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.
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 :-
- Colonial theory
- Syncytical theory
- Polyphyletic 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.
It was greatly accepted since it goes best with embryonic development. It was also supported by the following facts
- Plastic group
The flagellate are highly plastic groups means they have capacity for great changes in their body structures. Mostly it is assume that all plant and animal kingdom might be originated from them.
- Compact colonies
The flagellate shows a great tendency of forming compact colonies these resembles with various stages in embryonic development of metazoans.
- Flagellated spermatozoa
Tailed sperm cells of metazoans greatly resembles a modified flagellate. Hence it can be said that flagellated tail of sperm throughout the metazoan is a clear indication of their descent from flagellates.
- Flagellated body cells
Among lower invertebrates flagellated cells are quite common for example larvae of porifera bears flagella. Sponges also bear flagellated cells known as choanocytes that closely resembles choanoflagellates.
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.
- It was not accepted because
- Endoderm formation by primary delamination is very rare in metazoan.
- Formation of internal digestive tube or endoderm will be of no use without the mouth for taking in food.
[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.
- According to this theory the blastula and gastrula stages of embryonic development of higher metazoans represents repetition of adult stages of original metazoan ancestors that is blastaea and gastraea.
- Coelenterates are recently regarded as most primitive metazoan because of their radial symmetry. In this theory Haeckel also compare close structure similarity between ancestral gastraea and lower metazoans.
- In metazoan embryology endoderm is not always formed from invagination, even in coelenterates which are structurally nearest to hypothetical gastraea endoderm forms from multipolar ingression and the larva is parenchymula or planula rather than gastrula. It was the main criticism of Haeckel theory.
- According to this theory radial symmetry is regarded as primitive character echinoderms and some chordates ( tunicates) it is secondary condition derived after metamorphosis of larval form.
[D] METSCHNIKOFFS THEORY
- Metschnikof noted that primitive mode of gastrulation in coelenterates is by ingression and not by invagination, producing a solid gastrula. He also discovered that the digestion was intracellular and phagocytic in lower metazoans. According to him certain ectoderm cells of blastaea become phagocytic and wander into inner blastocoels which become filled with amoebioad cells embedded in gelateneous material. Therefore he argued that the diploblastic ancestral gastraea was solid rather than hollow organism and named it as parenchymula.
- It would not have required a digestive sac or mouth because of intracellular digestion.
- Formation of archenteron and blastopore were regarded as secondary processes.
- According to him his theory strong supported by discovery of proterospongia. It is choanoflagellate colony embedded in gelatinous matrix.
- Some outer collared cells frequently lose their collars and flagella and move into the jelly as amoeboid cells indicative of ingression.
- These theory was proposed b Hanson and Hadzi in 1958. Syncytial is multinucleated condition where cell membrane are absent between adjacent cells.
- It states that metazoan did not evolved from aggregation of unicellular protistan but by cellularization of primitive multinucleated syncytial ciliates.
- The ancestral metazoans were bilaterally symmetrical and syncytial in structure. It give rise to acoel flatworm by the process of cellularization. Turbellaria (acoel) are therefore regarded to be the most primitive metazoans and their bilateral symmetry is considered as primitive.
- Cnidaria especially anthozoa become radially symmetrical externally by adopting sessile mode of life and they were also derived from turbellarians. However internally they have still retain their primitive ancestral bilateral symmetry in their stomodaeum, mesenteries and in muscle bands. Radially symmetric Hydrozoa is structurally advanced than Anthozoa.
- Ctenphora and the mullers larva of Polyclad turbellian are both Planktonic exhibiting octaradial symmetry and their embryo is also similar in several ways. Therefore he mention that Ctenophore evolved from Polyclad.
- Theory of germ layer was rejected by this theory. According to that theory first ectoderm and mesoderm formed while mesoderm appeared later. But in the development of Acoela 3 alyer are formed insitu without development of any germ layer, thus he presume that germ layer were formed contemporaneously in the ancestral ciliates following cellularization.
- It was also assume that conjugation in ciliates is equivalent to copulation in metazoans.
- This theory compares Turbellarian such as Convoluta with certain multinucleated ciliate. Both are in same size range have an anterior posterior axis, both are ciliate and are bilateral symmetrical. Both lack digestive cavity.
- Central parenchyma of Acoela (Convoluta) looks syncytial due to imperfect cellularization he considered this similarities as evidence in support of the primitive position of Acoeleous.
- According to him it was all that needed to convert such a Protist to an Acoelous Turbellarian.
- Recent electron microscopic studies have revealed that Acoela is cellular and not syncytial.
- Parenchyma of Acoela may also prove to be cellular on closer examination.
- This theory considered biradial anthozoa to be primitive and radial hydrozoa be most specialized. This is base on the presumption that bilaterally symmetrical acoelous Turbellarian are most primitive living metazoan. But evidence shows radial symmetry of coelenterates to be primary and not secondarily evolve from a bilateral ancestor.
- Body plans of primitive groups are generally more variable or less fixed. Naturally Hydrozoa being more variable should be consider as more primitive than Anthozoa.
- Anthozoa with few exception are not hermaphrodite hence they seem improbable descendent of hermaphrodite Turbellarian.
- It fails to explain motility of flagellated sperm.
- Macronucleus is present in ciliates but absent in Acoels. It is assume that macronucleus may be evolve after origin of metazoans.
- Turbellarian, annelids and mollusc undergo spiral cleavage during embryonic development. On the other hand coelenterates shows various cleavage pattern other than spiral cleavage.
- More recently Greenberg and Preston have proposed that all metazoans have not originated from single ancestors. They have suggested a polyphyletic origin of metazoans
- It is fairly certain that sponges develop from the colonial flagellates.
- Whereas other multicellular group generated from syncytial protocilliates.
- Nursall in 1962 has given a theory that various multicellular invertebrate phylum have been evolved independently from different protists.
- Hanson and Sleigh have put forward a theory that sponges and cnidarians have evolved separately from flagellated protists and other group-ciliates has originated from flagellated protists and ciliates have given rise to Acoel-platyhelminthes and other metazoan phyla have evolved from Acoel-platyhelminthes.
- On an rough estimation life originated on earth about 3.7 billion years ago first cell to appear was a prokaryotic cell without a nucleus. With increase in complexity of life multicellular organism appear later on various theory were proposed to prove how metazoans were actually originated but none of them were able to fully prove its origin each theories were having certain drawbacks
- Nielsen and Norrevang also forwarded the trochaea theory in which the pelagic trochophore larva (trochaea) transforms into blastaea and gastraea, to form the ancestor of spiralians, and in turn gives rise to a pelagic tornaea as the ancestor of deuterostomes.
- The earlier unicellular or non multiecllular organisms were neither plants (protophyta) nor animal in nature Haeckel called them Protista which may be define as organism usually of small size, consist of nucleus apparatus in cytoplasm body which is not divided into cells some of the scientist consider them as a ancestor of metazoan.