INVERTEBRATA. 363 



enveloped in a spherical capsule formed by the secretion 

 above mentioned. The substance of the cyst is gelatinous 

 when first formed, but quickly solidifies and adheres to the 

 grass or leaf on which it is situated. In this condition the 

 cyst has a chance of being swallowed with the herbage by a 

 sheep, or occasionally by some other animal : it is possible 

 that such cysts may be attached to watercress or other 

 vegetable, and be eaten by human beings. 



11. Development of Cercaria into Adult. In the 



stomach of the sheep the cyst is dissolved and the cercaria 

 is set free to enter upon another phase of parasitic exist- 

 ence. It makes its way from the duodenum into the bile- 

 duct and its branches, and there feeds and develops into 

 the mature Fasciola hepatica. The sheep can tolerate the 

 existence of a small number of flukes in its liver for a long 

 time without suffering much in health, but if the parasites 

 are numerous the consequences to the host are serious and 

 often fatal. The disease is known to farmers as rot or 

 liver-rot, and large numbers of sheep are killed by it every 

 year in districts where it prevails. 



12. Characters of Trematoda. Fasciola is a member 

 of the phylum Platyhelminthes or Flat-worms, and among 

 those, of the class Trematoda. The Trematoda are usually 

 divided into two orders : I., the Digenetica, to which Fas- 

 ciola belongs, in which there are several generations in each 

 life-cycle and an alternation of hosts, the larval forms 

 usually living in an invertebrate host and the sexually 

 mature form in a vertebrate; II., the Monogenetica, in 

 which the egg of the sexually mature animal develops 

 directly into a form like the parent, and in which the para- 

 site lives always on one host. The majority of the Mono- 

 genetica are external parasites on fishes. 



13. Parthenogenesis and Alternation of Generations. 

 The life-history of the liver-fluke illustrates some very im- 

 portant phenomena, both physiological and morphological. 

 Physiologically, we have here an alternation of partheno- 

 genetic and sexual reproduction. The term digenetic is 



