76 



ANIMAL ECOLOGY 



fox dies, and this is also brought about by the food-cycle. 

 For the tapeworm produces vast numbers of eggs which pass 

 out with the excretory products of the fox ; some of these eggs 

 contaminate the vegetation which the rabbit is eating, or in 

 some other way get in with its food, and are then able ultimately 

 to grow up into more bladderworms in the body of the rabbit. 

 The diagram in Fig. 6 shows the way in which the food-cycle 



PLANTS 

 --EggD-- 



^RABBIT 



Larvae- 



+»POX 

 Adult 



Fig. 6. 



acts as a means of conveyance for the parasites, throughout 

 its life- cycle. 



7. This was a fairly simple case. There is always this 

 tendency for parasites to get transferred from one stage in a 

 food-chain to the next, like passengers on a railway. Many 

 parasites get out at the first station — in other words, they have 

 a direct Hfe-history, with no alternate host. An example of 

 this is a tapeworm {Hymenolepis) which occurs in mice, and 

 which has the larval and adult stages in the same host, although 

 in different parts of the body.^^ Or the parasite may get out 

 at the second station, like the rabbit tapeworm described above. 

 Or again, it may travel as far as a third host. The broad tape- 

 worm {Diphyllohothrium latum), which occurs occasionally 

 in man, causing severe ansemia, gets into the gut of some 

 small fresh-water copepod {e.g. Cyclops strenuus or Diaptomus 



WATER- 



'COPEPOD- 



, Egg ^ — Larva ->- Larva— ^— 



Fig. 7. 



gracilis) with the food of the latter, is then eaten by fish in 

 which it exists in the form of bladderworm cysts, and is finally 

 eaten by some carnivore or by man.^* The diagram (Fig. 7) 

 sums up this cycle. 



