124 



ANIMAL BIOLOGY 



by giving the patient liberal doses of thymol which kills 

 the worms, fortunately without greatly injuring the afflicted 

 person. 



Besides the numerous nematodes living within animals, 

 there are several serious parasites of plants. From our 

 human point of view the nematodes in general are a bad 

 lot, for there is scarcely any species for which a useful 

 function has ever been discovered. One 

 form which is sometimes classed with the 

 nematodes may be mentioned here, viz., 

 Gordius, or the so-called " horse-hair 

 snake." This worm which is popularly 

 but wrongly supposed to come from 

 horse-hairs that have fallen into the water 

 is parasitic during the earlier stages of 

 its life, generally in the body of some in- 

 sect, and only becomes free in the ma- 

 ture state. 



The flat worms comprise a large 

 phylum which is divided into three 

 The 8 rou P s > the Turbellaria, the Trematodes 

 hookworm, a, male; O r flukes, and the Cestodes or tape- 



b, female; o, mouth; , ... , 



v, opening for dis- worms. The most primitive group, the 

 charge of eggs. Turbellaria, are usually free-living 



(After Leuckart.) * 



animals, such as the planarians, which 

 are commonly found under rocks in lakes and streams. 

 The Trematodes are all parasitic forms characterized by 

 having a forked intestine, and usually one or more suckers 

 for attachment to the host. They are found in most verte- 

 brate animals from fishes to man. One of the most injur- 

 ious species is the common liver fluke Fasciola hepatica 

 which frequently infects sheep and sometimes occurs in 

 human beings. The flukes may reach a length of over an 

 inch, and when they are present in considerable numbers 



FIG 



