Simple Multicellular Organisms

Simple multicellular plants include the colonial forms like the volvox and the filamentous forms like the Spirogyra.

    A volvox colony Is made up of a large number of chlamydomonas like cells arranged as a single layer to form a hollow ball. The cells are connected by cytoplasmic strands.

   Each cell has flagella. These beat in a coordinated manner so that the colony is able to move in a definite direction, with one point of the colony always acting as the anterior end. Thus, unlike the chlamydomonas cell which can move about independently, the cells in the volvox have lost their ability to move about independently.

    When a colony is fully formed, there is no more growth by cell division (as is the case in most multicellular organisms). Cells division occurs when a daughter colony is to be formed. Only a few cells in a mature colony volvox colony can reproduce to form daughter colonies. Thus, the ability to reproduce is another characteristic that most cells In the volvox colony have lost.

    Other than these two featured lost through specialization, all the cells in a volvox colony function as independent living unites, just like a chlamydomonas cell.

     In Spirogyra , identical cells are joined end to end to form unbranched filaments. Each cell functions as an independent living cell. It can divide transversely into two and grow. This is how the filament grows in length indefinitely. If a cell breaks off from a Spirogyra filament, it can reproduce vegetatively to form a new filament. Spirogyra can also reproduce sexually.

   Simple multicellular animals include some colonial protozoa, sponges and Hydra.

    In most colonial protozoa, the individual cells are identical in structure and function, except that only a few may be able to reproduce.

    Sponges have a few special kinds of cells. For example, they have special collar cells for drawing in wager into their bodies; amoebiod cells for ingesting food; reproductive cells; and skeleton making cells. However. These special groups of cells are not recognized as tissues. This is because the cells within each group do not work together to carry out a particular function in a coordinated manner. Moreover, these cells are completely interdependent: any one of them can become modified to carry out some other functions.

   In hydra, there are several special kinds of cells such as sensory cells, primitive type of nerve cells, muscle cells and stinging cells. A group of each kind of cell works in a coordinated manner to carry out a particular function. For example, the muscle work together to shorten or lengthen the Hydra's body; the nerve cells form a king of nerve net throughout the body to transmit impulses. These special groups of cells are primitive tissues. Thus, the Hydra shows a tissue level of organization as some of its activities are performed in a coordinated manner by tissues. These tissues from the body wall which consist of 

• an outer layer of cells (ectoderm) that is sensitive and protective (like the skin in higher animals);

• an inner layer (endoderm) lining the gut and concerned mainly with digestion; and

• a thin middle layer of gelatinous material (mesogloea) separating the inner and outer layers.