PARASITES 75 



limited view of natural history. It is common to find parasites 

 referred to as if they were in some way more morally oblique 

 in their habits than other animals, as if they were taking some 

 unfair and mean advantage of their hosts. If we once start 

 working out such " responsibilities " we find that the whole 

 animal kingdom lives on the spare energy of other species or 

 upon plants, while the latter depend upon the radiant energy 

 of the sun. If parasites are to occupy a special place in this 

 scheme we must, to be consistent, accuse cows of petty larceny 

 against grass, and cactuses of cruelty to the sun. Once we 

 take a broad view of animal interrelations it becomes quite clear 

 that it is best to treat parasites as being essentially the same as 

 carnivores, except in their smaller size, which enables them to 

 live on their host. In other words, the resemblances between 

 the two classes of animals are more important than the 

 differences. 



6. We will now turn to a consideration of the place of para- 

 sites in the food-cycle of any animal community, and the ways 

 in which the food-cycle affects them. One of the greatest 

 questions which has to be solved by many parasites is what 

 to do when their host dies, as it is bound to do sooner or later. 

 This applies with especial force to internal parasites like 

 flatworms, which have become so specialised to a life of passive 

 absorption in the dark that they are unable to take any active 

 steps to deal with the situation created by the death of their 

 host. It is here that the food-cycle comes in and plays an 

 important part. Probably the commonest death for many 

 animals is to be eaten by something else, and as a result we find 

 that a great many parasites pass automatically with the prey 

 into the body of its enemy, and are then able in some way to 

 occupy the new host. Let us take the case of a tapeworm which 

 lives as a young larva or bladderworm in the muscles of a rabbit. 

 When the rabbit is eaten by some enemy, say a fox, some of 

 the bladderworms pass unharmed into the intestine of the fox, 

 and there continue their development and grow up into adult 

 tapeworms ; and, in this way, the problem created by the 

 death of the first host is solved. But foxes being also mortal, 

 the tapeworm has to get back again into the rabbit before the 



