PLATYHELMIA. 39 



The following table summarizes the succession of stages: 

 1. Sexiial Fluke 



(in Limnaea truncatula, <.- (in Limnaea truncatula, 



may produce may produce fresh 



daughter -redice). sporocysts by fission). 



Further Remarks on Flukes. 



"Alternation of Generations" includes a number of by no means equi- 

 valent cases. One of the simplest is that described (p. 27) for certain 

 Hydrozoa, where an asexual hydroid (A) produces a sexual medusa (S) by 

 budding or fission. The sequence is AS, AS, &e., and this simple alterna- 

 tion of asexual and sexual stages may be termed metagenesis. The 

 "internal budding" by which, in the Liver-fluke, redia and cercaria are 

 produced is not, strictly speaking, an asexual process, nor, on the other 

 hand, is it a normal sexual process. The germinal cells appear to be pre- 

 cociously formed ova which can develop without fertilization (partheno- 

 genetically). Such an alternation of normal sexual reproduction with 

 parthenogenesis may be termed heterogamy. Writing S for the sexual 

 fluke, s 1 for the sporocyst, and s 2 for the redia, this particular instance, 

 reduced to its simplest form is s 1 s 2 S, s 1 s 2 S, &c. The ciliated embryo 

 is included in the s 1 , since it develops into the sporocyst, and similarly the 

 cercaria is merged in the S. 



D. hepaticum is but infrequently found parasitic in man, a far more 

 important fluke, from a medical point of view, being D. hsematobium 

 (Bilharzia), which chiefly occurs in Egypt, Abyssinia, and S. Africa. It 

 infests the portal vein and its branches, and is remarkable among flukes in 

 having the sexes separate. The male is elongated and worm-like ; upon its 

 ventral surface is a groove in which the similarly shaped female is carried. 

 Life-history unknown. 



6. TJENIA (The tapeworm). 



Tapeworms are endoparasites, nearly all of which, when sexually 

 mature, inhabit the intestines of vertebrate animals. One of the 

 best known types is TsGnia sol in m, the Common Tapeworm, 

 which during the early stages of its existence is found in the 

 muscles of the pig (intermediate host), and, when mature, in the 

 small intestine of man (final host). 



