INFUSORY ANIMALCULES. 411 



mals seems to be exactly the same as that of Amoeba and Actino- 

 phrys ; and their mode of nutrition differs only in this, that their 

 food can only be drawn into that part of the body which is un- 

 protected by the carapace. Nothing positive is yet known as to 

 their Reproduction. Two individuals may not unfrequently be 

 seen with the apertures of their shells in contact with each other, 

 the pseudopodial prolongations being apparently common to 



Various forms of Simple Rhizopods: A, Difflugia proteiformis : B, Difflugia oblonga: c, Arcella 

 acuminata : D, Arcella dentata. 



both ; but whether these are in the act of conjugation, or whether 

 (as the younger aspect of one of the shells sometimes indicates) 

 the union results from the production of a bud not yet separated, 

 cannot yet be certainly affirmed. The most remarkable deve- 

 lopment of this type of organization presents itself among the 

 marine Foraminifera and Sponges; in which composite structures, 

 often of very large size, are developed by continuous gemmation, 

 and a complex skeleton is produced. These, not being in them- 

 'selves (for the most part at least) microscopic organisms, will be 

 more fitly considered under a separate head (Chap. X). 



265. Animalcules. Dismissing the Rhizopods for the present, 

 we have now to apply ourselves to the special subject of this 

 chapter; namely, the assemblage of minute forms of Animal 

 life, which are commonly known under the designation of Ani- 

 malcules. 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 

 characteristic peculiarities. In the infancy of Microscopic 

 knowledge, 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 hetero- 

 geneous assemblage of Plants, Zoophytes, minute Crustaceans 

 (water-fleas, &c.), larvae 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 capa- 

 bilities from the application of the principle of Achromatism 



