580 DISHARMONIES AND OTHER SHADOWS 



tive system is often highly developed and the multiplication 

 very prolific, which may be associated (a) with the fact that 

 the parasite is often living without much exertion, with abun- 

 dance of stimulating food at its disposal, and also (&) with 

 the probability that as the chances of death are often 

 enormous, non-prolific forms have been persistently elim- 

 inated. Parasites survive not because they are strong, but 

 because they are many. 



(e) While there are many different' types of parasites, it 

 is of interest to notice that some kinds of organisation are 

 not compatible with a parasitic mode of life. Among back- 

 boned animals the only parasite is the hag (Myxine) and 

 it is not thoroughgoing. There are very few parasitic Mol- 

 luscs or Ccelentera, and there are no parasitic Echinoderms, 

 partly perhaps because the life of these three types is very 

 dependent on the activity of ciliated cells which usually re- 

 quire fresh water-currents. Among plants, most of the para- 

 sites are Fungi and very few are Flowering Plants. 



(/) The life-histories of parasites are often very intricate 

 and full of risks. In many cases two hosts are required. 

 The embryos of the liver-fluke pass from sheep to water-pool ; 

 the hatched larvas enter a water-snail ; there are several asex- 

 ual generations in this first host ; minute flukes leave the snail 

 and encyst on blades of grass ; if these are eaten by a sheep 

 the second host the cycle recommences. There are curious 

 cases of hyper-parasitism where one parasite contains a sec- 

 ond which contains a third, and this gives rise to complicated 

 life-histories. 



(g) Thoroughgoing parasites, with a long evolution behind 

 them, are naturally enough well-adapted to the conditions 

 of their life. Thus a tapeworm in the intestine of its host 

 absorbs food by the whole surface of its body; it has mus- 



