192 FLEAS, FLUKES AND CUCKOOS 



their early larval development the ectodermal covering is cast off and 

 that the endoderm which would normally form the alimentary canal 

 has taken over the duty of a body covering. It has further been suggested 

 that the entire adult tapeworm is an endodermal sac, the lumen of 

 which has been obliterated by the encroachments of the mesodermal 

 tissue. In this case it would be more correct to say that a tapeworm is 

 an alimentary canal without a body, rather than a worm without a 

 digestive tract. 



The gut of vertebrates, which is the environment of all tapeworms, 

 presents certain unique features. To begin with it undergoes continual 

 peristaltic movement — that is to say rhythmical waves of contraction 

 pass along it. The tapeworm is continuously in danger of being swept 

 away — like a swimmer in a river with a powerful current forcing him 

 out to sea. The gut also contains protein, fat and carbohydrate splitting 

 enzymes and there is a wide range of pH. (1.7 in the stomach and 9.0 

 in the intestine). Moreover, the oxygen pressure is low and there are 

 regular physiological changes due to the feeding habits of the host. 

 Tapeworms have had to develop a series of adaptations to cope with 

 this particular situation. The cuticle of the cyclophyllidian cestodes 

 appears to possess a protective action which renders them immune to 

 digestion. Apparently no anti-enzyme is secreted. They have also 

 developed tolerance for a range of pH. varying between 4 and 11. 

 It has long been thought that cestodes were truly anaerobic and could 

 not make use of oxygen under any circumstances. Recently it has been 

 shown that, like roundworms and flukes, they will utilise it if it is 

 provided for them. In the gut, however, the most usual method is 

 anaerobic respiration. 



In order to avoid digestion in the host's stomach the larval forms of 

 tapeworms have also become highly specialised in certain directions. 

 For example, if a cysticercus larva is swallowed by the final host the 

 scolcx is invaginated into a sort of sleeve which shields it from the action 

 of the gastric juices. The scolex only evaginates when it reaches the 

 duodenum and is stimulated by the presence of bile. The "sleeve" 

 which has received both the acid gastric juices of the stomach, and the 

 alkaline juices of the pancreas, is digested, but the scolex which has been 

 untouched by the former remains undigested and develops into a 

 strobila. 



The health of the host and changes in its metabolism affect the 

 worms which parasitise it. In the laboratory it has been found that if 



