The Levels of Organization in animal complexity

1. Protoplasmic level of organsiation
Protozoans carry out all the metabolic functions of animals.
Organelles, or intracellular structures, carry out a variety of functions, such as digestion, excretion, respiration, and coordination of movement;
A single cell organism is said to be operating at the subcellular level of organisation, i.e. specialisation occurs within the confines of one cell


Multicellular (many celled) organisms have various levels of organization within them.
Individual cells may perform specific functions and also work together for the good of the entire organism.
The cells become dependent on one another
Multicellular organisms have 5 levels of organization of complexity ranging from simplest to most complex: 1. 2. 3. 4. 5.

Cellular level of organisation Tissue level of organisation Organ level of organisation System level of organisation Organ-system level of organisation

LEVEL 1 - The cellular level of organisation
Cells are the basic unit of structure and function in living things and may serve a specific function within the organism. examples
Blood cells: Red blood cells (RBC) are responsible for carrying oxygen around the body and the white blood cells (WBC) are responsible for fighting against pathogens. They engulf pathogens.
Nerve cells, bone cells
One multicellular organism still operating at the cellular level of organisation is the Phylum Porifera, subkingdom Parazoa

The Phylum Porifera (The Parazoa)


The sponges represent the lineage closest to the multicellular organisms (protists) that gave rise to the animal kingdom.
Members of the Phylum Porifera are considered to be just a loose aggregation of cells.
The cell layers of the sponges are loose association of cells, not really tissues because the cells are relatively unspecialized.


The Sponges are sessile (permanently attached, hence non-motile) sack-like animals of the Phylum Porifera (full of pores).
Sponges are the simplest of the multicellular animals.
They have no organ systems and are characterized by numerous canals and chambers that open to the outside by way of pores.
Many sponges are asymmetrical, but some exhibit radial symmetry.


Sponges are filter feeders.
Water (carrying suspended plankton and other potential food) enters numerous small pores called ostia (singular, ostium).
The ostia are surrounded by donut-shaped cells called porocytes that open and close to control water flow.
Water flows directly into an open chamber called the spongocoel (the term "coel" refers to an open space or body cavity in an animal).
Water leaves the spongocoel by a larger opening (the osculum).
The interior of the spongocoel is lined with flagellated cells called choanocytes (or collar cells).


The choanocytes have a tubular collar facing the spongocoel.
A flagellum extends from the center of this collar, the movement of which creates currents that force water through the sponge's "plumbing system".
Suspended food particles (plankton, larvae, etc) in the water are drawn through the collar from below, trapped on the outside of the collar, and then phagocytized.
The collar cells do not digest the captured food by themselves; instead it is passed to a second cell (an amoebocyte) waiting in the mesohyl (the acellular portion of the sponge).
Amoebocytes carry the food to other cells that require nutrition and thus partly compensate for the sponge's lack of a circulatory system.
Amoebocytes carry on other jobs.


They can, for example, undergo developmental changes to turn into any other cell type that may be required.
This allows for growth, repair, and reproduction of the sponge.
In addition, they are responsible for producing the sponge's skeleton (a network of fibers flexible protein (spongin) and needle-like spicules).
Spicules are usually made of calcium carbonate or oxides of silicon and the shape of the spicules is important in classification of the sponges.
Although sponges are classified as multicellular animals, they demonstrate less integration and specialisation of function than other animal groups.
They lack tissues organisation, and their cells are the primary units of structure and function.
The body is two layered and separated by a gelatinous mesohyl.

LEVEL 2 - Tissue level of organisation
The tissue level of organisation is a much higher level of organisation than the cell level of organisation.
Tissues are made up of cells that are similar in structure and function and which work together to perform a specific activity. Examples - blood, nervous, bone, etc.
Humans have 4 basic tissues: connective, epithelial, muscle, and nerve.
There are some multicellular animals that have attained this level of organisation as their highest operating level. e.g. the Radiata.
Radiata are group of animals which are characterised by radial symmetry.
Above the level of sponges, animals can be broadly divided into two classifications on the basis of body symmetry: radial and bilateral.


The radially symmetrical Phyla, the Radiata, possess a central mouth around which the rest of the organs are radially arranged.
Radiata consist of two phyla Cnidaria (Coelenterates) and Ctenophora (comb jellies).
Each of these phyla consists of individuals with two layers of cells, radial symmetry, and an inner gastrovascular cavity (coelenteron).
The gastrovascular cavity serves as a digestive structure.
Food is taken in through the mouth, and what is undigested passes out again through the mouth.
Whereas sponges are filter feeders, the coelenterates and comb jellies can ingest large food particles and pass the digested nutrients throughout the body.


The Radiata display a tissue level of organization.
Specialized nerve cells make up a nerve net (nerve tissue) which permeates the entire body of the organism and is responsible for sending nerve impulses all over the body.
Epitheliomuscular cells make up a contractile tissue (epidermal tissue), which is derived from the outer ectodermal layer of cells.
Epithelionutrive cells make a gastrodermal tissue specialised in digestion.

LEVEL 3 – The organ level of organisation
Organs are made up of tissues that work together to perform a specific activity.
Examples - heart, brain, liver, pancreas etc.
The rest of the animals except the sponges (operating @ cellular level of organisation) and Radiata (cnidarians and ctenophores -operating @ the tissue level) have three germ layers; ectoderm, mesoderm and endoderm and they have attained the organ level of organization.
An organ consists of 2 or more tissues that perform a particular function (e.g., heart, liver, stomach, etc).


Animals with three germ layers are said to be triploblastic and all triploblastic animals operate at the organ level of organization.
The simplest triploblastic animal is a flatworm, which belong to the Phylum Platyhelminthes.
The Phylum Platyhelminthes contains animals that are multicellular, bilaterally symmetrical, have distinct tissues, and simple organs for digestion, excretion, and reproduction.
Flatworms as the name imply are flat.
They exemplify an organ level of complexity compared with the tissue level shown by the radiata.
An example of a flatworm is Planaria.
There are four classes of flatworms; turbelaria, Trematodes, Monogenea and Cestodes.
Only Class Turbelaria is free living and the remaining three are parasitic.


The cnidarians are animals organized mostly on the tissue level.
From planarians to humans, animals are constructed on a more complex level of organization.
Not only do cells work together to form tissues, but tissues of various kinds are closely associated to form one structure, called an organ, adapted for the performance of some one function.
E.g. human stomach is an organ composed of epithelial tissue, connective tissue; muscle tissue and nervous tissue.
The epithelial tissue layer lines the cavity and contains the gland cell that secretes gastric juices;
the muscle tissue layers produce the stomach contractions;
the nervous tissue coordinates muscle contractions and the connective tissue binds various layers together.

LEVEL 4 - Organ-system level of organisation)
All triploblastic animals operate at the system level of organisation.
Organs with a common function associate and they form the organ-system.
Organs are groups of two or more tissues that work together to perform a specific function for the organism.
Examples - circulatory system, nervous system, skeletal system, etc.
The Human body has 11 organ systems - circulatory, digestive, endocrine, excretory (urinary), immune (lymphatic), integumentary, muscular, nervous, reproductive, respiratory, and skeletal.


An organ usually cooperates with other organs in performance of some life activity and such a group of structure devoted to one activity is termed an organsystem.
Thus the stomach is part of the digestive system; and all other parts such as the eosophagus, liver, pancreas and intestine are necessary for proper performance of digestion.
Flatworms have the simplest version of these and are the simplest phylum of animal built on the organsystem level of construction