38 



ZOOLOGY 



and derive their nourishment from the tissues of the animals 

 which they infest. Such predatory forms are termed 

 parasites; and the unfortunate animals which have to 

 tolerate their attacks are termed hosts. Some of the parasitic 

 Platyhelminthes cause disease and death to their hosts, in parti- 

 cular the liver-fluke, which lives in the liver of sheep, has in 

 certain years inflicted losses amounting to millions of pounds on 

 the British farmers. This animal possesses suckers, by means of 

 which it adheres to the animal which it infests. Other para- 

 sitic Platyhelminthes find their home in the intestines of man 

 and the higher animals these, the 

 so-called tape-worms, live on the 

 digested food of their hosts, and have 

 in consequence no digestive apparatus 

 of their own : they consist merely of 

 a skin, enclosing nerves, muscles, and 

 reproductive cells the inner set of 

 tubes lined with digestive cells, so 

 characteristic of other Platyhelminthes, 

 being totally absent. These creatures 

 cause only a minor amount of incon- 

 venience to their hosts when they 



T? AT are f u ^ g rown > but many of them, 



IG View7d from bl when ?% and immature, infest the 

 low ; 0, branches of flesh and internal organs of animals and 

 vs, cause disease and death. These parasites 

 have to take their chance of their first 

 host being devoured by a carnivorous 



animal : if this occurs, they complete their development in the 

 intestine of the devourer, and become adult. One tape-worm, 

 in its immature stage, infests the brain of domestic animals, and 

 causes the disease called staggers another attacks the liver 

 of domestic animals and of man himself, and causes a dangerous 

 tumour which is often fatal. 



Another phylum whose members are termed " worms " is 

 the Nemertinea. In these animals we also find a type of 

 structure which consists of a skin and internal digestive tube, 

 and between them a mass of muscular tissue*. There is 

 likewise a central nervous system consisting of two large 

 ganglia termed a brain and two lateral cords. But in one 



gut; o, mouth; 

 sucker. 



