Response to Stimuli.--Many of these little creatures may be found collected around masses of food, showing that they are attracted by it. In another part of the slide we may find a number of the paramoecia lying close to the edge of an air bubble with the greatest possible amount of their surface exposed to its surface. These animals are evidently taking in oxygen by osmosis. They are breathing. A careful inspection of the jar containing paramoecia shows thousands of tiny whitish bodies collected near the surface of the jar. In the paramoecium, as in the one-celled plants, the protoplasm composing the cell responds to certain agencies acting upon it, coming from without; these agencies we call _stimuli_. Such stimuli may be light, differences of temperature, presence of food, electricity, or other factors of its surroundings. Plant and animal cells may react differently to the same stimulus. In general, however, we know that protoplasm is _irritable_ to some of these factors. To severe stimuli, protoplasm usually responds by _contracting_, another power which it possesses. We know, too, that plant and animal cells take in food and change the food to protoplasm, that is, that they _assimilate_ food; and that they may waste away and repair themselves. Finally, we know that new plant and animal cells are _reproduced_ from the original bit of protoplasm, a single cell. [Illustration: A paramoecium. _c.v._, contractile vacuole; _f.v._, food vacuole; _m_, mouth; _ma.n._, macronucleus; _mi.n._, micronucleus; _w.v._, water vacuole.] The Structure of Paramoecium.--The cell body is almost transparent, and consists of semifluid protoplasm which has a granular grayish appearance under the microscope. This protoplasm appears to be bounded by a very delicate membrane through which project numerous delicate threads of protoplasm called _cilia_. (These are usually invisible under the microscope).