ANIMAL PARASITES 253 



digestive juices free the larva, it then becomes attached to the 

 intestinal mucous membrane and develops into the adult thorn- 

 headed worm (Fig. 74). This parasite is characterized by a 

 hooked proboscis or thorn at its anterior extremity, and the 

 absence of a distinct digestive tract. The male is much smaller 

 than the female. The eggs are passed out of the intestine with 

 the ftpces. 



Fig. 74. — Thorn-headed worms. 



The Round-worms or Nematoids, — Eound-worms are very 

 common parasites of domestic animals (Fig. 75). This group 

 of worms is characterized by their cylindrical form, the presence 

 of a true digestive canal and the separation into two sexes, male 

 and female. The life history is more simple than in the flat 

 wonns. Intermediate hosts are not required for the develop- 

 ment of the common forms. The eggs and embryos are deposited 

 by the female in the intestinal tract, air passages, or excretory 

 ducts of the kidneys of the host. Development may be com- 

 pleted here, or the eo-gs and embryos are passed oif with the body 

 excretions. They may live for a short time outside the animal 



