66 



PLATYHELMINTHES TREMATODA 



CHAP. 



nestlings by the unintentional agency of their parents. 1 The 

 snail regenerates its lost tentacles only for the sporocyst to again 

 bud off fertile branches into them. 



The egg of this Distome thus gives rise to a larva which 

 enters the tissues of one particular Mollusc. Here it becomes 

 a branched sporocyst within which the sexual worms are formed, 





Fig. 33. A, Succinea 

 putris, infested 

 by B, Leucochlori- 

 dium paradoxum, or 

 the fully - formed 

 sporocyst of Disto- 

 mum viacrostomurn. 

 (After Heckert.) A, 

 Natural size ; B, x 



apparently each from a single embryonic blastomere (" Keim 

 zelle "), by a process comparable with the development of a 

 parthenogenetic ovum, and the whole cycle has been termed 

 Alloiogenesis, i.e. alternation of sexual and parthenogenetic genera- 

 tions (Grobben). 2 Leuckart 3 and Looss, 4 however, consider that 

 what was once a metamorphosis of an individual (as in the 



1 Heckert, Bibliotheca Zoologica (Leuckart and Chun), Heft 4, 1889. I am not 

 aware that Leucochloridium has been noticed in England. 



2 "Heterogamy" usually means the alternation of bisexual and unisexual 

 generations (e.g. Rhabdonema nigrorcnosum), but is, unfortunately, also used in the 

 sense of Alloiogenesis, as defined above. See Grobben, Arbeit. Zool. zoot. Juts. 

 Wien, Bd. iv. 1881, p. 201. 



3 Parasiten, Bd. i. Abth. II. p. 152. 



4 Festschrift f. Leuckart, Leipzig, 1892, p. 167. 



