PART II 



HELMINTHOLOGY 

 INTRODUCTION 



The object of the following exercises is to acquaint the student 

 with the commoner parasitic worms infesting man and the do- 

 mesticated animals. 



Formerly the term Vermes was used to cover all groups of 

 worms. This group is now divided into a number of phyla, of 

 which the following include parasitic forms: 



a. Nemathelminthes. Commonly called thread worms, or 

 roundworms, they are cylindrical, non-segmented animals, usu- 

 ally more or less tapering at both ends. The alimentary 

 canal is simple and well developed, except in the Acantho- 

 cephala. The water-vascular canals are usually conspicuously 

 located on both sides, longitudinally. The sexes are separate and 

 development is usually direct, i. e. without a necessary inter- 

 mediary host. Examples are the roundworm of the horse, 

 (Ascaris megalocephala), and the trichina of swine, rats and man, 

 (Trichinella spiralis). 



b. Annelida. Worms composed of a series of rings, this seg- 

 mentation often affecting the internal organs. The number of 

 segments is usually large, a fact that will differentiate these 

 worms from certain parasitic insect larvae, in which the seg- 

 ments are generally n or 12, never more than 19 or 20. In- 

 ternally, of course, the presence of a tracheal respiratory system 

 will differentiate the latter. Examples are the common earth- 

 worm (Lumbricus terrestris) and the medicinal leech (Hirudo 

 medicinalis). (The latter only need be considered here.) 



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