THE WOMBAT. 19 



A study of the life of insects and the important part they 

 play in the economy of nature is useful and of practical im- 

 portance. To deal with the subject in anything like a systematic 

 manner would be beyond the limits of this paper, and so I 

 shall only refer to animal parasites. A parasite is a creature 

 which inhabits another living creature, and derives its nourish- 

 ment from its tissues ; they are found widely distributed among 

 animals from the lowest order up to man himself. 



They are to be found in the cavities and tissues of all the 

 organs of animals, and dwelling on the skin like ticks and lice; 

 the latter are known as Epizoa, and the former as Entozoa. 



I shall confine my remarks to the Entozoa. There can 

 be no doubt that parasites play an important part in the 

 •economy of their hosts. Their life history is a very curious 

 one indeed : it is a continual round of danger. But in all 

 animals the number of eggs laid is proportional to the amount 

 of danger they undergo during development ; indeed, if this 

 were not so such creatures would soon become extinct. A 

 single female nematoid worm produces enough ova to develop 

 into hundreds of thousands of such worms, but few indeed 

 reach maturity. Nature, during a long system of evolution, 

 has had to contend with complexity and uncertainty of develop- 

 ment, and has it now so balanced that there shall be prolific 

 births to allow for the fact that only some few individuals 

 will survive. No subject perhaps is of greater interest to 

 stockowners, or provokes more discussion than the life history 

 •of these parasites. In Australia the summer sun destroys 

 immense numbers of ova, and the dryness of the climate is 

 also a check to their development. 



In some species of round worms the ova undergo a certain 

 stage of development without having to gain access to the 

 body of an intermediary host. An intermediary host is a 

 creature in the tissues of which part of the life of another 

 creature must be spent before development to the adult form 

 ■can occur. The intermediary hosts of many of the nematoda 

 are grubs, snails, caterpillars, etc. The embryos of the round 

 worms find their way into the bodies of these hosts and, after 

 passing a limited period of their existence therein, find their 

 way out again into the water. Animals drinking at pools 

 sometimes pick up the embryos, and it is evidently from water 

 pools, damp places, and vegetable products that animals 

 (including man) become infested with round worms. It must 

 be understood that they are quite incapable of prolonging 

 existence without reaching the tissues of their destined hosts. 

 Some indeed will absolutely perish unless they reach particular 

 animals, and also reach particular organs and tissues of those 

 animals. Thus we see that the study of the life -history of 



