CHAPTER XX 



FLATWORMS (PLATYHELMINTHES) AND THREADWORMS 

 (NEMATHELMINTHES) 



CONSIDERED systematically the flatworms 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 earthworm 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. 



With the exception of the leech (Hirudo medicinalis) commonly 

 used to draw blood, the annelids are of little importance from a medical 

 standpoint. However, the flatworms 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 crawl about among aquatic plants to seek food. The cilia 

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

 contracts and expands its body. As soon as Planaria 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. 



