OF THE PROTOZOA. CILIATA. 



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prehended under the appellation Ciliated Protozoa are indubitably animals, 

 has never been called in question ; nevertheless their claim to be considered 

 independent organisms has been challenged by a few naturalists, who insist 

 on their being generally nothing more than phases of development of animals 

 more or less elevated in the scale. These objectors have, however, hitherto failed 

 to produce sufficiently direct and exact observations in proof of this general 

 assertion, which rests mainly upon presumed external resemblances, and on 

 analogy with many of the inferior animals, among which the so-called 

 ^' alternation of generations " is the rule. In the foregoing pages there is 

 certainly sufficient evidence that some Infusorial forms are merely stages of 

 development of others ; and nothing is more probable than that some may 

 similarly be phases of animals belonging to other classes than the Ciliata ; 

 yet, on the other hand, the independent character of several families (for 

 example, of Vortlcellina, Ophrydina, and Colejpina) has not been at all shaken 

 by the researches of naturalists. 



There is, we believe, a trae typical organization appertaining to the Ciliata, 

 of a distinct character fi'om that of other animal organisms, and inconverti- 

 ble. It may be more or less perfected, or more or less degraded, and may, 

 in the process of development, be put in abeyance for a time, though not 

 replaced by another ; and under this impression, Stein's views of Acineti- 

 form metamorphosis have, to our mind, an air of improbability. 



The very distinguished naturalist M. Agassiz stands among the foremost 

 in advancing the sweeping conclusion, that the Ciliated Protozoa have no 

 existence as a class. Most of the Enterodela of Ehrenberg, he says (A. N. If. 

 1850, vi. p. 156), " far from being perfect animals, are only germs in an 

 early stage of development. The family of Vorticellce exhibits so close a 

 relation with the Bryozoa, and especially with the genus PediceUina, that I 

 have no doubt that wherever Bryozoa should be placed, Vortkella should 

 follow, and be ranked in the same division with them. The last group of In- 

 fusoria — Bursar la, Paramecium, and the like — are, as I have satisfied myself 

 by direct investigation, germs of fresh-water worms, some of which I have 

 seen hatched from eggs of Planaria laid under my eyes." In these statements 

 Mr. Girard {Proceedings of American Association, 1848, p. 402) coincides, 

 and adds that CoJ-poda CucuUulus is one of the embryonic stages of fresh-water 

 Plana7'ia. 



To these statements it may veiy fairly be objected, that the embryonic 

 animalcules presumed to be identical with certain Ciliata may possess merely 

 a deceptive outward resemblance, and, again, that in the case of the assigned 

 affinity of the VorticeUina, an exact comparative examination of the organi- 

 zation of this family with that of Bryozoa will show that there is no true 

 homology, but simply some general points of similarity, between them. 



When Schleiden and others unfolded the cell-theory as a general fact in 

 organic beings, attempts were at once made to apply it to the simplest animal 

 structures, among which the Ciliated Protozoa are numbered. The Protozoa 

 were called unicellular animals ; a cell- wall, more or less modified, was 

 everywhere discovered or supposed ; and the more solid body, the testis of 

 Ehrenberg, was at once assumed as the "nucleus." This name we have for 

 convenience' sake retained, although its special relation with ceU-structure 

 and the cell-theory cannot, in our opinion, be sustained. 



The cell-theory, in its application to Protozoa, found a very able advocate 

 in KoUiker (J. M. S. 1853, i.), and was upheld by many others ; its simpli- 

 city, and the generalization as to structure and function it suggested, recom- 

 mending it to philosophic minds. Latterly, however, a more exact apprecia- 

 tion of the true organization and functional history of animalcules has caused 



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