_Contracting_ or _pulsating vacuoles_, small, clear spots in the cell, filled with water. In the living cell these disappear at intervals and then appear again. _Oral groove_, a funnel-shaped groove in one side of some one-celled animals, conducting food to the mouth. In paramecium it often shows as an oblique line when the animal rolls. _Gullet_, the inner end of the oral groove. _Cilia_, numerous minute, vibrating, protoplasmic hairs on the surface of many cells. _Respiration_, the passage of oxygen into the tissues of a living organism and of carbon dioxide out of them. These gases can pass through any thin, moist, organic membrane. When such a membrane separates two fluids which differ in the amount of oxygen they contain, oxygen passes to the fluid containing the smaller amount.[2] The same is true of carbon dioxide. Respiration is believed to occur in all living organisms. [2] This passage of fluids through membranes is known as _osmosis_. _Digestion_, the process of making food materials soluble, so that they can pass through membranes and be used to build up protoplasm. A few forms of cells are able to take in solid food and digest it in their protoplasm, but most cells can admit only fluid food. _Fission_, a method of reproduction used in all cells, by which a cell divides itself into two, usually through the center. In some one-celled animals this may be preceded by _conjugation_, when two animals unite temporarily and exchange nuclear substance; or in some forms two cells may fuse and the resulting cell may divide. _Budding_ is a form of fission in which a small projection is formed on the parent cell and then cut off, making a new individual. _Protozoa_ (first animals), animals of one cell, existing alone or in loose colonies. _Observations._ 1. Examine a single cell, stained to show structure. Identify the nucleus, cytoplasm, and, if present, the nucleolus or the micronucleus, and the cell wall. Draw to show the form of the cell and the details of its structure. Label all details. 2. Examine some stained paramecia. Select a typical one and identify in it nucleus, micronucleus, cytoplasm, and cell wall or cell membrane. You may also be able to see vacuoles, looking like holes in the stained protoplasm. Give reasons for considering this animal to be a single cell. Draw one, to show its cellular structure. Label all details. 3. Clean a slide and cover glass, place a drop of water containing living paramecia on the slide, cover it, and examine. What structures do you see which you saw in the stained paramecia? What structures do not show? Identify any new structures you may observe. Identify also the leading end and the side containing the oral groove. 4. Describe the shape of the animal. What is the actual length of the animal?