CHAPTER XXIV 



THE PLANARIANS, FLUKES, AND TAPEWORMS: 

 PLATHELMINTHES (FLATWORMS) 



He prayeth best, who loveth best 

 All things both great and small ; 

 For the dear God who loveth us, 

 He made and loveth all. 



Coleridge, The Rime of the Ancient Mariner 



A Planarian. On the lower surface of submerged stones, 

 near the margin of ponds, there are many little black, or 

 white, flatworms. The most of these belong 

 to a genus called Plana'ria. Quite often a 

 larger specimen (10 mm. to 15 mm.) is found 

 among the others. This is most likely to 

 belong to the species Procot'yla fluviat'ilis 

 (Fig. 136). It is nearly colorless except for 

 a clouded middle region. Upon examination 

 with a simple lens an observer may distinguish 

 the organs of the interior with remarkable 

 clearness. The mouth is in the middle of the 

 under surface. It is at the end of a short pro- 

 boscis (the pharynx) which may be rolled 

 inside out. Microscopic food entering the 

 mouth may pass forward through a slender 

 tube, and into all the many branches, or it 

 may pass backward by two tubes and into 

 their branches. Branches from the digestive system carry 

 food directly to every part of the body. No separate cir- 

 culatory system has been developed. At the anterior end 

 beneath, there is a shallow sucker for holding on to stones 



261 



Fig. 136. A pla- 

 narian. (x 10) 



