PARASITES 77 



Perhaps the most striking examples of the way in which 

 parasites may pass along food-chains are afforded by the worms 

 of the genus Echinorhynchus y which in some cases go on being 

 transmitted from one host to another until they end up in 

 the animal which forms the end of a whole chain, and so can 

 get no farther, and indeed probably never get back again to 

 their first host. They are like passengers who forget to get 

 out at the right station and travel by mistake on to the terminus. 

 8. There is often a similar tendency amongst ectoparasites 

 to become transferred from their first host to its enemy. This 

 is well known to be the case amongst some species of fleas. 

 Ceratophyllus sciurorum, which lives commonly upon the 

 squirrel and dormouse, occurs also occasionally on the pine- 

 marten, weasel, and stoat, while C. walkeri comes commonly 

 on the bank- vole, weasel, and stoat.^^ 



All the cases in which parasites are transmitted from one 

 host to another by blood-sucking insects are also examples 

 of the role played by the food-cycle in the lives of parasites. 

 There are parallel cases of plant parasites being carried from 

 plant to plant by insects. The protozoan PhytomonaSy which 

 is found very widely in the tissues of various species of spurge 

 (Euphorbia), mainly in warmer countries, is carried from one 

 plant to another, in some places at least, by insects. It has 

 been shown that Phytomonas davidi, which occurs in Euphorbias 

 in Portugal, is carried from plant to plant by a bug, Steno- 

 cephalus agilis, which lives upon the juices of the spurge, and 

 in which the Phytomonas has a definite life-history stage. ^^* 



Since the food-cycle is so important in determining many of 

 the possible modes of transference of parasites from one host 

 to another, it is plain that biological surveys carried out along 

 food-cycle lines would be of great value in narrowing the field 

 of inquiry when the Hfe-history of any particular parasite is 

 being studied. 



9. Having shown the relation which exists between parasites 

 and food-chains formed by herbivorous and carnivorous 

 animals, we may now turn to a consideration of food-chains 

 among parasites themselves. Just as the carnivores in a 

 food-chain usually become progressively larger and larger, 



