Using the microscope, we saw a creature eating some leaves of lettuce. It was transparent and nearly 0.1-0.5 mm long. It was a rotifer. It is a microscopic aquatic animal which can be found in freshwater environments like lake bottoms, or flowing water environments such as rivers or streams.

Rotifers have a long body made of three sections: a head, a body, and a foot. The foot usually has two toes. The head of a rotifer has a large, mouth, surrounded by cilia. The rotifer's cilia are used to take food and to move in the water.
Rotifers are not favored for fossilization because of their very small size.  They are mostly used in aquaculture and aquariums because of their very high reproductive rates.

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Diatoms are a major group of algae, and are among the most common types of phytoplankton. Diatoms are unicellular, although they can form colonies in the shape of filaments or ribbons.Only male gametes of centric diatoms are capable of movement.Diatom communities are a popular tool for monitoring environmental conditions, past and present, and are commonly used in studies of water quality.

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pH is  the  measurement of the acidity or the alkalinity in the solution. If you want to take pH measurement, you must have a pH-meter and you must keep the sensor in a bottle of a KCl saturated solution to protect it.
Before using it, you must clean the sensor with deionized water before drying it.
You must put the sensor in a solution of pH=4, then in a solution of pH=7 to calibrate the pH-meter and having the right settings.
Finally, you have to put the sensor in the stagnant water and wait for the reading to stabilized to have your data.

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