Origin of metazoa 

WHAT ARE METAZOANS?

Metazoans can be define as multicellular organisms or as cellular animal or more accurately simply as animals.

They are holozoic multicellular organisms. Which are develop from embryo. Their body cells are generally organized in tissues and organs. [exception, porifera have cellular level of organization and coelenterates have tissue level of organization.] There gametes are never formed within unicellular structures but are produced within the multicellular sex organs or atleast within the surrounding cells.

DIFFERENCES BETWEEN METAZOANS AND PROTOZOA

Animal kingdom is generally divided into two subkingdom metazoan and protozoa which can be differentiate on the basis of number of cells present to form a their body. Subkingdom protozoa are unicellular or acellular animals composed of single cell or colonies of like cells.

On the other hand subkingdom metazoan are multicellular animals usually arrange in different layers, tissues and organs.

PREMATAZOANS ANCESTORS

A unicellular protistan perform all of the function of living from such functionally complex single cell protistan arouse the multicellualr animals. But how single cell protozoa which evolve possibly 2,000 million years ago, gave rise to the multicellular metazoans remains fundamental mystery of evolution.

To resolve this mysteries various theories were put forward. All of these theories can be categorised in three main theories which are as follow :-

1. Colonial theory
2. Syncytical theory
3. Polyphyletic theory

COLONIAL THEORY

Colonial theory can also be refer as flagellate theory. It was first purposed by Butschli, Lankester and Haeckel in 1874 later it was modified by Metschnikoff in1866. It is a classical theory according to which hollow, colonial, flagellates , like volvox were probable ancestors of metazoans.
It states that multicellular organisms came from association of many unicellular flagellate individual which forms a colony. With increase in cell they become more and more specialized in structure and function. Later on individuals in the cell was lost and whole itself became a single multicellular individual or a metazoans.

Criticism

According to this theory metazoans ancestor were like existing freshwater volvocid phytoflagellates. But these plant like organisms having cellulose cell wall, chlorophyll autotrophic mode of nutrition and undergoes reduction division following fertilization. The theory fails to explain that how these plant like characters were lost during the course of evolution. However, this objection is met with by holding that probably metazoan arose from some zooflagelates whose colonial organization must be similar from the volvocid phytoflagellates.

The flagellated colonies are of many type for example linear, tree like, plate like spherical solid as well as hollow. Of these which colony give rise ancestral metazoan has been subject of great speculation among scientist. Some of noteworthy views are listed below.

[A] BUTSHLIS THEORY

The plate like form such as Gonium was favoured by Butschli as the ancestral metazoan type. He postulated that first they become two layered by cutting of lower plate of cells and later curved into sphere. But this idea got no support from embryonic development of metazoans

[B] LANKESTERS THEORY

According to lankester, the ancestral metazoans was a morula like solid colony. Food taken by the outer surface of the external flagellated cells and further pass to their inner cells, this process is also seen in sponges and coelenterates. He named this as planula which was initially solid organism later on it develop mouth and digestive cavity.

[C] HAECKELS THEORY OF BLASTEA AND GRASTRAEA

According to this theory metazoan are formed from single protistan cells aggregation into a little hollow, spherical , flagellate colony. Similar to volvox. It possessed a anterio posterior axis. This hypothetical organism was named as blastea and was considered similar to blastula or coeloblastula stage in embryogeny of living metazoans.

The blastaea invaginated at posterior pole to become double walled sac termed as gastraea. The inner endodermal sac thus formed was termed as primitive intestine or archenteron and its single opening to exterior as primitive mouth, protostoma or blastopore. The endodermal cells lining the digestive cavity were relieved from perceptive and locomotory functions and become specialised for engulfing and digesting food.