Origin of Metazoa; Metamerism & Symmetry

• Describe colonial hypothesis of metazoan origin.


The colonial theory or flagellate theory is the classic & still most widely acceptable theory of the origin of metazoans.
This theory was 1st put forward by Haeckel (1874), later modified by Metschnikoff (1886) & revived by Hyman (1940).
Proposition:-
Colonial theory holds that multicellular animals came about through association of many unicellular flagellates, forming a colony.
With increase in the no. of individual cells, they became more & more specialized in structure & functions.
Soon, individuality in the cells was lost & the whole colony itself became a single multicellular individual or a metazoan.
Support;-
This hypothesis is supported by the following facts:-
1. Plastic group:- the flagellates are a highly plastic group, i.e. capable of great changes.
2. Compact colonies:- the flagellates show a great tendency of forming compact colonies. These clearly resemble various stages  in the embryonic development of Metazoa.
3. Flagellated spermatozoa:- the metazoan flagellated sperm cells very closely resemble a modified flagellate.
4. Flagellated body cells:- lower metazoans like sponges possess flagellated cells or choanocytes &  also produce flagellated larvae (parenchymula, amphiblastula) that closely resemble choanoflagellates (Proterospongia).
Criticism:-
1. According to Colonial theory, the metazoan ancestors were Volvox like phytoflagellates. But these are plant like organisms have cell walls, chlorophyll etc. The theory doesn’t explain how these plant characters were lost in metazoan ancestors during the course of evolution.
2. The flagellate protozoan colonies are of several types such as linear, spherical, tree like, plate like & solid as well as hollow. Of these, which type gave rise to the ancestral Metazoa, has been a subject of great speculation.


• Write a note on cyclomerism theory of the origin of metamerism.
The cyclomerism theory was originally proposed by Sedgwick (1884) & greatly supported by Remane (1950, 1963).
This theory assumes that coelom originated in some ancestral radiate actinozoan coelenterate, through the separation of 4 gastric or enterocoelic pouches from the central digestive cavity or gut.
Division of 2 pouches resulted into 3 pairs of coelomic cavities- protocoel, mesocoel & metacoel in the protocoelomate or ancestral coelomate.
Loss of protocoel & mesocoel led to the unsegmented coelomates such as molluscs.
Later subdivision of metacoel produced primary segments leading to the segmented annelids.
The phylogenetic implication of this theory is that all bilateral metazoans were originally segmented & coelomate & the acoelomate groups ( flatworms, nemerteans) have lost these characters secondarily.  


• How does the segmentation of an annelid worm differ from the repeating units of the body of a tapeworm?
The segmentation of annelids is called true segmentation or metamerism whereas the repeating body units of a tape worm represents pseudometamerism.
1. In annelids the no. of segments is generally constant for each species while the no. of body segments in a tapeworm is not fixed as new segments are continuously added throughout life.
2. In an annelid growth occurs due to simple elongation of pre existing segments whereas in a tapeworm growth occurs due to addition of new segments from a region of proliferation, just behind the scolex.
3. All segments are of the same age & at the same stage of development in an annelid while the proglottids of a tapeworm differ from one another in age & in the stage of development.
4. In an annelid the body segments are functionally interdependent & integrated whereas segments or proglottids of a tapeworm are independent & self contained units each having a full set of sex organs & a portion of excretory & nervous systems.


• What do you mean by symmetry? Mention the different types of symmetry in the kingdom Animalia & their importance.
Symmetry is the arrangement of body parts in a balanced, geometrical design, divisible into equal halves by planes of division.
There are 4 types of symmetry present in kingdom Animalia.
1. Spherical symmetry:-
Spherical symmetry implies that any plane of division passing through the centre of the animals body will cut it into 2 equal halves.
Some protozoans like Actinophrys, Volvox etc. have spherical symmetry & it is adapted for free floating or rolling movements.
2. Radial symmetry:-
Radial symmetry suggests that any plane passing through the longitudinal axis of the body of the animal will divide it into equal halves or antimeres.
Examples are echinoderms & most cnidarians.
Radial symmetry is best suited for a sessile existence. Most of the animals are attached forms. Due to the similarities of antimeres, there sensory receptors are equally distributed all around the periphery. This enables them to receive stimuli, obtain food, repel enemies from all sides.
3. Biradial symmetry:-
The term biradial symmetry denotes a mixture of bilateral & radial symmetry in these animals. This type is found in Ctenophora.
Animals such as Pleurobrachia have oval body with 8 comb plates arranged radially, & are used for swimming.
It also have a pair of retractile tentacles which help them in capturing food.
Tentacles demonstrate bilateral symmetry while comb plates show radial symmetry. Thus the animal takes advantage of both symmetries for active swimming & food hunting.
4. Bilateral symmetry:-
Bilateral symmetry implies that the median longitudinal or sagittal plane passing through the animals body divides it into 2 externally identical halves which are rough mirror image of each other.
Bilateral symmetry is characteristic of most successful & higher animals. First phylum of animal kingdom to exhibit bilateral symmetry is Platyhelminthes.
Bilateral symmetry permits streamlining, favors the formation of a central nerve center, contributes to cephalization and promotes actively moving organisms.