272 



ZOOLOGY 



SECT. 



tinguished from the surrounding mesodermal syncytium by its 

 enclosing the remains of the yolk. No metamorphosis is known 

 to occur. 



An account has already been given (p. 240, Fig. 193) of the 

 development and metamorphosis of the Liver-Fluke (Fasciola 

 hepatica) which may be looked upon as typical of the Digenetic 

 Trematodes in general. There is thus to be recognised in the 

 Digenetic Trematodes an alternation of generations comparable to 

 that which has been described as so general in the Ccelenterata. 

 In the Trematoda, however, it is to be observed, it is an alterna- 

 tion of a sexual, not with an asexual, but with a parthenogenetic 

 generation (the sporocyst), the ova of which develop into a second 



Fia. 222. Sections through embryos of Dendrocoelum lacteum (somewhat diagrammatic). 

 dz, yolk-cells ; ec, ectoderm ; en, endoderm ; ph,, embryonal pharynx ; ph a , permanent 

 pharynx ; wz, wandering cells. (From Korschelt and Heider, after Hallez.) 



parthenogenetic generation (the redise) ; and these finally produce 

 larvae (the cercariee) capable of developing into the sexually 

 mature form. The term heterogeny is applied to a life-history 

 of this kind, in which several distinct generations succeed one 

 another in a regular series. 



In some of the Distomidse the eggs, instead of becoming free as 

 in the case of the Liver-Fluke, are taken directly into the digestive 

 canal of the intermediate host, and there hatched out. The 

 sporocyst stage may take the form of a branching tube in the 

 interior of which cercariao are developed the redia stage being 

 omitted. Sometimes the sporocyst becomes directly developed 

 into a redia instead of giving rise to a generation of the latter by 



