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 The Closterium acuminatum presents an other instance of this exchange. In all the Closteria, seen sidewise, and where the two half cuirasses join, we discover two holes (fig. 57. 67. d. d. fig. 59. d. d. fig. 61. c.) opposite one another. The destination of these two openings was long for me a matter of doubt, till I could at last distinguish the junction of the Closterium acuminatum, in which two animalcules (fig. 59.) placed themselves obliquely one against the other, and till I saw, through these openings, the exchange of a transparent and scarcely perceptible fluid. In this union, we see also upon the cuirass an opening, both on the clear and on the convex side. This opening, however, is only visible during the union, described (fig. 59. x.). At all other times, in spite of all sorts of trials, I never could distinguish any thing.

The genus Echinella Ehrenb. (fig. 54—55.), beside the orifice (fig. 55. a.), has upon each cutting surface of its body, three lateral pores, consequently sex in all (fig. 54. 55. b. b. b.), passing through the cuirass. Though closely adhering together by these small cutting surfaces, I never could discover any exchange of fluids operated by these pores. The propagation of the Oscillatoriae and Diatomeae is probably performed by decomposition of the parts; I never saw, however, propagation take place by division, admitted and even described by several naturalists, such as it may be seen in Infusoria of a higher category, and such as it has been admirably *