826 



ANIMAL LIFE 



The flat worms are parasitic in all classes of vertebrates. 

 The liver fluke, annually causing the death of thousands 

 of sheep in Great Britain and South America, inhabits the 

 bile ducts in its adult stage, while during its early develop- 

 ment it swims free in the water and later becomes para- 

 sitic in the pond snail. After a time it escapes from the 

 snail and becomes encysted on a grass blade, where it re- 

 mains until eaten by the sheep. 



The tapeworms, various species of which inhabit the ver- 

 tebrates from fish to man, attain the largest size of any of 



the parasites, speci- 

 mens having been 

 reported over fifty 

 feet in length. They 

 are characterized by 

 a small head (scolex) 

 bearing hooks or 

 suckers for anchor- 

 age in the wall of 

 the alimentary tract, 

 a tapelike body com- 

 posed of numerous 

 segments, called 

 proglottides, and 

 the absence of a 

 digestive canal. They are nourished by the absorption 

 through their body walls of the digested liquid food of 

 the smaller intestine. The eggs, which the worms mature 

 by the thousands daily, can not develop within the host, 

 but pass out, and upon being taken in with food or drink 

 by certain animals hatch into spherical forms with six 



FIG. 366. A tapeworm from the cat. h, head; 

 p t posterior end. Nearly life size. 



