STEENSTEDP'S THEORY OF ALTERNATION OF GENERATIONS. 35 



material was to hand, but there was no one capable of using it. In 

 spite of the close similarity between the Cercaria and the Distomum, 

 no one ventured to state that one was the young of the other, since 

 they had been found in the bodies of quite different animals. 



The phenomenon of alternation of generations threw a fresh light 

 upon the well-known "sexless" Entozoa. According to earlier opinions, 

 these were either independent, spontaneously generated organisms, or, 

 as Eschricht thought, immature forms. The theory of alternation of 

 generations rendered it possible that they played the part of an inter- 

 mediate generation, the "nurse." In fact, Steenstrup^ liad no hesi- 

 tation in speaking of many of these forms, especially the bladder- 

 worms, as " nurses." 



What stress was laid upon the migrations of the embryos by 

 Steenstrup is sufficiently shown by his statement, based upon firm 

 conviction, that the Entozoa are generally only parasitic for a longer 

 or shorter period ; and tha.t at other times, perhaps in different stages 

 or generations, they lead an independent existence, or, as it is termed, 

 " have a geographical distribution in nature (e,g., in water) outside the 

 body of a host."^ This opinion was strikingly confirmed by a new 

 discovery. 



Dujardin^ frequently discovered on the ground, and especially 

 after a sudden rainfall, masses of a Filaria-like JSTematode (Mermis), 

 resembling very closely Gordius aquaticus, which had been known for 

 some time as an inhabitant of water. This appearance (the so-called 

 "worm-rain") could only be explained by supposing that these 

 creatures had left the bodies of insects or snails, upon which they are 

 parasitic, for the purpose of laying their eggs in the damp earth. Von 

 Siebold proved that this explanation was the right one, by finding not 

 merely these Mermithidae in the bodies of insects and insect larvfe, 

 and observing them in the act of wandering away ; but also by the 

 discovery that Gordius was also sometimes parasitic* At the time of 

 their migration the Gordiacese are mature ; but copulation and ovi- 

 position take place subsequently, in water in the case of Gordius, and 

 in damp earth in the case of Mermis. Von Siebold succeeded later ° 

 in tracing the development of the embryo within the egg during the 



' Loc. cU., p. 111. ^ Loc. cit., p. 116, Note. 



" Ann. Sci. Nat, t. xviii., p. 129, 1842. (Similar observations have been fre- 

 quently made since the publication of this paper by numerous observers, and among 

 others by myself. ) 



* Entomolog. Zdtung, p. 77, 1843. VUlot has recently stated that the presence of 

 Gordius in insects is an accidental wandering, while he believes that minnows and loaches 

 are its normal hosts. See Archives de Zool, ExpSr., t. iii., p. 182 et seq., 1874. 



' Ibid., 1848, p. 290 ; 1850, p. 239, — Jahresb. d. schlesischen GeseUsch. fur vaterl. 



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