CHAPTER XX. 



FLATWORMS (PLATYHELMINTHES) AND THREADWORMS 

 (NEMATHELMINTHES) 



Systematically the flat worms and round worms should be placed 

 before the earthworm as they are not coelomates, but, as the average 

 man always thinks of a sort of segmented animal similar to an earth- 

 worm when worms are mentioned, and medical men likewise are not 

 very accurate when they discuss these animals, the student is more likely 

 to remember the three types of worms if he thinks of them all at once 

 and notes their similarities and differences. 



The Annelids are of little importance from a medical standpoint 

 with the exception of the leech (Hirudo Medicinalis) commonly used to 

 draw blood, but the flat worms and round unsegmented worms have 

 come to have a very considerable bearing on the human being from a 

 pathological standpoint. 



THE FLATWORMS 



The flatworms (which constitute the phylum Platyhelminthes) are 

 subdivided into the following three classes : 



Class I. Turbellaria (Lat. turbo, I disturb), with ciliated ectoderm; 

 free-living habit, example : Planaria. 



Class II. Trematoda (Gr. trema, a pore; eidos, resemblance), with 

 non-ciliated ectoderm ; suckers ; parasitic habit, example : Fasciola 

 hepatica (liver fluke), and 



Class III. Cestoda (Gr. kestos, a girdle; eidos, resemblance), with 

 body of segments ; without mouth or alimentary canal ; parasitic, exam- 

 ple Taenia (tapeworm). 



TURBELLARIA 



Turbellaria are the only flatworms which are not parasitic. They 

 live on the lower surface of submerged stones and debris close to the 

 margin of ponds, springs and lakes. Most of these are Planaria (Fig. 

 175), but often a longer worm is found (from ten to fifteen millimeters) 

 which is called Dendrocoelum lacteum. 



Planaria crawls about among aquatic plants to seek its food. The 

 cilia covering the ectoderm assist in this movement, though the animal 

 also contracts and expands its body. As soon as a planarian finds a 

 small animal suitable for its food, the proboscis, lying near the center 

 of the body, is practically turned inside out through the mouth. This 

 proboscis grasps the food and draws it into the body. As the mouth 

 is near the center of the ventral surface, the proboscis can be extended 

 in any direction. 



