LEVELS OF ORGANISATION
All members of kingdom— Animalia are multicellular. heterotrophic eukaryotes
But, all of them do not exhibit the same pattern of organisation of cells. The
cells in their body arc of types. These are organised into many functional
units of progressively increasing complexity.
The animal body shows four basic plan of structural organisation as given below
l. Cellular
In this level, the body shows some division of labour among cells. They are
remarkably independent and can change their form and function. It is found in
sponges. The body consists of many cells arranged as loose cell but,
the cells do not form tissues rather each cell functionally active independently,
2. Tissue Level
The cells performing the same function are arranged into
tissues, hence is called tissue level of organisation, e.g., in
coelenterates. the arrangement of cells is more complex.
3. Organ Level
In Platyhelminthes and other higher phyla, tissues are grouped
together to form organs. each specialised for a particular
function. i.e.. organ level organisation is present.
4. Organ System Level
In animals like annelids, arthropods, molluscs, echinoderms
and chordates, organs have associated to form functional
systems, each system concerned with a specific physiological
function. This is called organ system level of organisation.
Organ systems in different groups of animals exhibit various
patterns of complexities.
The digestive system in Platyhelminthes has only a single
opening to the outside of the body that serves as both mouth
and anus and is thus, called incomplete.
A complete digestive system has two openings, i.e., mouth and
anus.
Another example is the two types of circulatory systems. Open
type circulatory system, in which blood is pumped out of the
heart and cells and tissues are directly bath in blood.
In closed type system, the blood is circulated through a network
of blood vessels of varying diameters (arteries, veins and
capillaries) and tissues remain in touch with.