DIVERGENCE AND ADAPTATION 365 



a living and animals from a great variety of classes have 

 taken advantage of this means of obtaining it. There is a 

 natural check to the number of possible parasites, for if 

 the hosts, or organisms preyed upon, were to be killed 

 off, the parasites would starve. As it is, most animals 

 harbor *a number of these dependent creatures. Man 

 for instance is infested with a considerable number of 

 these parasitic Protozoa to say nothing of the numerous 

 disease-producing bacteria. Of the trematodes, cestodes 

 and round worms that attack him there are somewhat over 

 fifty species. And then there are various species of ticks, 

 mites, fleas, lice, bed bugs and other creatures which 

 infest his person with more or less regularity according 

 to his location or manner of life. 



Parasites are classed as external, such as ticks and fleas, 

 and internal such as tape- worms. They differ as to the 

 degree of dependence upon their host, some, the obligatory 

 parasites, being like the tape-worm entirely dependent 

 on their host; others called facultative parasites being 

 only occasionally parasitic, such as mosquitoes and biting 

 flies. Parasitism almost always entails a certain amount 

 of degeneration. Where parasites live in or upon their 

 host there is often a loss of the higher sense organs, a 

 degeneration of the nervous system, a loss of organs of 

 locomotion or a conversion of them into organs of attach- 

 ment, and sometimes a loss of the organs of digestion where 

 the parasite lives upon the digested food of its host. An 

 extreme case, as we have seen in the chapter on the crusta- 

 ceans, is furnished by the parasite Sacculina, in which the 

 animal has lost sense organs, appendages, digestive tract 

 and has become converted into an irregular mass presenting 

 no recognizable points of similarity to the barnacles to 

 which it is related. Its life history affords an interesting 

 illustration of the extent of degeneration to which para- 



