ANIMALCULES I — INFUSORIA : — ROTIFERA. 451 



ANIMALCULES. 



337. "We have now to apply ourselves to the special subject of 

 this Chapter, namely, the assemblage of those minute forms of 

 Animal life which are commonly known under the designation of 

 Animalcules. Nothing can be more vague or inappropriate than 

 this title, since it only expresses the small dimensions of the 

 beings to which it is applied, and does not indicate any of their 

 charactei-istic peculiarities. In the infancy of Microscopic know- 

 ledge, it was natural to associate together all those creatures which 

 could only be discerned at all under a high magnifying power, and 

 whose internal structure could not be clearly made out with the 

 instruments then in use ; and thus the most heterogeneous 

 assemblage of Plants, Zoophytes, minute Crustaceans (water-fleas, 

 &c), larva? of Worms and Mollusks, &c, came to be aggregated 

 with the true Animalcules under this head. The Class was being 

 gradually limited by the removal of all such forms as could be 

 referred to others ; but still very little was known of the real 

 nature of those that remained in it, until the study was taken up 

 by Prof. Ehrenberg, with the advantage of instruments which had 

 derived new and vastly improved capabilities from the application 

 of the principle of Achromatism. One of the first and most 

 important results of his study, and that which has most firmly 

 maintained its ground, notwithstanding the weakening of Prof. 

 Ehrenberg's authority in other respects, was the separation of the 

 entire assemblage into two distinct groups, having scarcely any 

 feature in common excepting their minute size, one being of very 

 low and the other of comparatively high organization. On the 

 lower group he conferred the designation of Polygastrica (many 

 stomached), in consequence of having been led to form an idea of 

 their organization which the united voice of the most trustworthy 

 observers now pronounces to be erroneous ; and as the retention of 

 this term must tend to perpetuate this error, it is well to fall back 

 on the name Infusoria, or Infusory Animalcules, which simply 

 expresses their almost universal prevalence in Infusions of Organic 

 matter. For although this was applied by the older writers to 

 the higher group as well as to the lower, yet as the former are now 

 distinguished by an appropriate appellation of their own, and are, 

 moreover, not found in infusions while in that state of rapid 

 decomposition which is most favourable to the presence of the 

 inferior kind of Animalcules, the designation may very well be 

 restricted to the forms essentially constituting the Polygastrica of 

 Ehrenberg, which is the sense wherein it has been used by many 

 recent writers. — To the higher group, Prof. Ehrenberg's name 

 Rotifera or Rotatoria is on the whole very appropriate, as signi- 

 ficant of that peculiar arrangement of their Cilia upon the anterior 

 parts of their bodies, which, in some of their most common forms, 

 gives the appearance (when the cilia are in action) of Wheels in 

 revolution ; the group, however includes many members in which 



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