THE ART OF PAIUSITISM 219 



worm, and the spiny-headed worm, for example, there is no trace 

 whatever of a digestive tract in the adult. Such worms, however, 

 have access to various digested foods which are ready for absorption 

 by the host and it appears certain that these gutless forms must be 

 able to absorb and utilize materials from the alimentary canal of 

 their benefactor. Other worms, such as the flukes or trematodes, and 

 roundworms, possess a well-developed alimentary canal, the secretions 

 of which, in some instances at least, cause a liquefaction of the tissue 

 in the immediate vicinity of the parasite, thus making it available as 

 food for the organism. 



Another problem which parasites have had to solve is that of respira- 

 tion. In the case of cellular or blood-inhabiting forms the parasite 

 obviously has access to plenty of oxygen, whereas intestinal parasites 

 face a difficulty, since the alimentary canal is known to contain little 

 oxygen. Many investigators now believe that these worms secure 

 their energy from the breakdown of dextrose. This substance results 

 from the hydrolysis of more complex carbohydrates and is the form 

 in which it is absorbed from the intestine into the blood stream. 

 Presumably oxygen is secured during the process of anaerobic fermen- 

 tation that results in the splitting of dextrose or glycogen (if the 

 carbohydrate has been converted into glycogen during the metabolism 

 of the parasite) into fatty acids and carbon dioxide. This type of 

 metabolism is characteristic of some bacteria and yeasts. 



One of the most striking effects of the parasitic habit lies in the 

 tremendous development of the reproductive capacity of the parasite, 

 a process undoubtedly correlated with the numerous hazards which 

 must be met if its life cycle is to be completed. The development 

 occurs in two ways, — first by the production of enormous numbers 

 of eggs, and secondly by the interpolation of asexual stages in the cycle. 

 Thus it has been estimated that a single free-swimming, ciliated stage 

 {miracidium) of a fluke may be the indirect parent of as many as 

 10,000 free-swimming, tailed larvae {cercariae). 



External or ectoparasites also show marked evidence of adaptation to 

 their type of existence, as shown by the piercing and sucking mouth 

 parts of the parasitically inclined arthropods or the degeneration 

 of the mouth parts in the case of the adult botflies, as well as by 

 the laterally compressed body of the flea, and the loss of wings in 

 lice and bedbugs. Limitation as to the host and as to the location 

 on the host shows specialization among this group. These factors 

 tend to illustrate stages in the development of ectoparasitism. 



