The only absolutely essential parts of the cell are the inner nucleus or kernel and the tiny mass of living jelly surrounding it, called the protoplasm.
The most powerful microscopes disclose in this protoplasm a certain definite structure, a very fine, thread-like network spreading from the nucleus throughout the semi-fluid albuminous protoplasm. It is certainly in line with the broad analogies of life, to suppose that in each cell the nucleus with its network is the brain and nervous system of that individual cell.
All living organisms consist, then simply of cells. Those consisting of but one cell are termed unicellular; those comprising more than one cell are called pluricellular.
The unicellular organism is the unit of life on this earth. Yet tiny and ultimate as it is, every unicellular organism is possessed of an independent and "free living" existence.
To be convinced of this fact, just consider for a moment the scope of development and range of activities of one of these tiny bodies.
"We see, then," says Haeckel, "that it performs all the essential life functions which the entire organism accomplishes. Every one of these little beings grows and feeds itself independently. It assimilates juices from without, absorbing them from the surrounding fluid. Each separate cell is also able to reproduce itself and to increase. This increase generally takes place by simple division, the nucleus parting first, by a contraction round its circumference, into two parts; after which the protoplasm likewise separates into two divisions. The single cell is able to move and creep about; from its outer surface it sends out and draws back again finger-like processes, thereby modifying its form. Finally, the young cell has feeling, and is more or less sensitive. It performs certain movements on the application of chemical and mechanical irritants."
The single living cell moves about in search of food. When food is found it is enveloped in the mass of protoplasm, digested and assimilated.