274 DARWINISM AND THE PROBLEMS OF LIFE 



how can this be done on land, and in the vertebrates 

 that shelter most of the parasites ? In some cases, such 

 as the gourd- worm {distomum) and the broad tape- worm, 

 an active larva develops from the egg, falls into the 

 water and enters a mollusc or a fish. But in this the 

 animal has not reached its definitive host, the sheep or 

 man. The larva has to be conveyed from the interior 

 of the aquatic animal to the stomach of the mammal, 

 and this is done by the former being eaten, consciously 

 or unconsciously, together with the larvae by the eater. 



This passive transition, which we shall study more in 

 detail afterwards, is the only possible means of surviving 

 for most parasites. In only a few cases is an active 

 larva developed from the &^^ ; as a rule the eggs must 

 be licked up and taken into the mouth of a host, and 

 even then the end of the parasite's development is not 

 reached. The host must be eaten, and the larvai pass 

 in its flesh into the interior of their principal host, where 

 they come to sexual maturity. Thus chance plays a 

 great part in the maintenance of parasitic species, and 

 in view of the slender prospect that the individual egg 

 has of ever being developed, we can understand why 

 they are laid in such vast numbers, sometimes up to 

 100,000,000. 



The eggs, which lie on the earth after leaving the 

 host and usually have to wait a long time before they 

 reach the stomach of an animal, need a high power of 

 resistance. As a rule they have a shell and plenty 

 of yolk ; this in turn requires organs in the mother's 

 body that are capable of meeting these demands. 

 Hence there are yolk-bodies and shell-glands, as well as 



