CHAPTER VI 



PARASITES 



As opposed to carnivores, parasites are (i) much smaller than the hosts 

 upon which they prey, but (2) feed in essentially the same way as carni- 

 vores, the chief difference being that the latter live on capital and the 

 former on income of food ; but (3) a complete graded series can be 

 traced between typical parasites and typical carnivores, both among 

 animals which eat other ones, and (4) among those animals which live 

 by robbing other animals of their food. (5) We can therefore apply 

 the same principles to parasites as were applied to carnivores in the last 

 chapter, making certain alterations as a result of the different size- 

 relations of the two classes of animals. (6) The food-cycle acts as an 

 important means of dispersal for internal parasites, so that they often 

 have two, or (7) more, hosts during their life-histories, and (8) this means 

 of dispersal is also used to some extent by external parasites. Animals 

 which suck blood or plant-juices often play an important part in such 

 life-histories. (9) In food-chains formed by parasites, the animals at 

 each stage become smaller, and at the end of a chain are so small that 

 bacteria are often found to become important organisms at that point. 

 (10) The parasitic hymenoptera occupy a rather special position in the 

 food-cycle. (11) When parasites and carnivores are both included in 

 the same scheme of food-cycles, the latter become very complex, as 

 is (12) shown by an example ; but (13) in practice, a number of parasites 

 can be considered as forming part of their host, as far as food is concerned, 

 although when numbers are being studied the parasites must be treated 

 separately. 



I . If you make a list of the carnivorous enemies and of the 

 parasites of any species of animal, you will see (although 

 they are so obvious that they easily escape notice unless 

 pointed out) certain curious facts about the sizes of the two 

 classes of animals relative to their prey or host. For instance, 

 a frog would have for enemies such animals as otters, herons, 

 or pike, which would be anything up to fifty times the size 

 of the frog, while the parasites would be animals like flat- 

 worms, nematodes, or protozoa, which would be five hundred 

 or five thousand times smaller than the frog. The same thing 

 applies to any other animal. A mouse is preyed on by 



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