PARASITIC WORMS. 161 



field is required. The process is parasitic transmigra- 

 tion. 



2. Nematoidea. The parasitic worms of this class are 

 more or less thread-like. 



The hair-worm or dracunculus is an example. This 

 is a sort of long thread-like creature which penetrates 

 under the skin, and produces symptoms of less or greater 

 gravity. It is viviparous, and in this respect resembles 

 the trichina spiralis, a small filiform worm that produces 

 the disease known as trichinosis, that is communicated 

 to man by the pig. In the embryonic state the trichina 

 inhabits the muscles of the pig, so much so that the in- 

 fected flesh is literally crowded with them. Under the 

 microscope the parasite appears as a little worm coiled in 

 a spiral in a little sac or cyst. When man eats such flesh 

 the young trichinae are developed in his intestine, and 

 thousands of these creatures perforate the intestines and 

 pass into the muscles. Trichinosis is a dangerous disease, 

 very common in sections of countries where raw pork is 

 eaten. The trichina is destroyed by thorough cooking. 



The other nematoidea are all oviparous. Such are, for 

 example, the pin-worms or seat-worms of children, the 

 oxyurus vermicularis, that inhabits the rectum, the as- 

 carides that live in the small intestine, etc. 



3. Trematoidea. Under this title are considered those 

 worms that arc flattened in the form of a heart or leaf, 

 and that are generally known as gourd-worms, Their 

 digestive apparatus usually consists of two branc'hing 

 tubes. Two suckers, one oral and one ventral, serve to 

 fix the creature in position. These parasites pass through 

 various stages in the course of their development. They 

 have been found in the kidneys and in the liver of man, 

 and they are not uncommon in cattle. 



I 14* 



