12 THE PROTOZOA 



with their canals, show the strongest similarity to the corresponding 

 structures in flatworms. The trichocysts were particularly difficult 

 for the opponents of Ehrenberg to explain. Stein ('56) regarded 

 them as "taste-bodies" (Tastkorperchen), and we find even Leydig 

 ('57) regarding them, together with the microsomes in the stalks of 

 Vorticella, as the nuclei of very minute cells. 



Kolliker ('48, '49), following von Siebold, but at first almost alone, 

 strenuously maintained that all Protozoa are single-celled animals, in 

 spite of severe criticism, especially by the ardent and brilliant young 

 naturalists, Claparede and Lachmann, who, not able to make out the 

 cell-membranes and nuclei in many cases, placed the Protozoa with 

 the Hydroida, making them a subdivision of the Ccelenterata. It 

 should be noted, in justice to Claparede, that later he admitted his 

 error. 



The opponents of Kolliker were, however, gradually convinced. 

 Max Schultze ('63) showed the identity of Dujardin's sarcode with 

 protoplasm; Stein ('67) vigorously assailed the objections of Leydig 

 and Haeckel, while the latter ('73) brought back his Infusoria from 

 the Articulata, to which he had consigned them in 1866, and became 

 an ardent advocate of Kolliker's theory. The next few years saw 

 the remarkable researches of Biitschli, Engelmann, and Hertwig, upon 

 the physiology and finer organization of the Protozoa, and their work, 

 with that of the hosts of others since them, aided by modern tech- 

 nique, has fully demonstrated the unicellular nature of all Protozoa. 1 



The theory of alternation of generations (the alternation of a sexual 

 with an asexual method of reproduction) also became curiously in- 

 volved in the foregoing controversy. Steenstrup ('42) applied his 

 discovery of alternation of generations to the Protozoa, regarding the 

 parasitic Infusoria which he found in certain molluscs, as the larval 

 form of the liver-fluke, Distomum. The same view was found in 

 various forms in the works of Claparede and Lachmann, Perty, and 

 Kolliker, and finally as the " Acineta-theory " in the works of Fr. 

 Stein. This theory was based upon the supposed metamorphosis of 

 one form of Protozoa into another. The first suggestion of such a 

 metamorphosis seems to have been given by Pineau ('45), who ob- 



X L. Agassiz ('57), adopting a point of view which has appeared sporadically since von 

 Siebold announced that the Protozoa are single-celled animals, advocated the abandonment 

 of the Protozoa as a group, placing some divisions with the lower plants, others with the 

 larvae of worms, and still others with the Bryozoa. His most curious error was in placing 

 J^richodina pediculata, one of the higher forms of Protozoa, as the medusa-generation of the 

 fresh-water polyp Hydra. "I have seen for instance," says Agassiz, "a Planaria lay eggs, 

 out of which Paramcecium were born, which underwent all of the changes these animals 

 are known to undergo up to the time of their contraction into a chrysalis state; while the 

 Opalina is hatched from Distomum-eggs " ('57, p. 182). Similar views were held by Alder 

 ('51), Burnett ('54), and even recently by Lameere ('91), and by Villot ('91). 



