46 HELMINTHOLOGY 



c. Platyhelminthes. In this group of worms the body is more 

 or less flattened dorso-ventrally. The digestive tract may be 

 entirely lost, as in the tapeworms, or may consist of a blind sac, 

 more or less branched, as in the flukes. An anal aperture is 

 never present. The flat-worms are usually hermaphroditic, and 

 development in the parasitic species is usually indirect, i. e. 

 through an intermediary host. Examples are the pork tape- 

 worm of man (Tania solium) and the liver fluke of sheep [Fas- 

 ciola (Distoma) hepatica}. 



To distinguish worms from insect larva. When insect larvae 

 (parasitic in the body of higher animals) are encountered, there 

 may be some difficulty in differentiating them at once from 

 worms, because of their environmental setting. Instances of 

 this are bots and warbles ((Estridae), screw worms (Chrysomyia 

 macellaria) and flesh fly larvae (Sarcophagidae) in intestinal 

 myiasis. Usually these larvae, are short and plump, with well- 

 marked segments, few in number, usually n or 12, certainly not 

 more than 20. Furthermore, microscopic examination will re- 

 veal a system of tubules in the insect, extending internally to 

 all parts of the body, the tracheal breathing system. 



EXERCISE 25a 



THE ROUNDWORMS 



PHYLUM NEMATHELMINTHES, CLASS NEMATODA, FAMILY 

 ASCARID^E 



The roundworm of the horse Ascaris megalocephala. 



A. Characteristics of Nematoda. The roundworms or thread- 

 worms may be recognized by their cylindrical, non-segmented 

 or non-jointed form, covered with a rather thick cuticle, and 

 the presence of a simple, well-developed alimentary canal; also 

 a pair of lateral longitudinal water-vascular canals, usually 



