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 transparent, is placed over the animalcules, which are flattened by the pressure of the glass, so long as the observation lasts. The second, requiring still more dexterity, takes place by separating the larger species, with a very sharp and cutting needle, but it requires an extremely good eye and patience.

Perseverance, so necessary in all sorts of researches, is indispensable in microscopic observations, which must be often repeated and varied; but that trouble is amply rewarded, and the pleasure felt by the investigator, who discovers new forms, so beautiful, so surprizing and unexpected, is beyond description.

According to the organisation of these animalcules, we can divide them into two classes, subdivided into six families, viz, the Naviculae, the Cosmaria, Euastra, Fragilariae, Closteria and Arthrodiae. The three first families have a brittle tegument, the three others a flexible one; but, in order to recognize exactly the form of these creatures, it is necessary to describe each of the organs which compose them.

The external tegument (epidermis) of these animalcules is of two species. It is a siliceous, transparent and vitreous shell, called cuirass (lorica).