parasitic) flatworms 

 or turbellarians are found under stones, leaves, or other sub- 

 merged objects. 



Behavior and External Structure. — Make observations upon 

 a living specimen in a watch glass, using a hand lens or dissecting 

 microscope. Study the behavior, form, and surface structure 

 of the animal, making use of the following as a guide: (1) form 

 of body, making specific references to the relative length of the 

 three axes (longitudinal, vertical, transverse); (2) relation of 

 body to the substratum (surface over which the animal crawls) ; 



(3) relations of the ends of the body to direction of locomotion; 



(4) movements which involve muscular activity and the normal 

 glidiiig form of locomotion due to the action of cilia covering the 

 body; (5) muscular movements as a means of crawling, determin- 

 ing the nature and progress of the waves of contraction; (6) 



82 



