420 The Animal Kingdom 



other carnivores. It is not a particularly dangerous tapeworm; 

 however, the cysticercus is able to form cysts containing large numbers 

 of scolices. These large cysts are known as hydatid cysts. Daughter 

 cysts may also be produced and scattered to other areas. Often these 

 attain tremendous sizes and can cause a great deal of damage, and may 

 even be fatal. The normal secondary host is sheep or cattle, but man may 

 become an accidental secondary host by too close association with his 

 domesticated dog or cat. Frequently these cysts develop in the liver, 

 kidneys, spleen, heart, brain, or muscle. The only possible treatment 

 is removal by surgical means. Prevention consists chiefly of keep- 

 ing pets from eating waste parts of slaughtered animals from which 

 they can normally become infected. 



All of the tapeworms so far discussed have required intermediate 

 hosts for the completion of their life cycles. There is one form, 

 Hyinenolepis nana, commonly found in children which is able to com- 

 plete its life cycle in a single host. This is a very tiny tapeworm, about 

 10 to 45 mm. in length. The eggs when ingested hatch into larvae 

 which quickly burrow into the tips of the anterior villi. Here they 

 develop into the cysticercoids which, when mature, escape into the 

 lumen of the intestine. Here they attach and grow to maturity. This 

 worm is more common in children than in adults and immunity to it 

 develops readily. 



ADAPTATIONS TO PARASITIC LIFE 



The flukes and tapeworms show vividly the adaptations which 

 an animal must make to a completely parasitic life. They lack sense 

 organs, organs of locomotion, and possess suckers or hooks for at- 

 tachment. Their digestive systems are often reduced, or in the case of 

 the tapeworm, lacking entirely. Complicated life cycles ensure the 

 reaching of the correct host. Along with this, they have developed 

 tremendously their powers of reproduction. Thus while their complex 

 life cycles mean that most eggs will never reach maturity, the tre- 

 mendous number produced ensures the continuation of the species. 

 They have developed mechanisms to enable them to live bathed in the 

 digestive juices of their hosts. This they do either by cuticula im- 

 pervious to the digestive juices or by enzymes which counteract the 

 effect of these destructive juices. 



From this brief survey of the parasites, it would appear that 

 control of them would be essentially a matter of improved sanitation. 



