(63). If we examine a drop of water taken from any pond or ditch in which vegetable or animal substances have been permitted to undergo incipient decay, with a microscope even of very limited power, we must soon perceive that it swarms with innumerable organisms, which are evidently endowed with life and exhibit considerable activity. Prom the circumstance of their extreme minuteness, these microscopic beings were designated by their first discoverers "Animalcules" to which appellation, from the fact of their generally making their appearance in vegetable infusions, the term "Infusorial" was very generally superadded by their earlier investigators. Progressive improvements in the structure of the microscope, however, soon made it apparent that the so-called Infusorial or Microscopic Animalcules embraced a vast variety of different forms of living beings possessed of little in common except their invisibility to ordinary observation: the larvae and even the adult states of innumerable Insects, Crustaceans, "Worms, and Zoophytes were all comprehended under a term so general; and even microscopic Algae, Desmidieae, and Diatomaceae, now universally acknowledged to be members of the vegetable domain of Nature, were included in this chaotic assemblage - organisms widely dissimilar from each other both in their shape and structure.

It would be foreign to our present purpose to analyse the successive steps whereby something like order has at length been established in a scene of such apparently inextricable confusion, and how pari passu with the improvement of the microscope has been the rapid advancement of knowledge in connexion with these until so late a period unknown existences; suffice it to say that, in accordance with the more refined characteristics now adopted in zoological classification, Crustaceans and Insects, as well as the larvae of Annelidans, Zoophytes, and Echinodenns, have been successively withdrawn from the group and located in their appropriate stations, while innumerable zoospores and embryonic plants, together with the Desmidiaceae and the Diatomaceae generally, are by common consent conceded to the botanical series of Creation*. Still, as it would appear, the zoologist is reluctant to dissever forms of life which habit has accustomed the microscopical observer to associate with each other; and even M. Dujardin, one of the latest and most unprejudiced writers upon the history of these living atoms, includes under the term Infusoria, animalcules essentially remote both in their general characters and intimate organization.

* Vide Muller, 1786: Ehrenberg, Infusionsthierchen, 1837: Dujardin, Hist. Nat. des Zoophytes: Pineau, Ann. Sc. Nat. 3e ser. tomes iii. v. ix.: Stein, Wiegm. Archiv, 1849; id.Sieb. and Ko1. Z. iii.; id. Die Infusionsthierchen, Leipzig, 1854: Peltier, l'Institut, 1836: Focke, Isis, 1836, and Physiolog. Studien: Kutorga, Naturgesch. d. Infusionsthierchen: Meyen, Muller's Archiv, 1839: Pritchard, Infus. Anim.: R. Jones, Ann.Nat. Hist. 1839: Werneck, Ber. d. Berl. Akad.1841: Erdl, Mull. Archiv, 1841: Griffith, Ann. Nat. Hist. 1843, xii.: Siebold, Lehrbuch Vergl. Anat.: Cohn, Sieb. and Ko1. Z. iii. 260: Kolliker, Sieb. and Kol. Z. i. 198: Claparede, Wiegm. Arch. Dec. 1854, translated in Ann. Nat. Hist. 2 ser. xv. 211: Schneider, ibid. p. 191, translated ibid. xiv. p. 322: Carter, Notes on the Infusoria of Bombay, Ann. Nat. Hist, for 1856 and 1857: A memoir by Dr. N. Lieberkuhn in Midler's Archiv for 1856, translated in Ann. Nat. Hist, for Oct. 1856.

* It is by no means an easy task to indicate the boundary-line which separates the animal from the vegetable kingdom. The most important difference, that the vegetable cell-membrane contains no azote, while the animal cell-membrane does, cannot be applied in doubtful cases, the tenuity of the membrane not allowing of the investigation. That animals possess the power of locomotion, but plants not, is incorrect as applied generally, and is still less applicable here, because many unicellular Algae exhibit motion, frequently very energetic motion (when swarming), whilst the ova of multicellular Algae are quiescent. The unicellular Algae differ from the Infusoria in this, that their membrane and its appendages are not motile, and that consequently they have a rigid form, whilst the latter in some instances change their figure, and in others are furnished with motile cilia. The presence of starch is, further, not invariably decisive as to the vegetable nature of a cell. The ova of multicellular animals, the figure of which is rigid and unchangeable, may also be recognized as not belonging to the unicellular Algae from their want of colouring matter, which is present in the latter.

We can scarcely expect Chemistry to decide what is animal and what plant. The non-nitrogenous cellulose, which at first sight appears to be an exclusive attribute of the vegetable, is also found pretty generally in the animal kingdom, as we learn from the researches of Or. Schmidt on Cynthia mammillaris, and those of Kolliker and Lowig on a great number of the most various of the lower animals. Just as little does chlorophyll appear to be exclusively characteristic of the vegetable world, since the green granules and vesicles which occur imbedded in the parenchyma of Hydra viridis, of various Turbellariae (Hypostomum viride and Tryphoplana viridata, Schm.), and of Infusoria (Stentor polymorphus, Bursaria vernalis, Loxodes bursaria, etc.), are probably closely allied to chlorophyll, if not identical with it. Erythro-phyll also might be said to occur in the lower animals (for instance in Leucophrys sanguinea and Astasia hamatodes), in which latter the red colour frequently passes into green, as does the erythrophyll of unicellular Algae1.

1 The colouring matter of plants is distinguished by botanists into Chlorophyll, Erythrophyll, Phycochrom, and Diatomin. The Chlorophyll is of a grass- or yellow-green colour, little or not at all affected by diluted acids and alkalies, and frequently turns brown upon the death of the plant. The Erythrophyll presents a red or purple colour, not changed by diluted acids, but becoming green on the addition of alkalies, and also most usually after death. The Phycochrom is verdigris-green or orange, changed into orange by the action of diluted acid, and into a brown-yellow by that of alkalies. The Diatomin is brownish yellow, not altered by diluted alkalies, but changed into verdigris-green by diluted hydrochloric acid, and usually after death. Vide Nageli, Gattungen einzelliger Algen, physiologisch und systematisch bearbeitet: Zurich, 1849.