Metazoa-Origin and Evolution, Biology tutorial

Levels of body organization:

The larger units are atoms and several atoms get together to create combinations known as compounds that are variously attached together to provide higher level of organization known as complexes of compounds. Such levels of matter can be seen as the pyramid. In this pyramid any given level has all lower, levels as its component and itself is also component of all higher levels. Organisms which are composed of just one cell are simplest and the most primitive creatures known as unicellular organisms. Their level of body organization is at the lowest and is known as protoplasmic level of body organization.

Phylum Placozoa has single species of minute marine animal Trichohlar adharens made up of dorsal and ventral epithelial layer enclosing loose mesenchyme like cells. Mesozoans have some 50 species of small parasitic worms which have simple structures composed of 20-30 ciliated cells covering few reproductive cells. The next higher level of body organisation in pyramid is organ. Organs are usually made up of more than one kind of tissues.

When organs work together to carry out specific function there is highest level of body organization that is organ system level of body organization. Systems are related with basic body functions. This kind of body organization is seen for first time in Platyhelminthes. The next higher level of organisation is cellular level. This is actually an aggregation of cells which are functionally differentiated. The division of labor is apparent so that some cells are specialized for reproduction some for nutrition.

Characteristics of metazoa:

1. In life history of metazoans, typically the fertilized egg passes by blastula stage in course of its early embryonic development before changing into adult.

2. As metazoans are multicellular they are comparatively larger in size than unicellular protozoans. Nutritional requirements are more and they have to look for food.

3. Ability for locomotions has affected shape of metazoan animals which in turn has conferred specific kinds of symmetries to metazoan groups.

4. Most of the metazoans illustrate differentiation of anterior end or head (cephalization); related with cephalisation, there is centralization of nervous system in head region.


All living organisms possess some body shape and form. General body plan of animals may be arranged in one of many ways. Arrangement of parts or organs on either side of the imaginary dividing line or around common axis or radially around point so that opposite parts are mirror images of one another is known as symmetry. There are 2 broad divisions of symmetry,

(i) Primary or embryonic (ii) Secondary or adult.

Primary symmetry is bilateral and secondary symmetry is radial. With regard to symmetry animals can be basically of five types:

(i) Asymmetrical

(ii) Spherical

(iii) Bilateral

(iv) Radial and

(v) Biradial.

Asymmetrical and Spherical:

These are animals that can't be cut in 2 identical halves via any plane or axis (longitudinal, sagittal or transverse). Amoeba and most of poriferans are examples. At the other extreme, is spherical symmetry. Animals having spherical symmetry can be split in identical halves along number of planes that pass through centre or in other words every plane by centre will yield two halves that are min images of each other. E.g. protozoa and i rare in other groups of animals. Actinophrys and colonial Volvox are typical, examples.

Radial and Biradial:

Radial symmetry is symmetry in which parts are so given around the central axis or shaft, like spokes of wheel, that any vertical cut by axis would split whole animal in two identical halves. Common jelly fish and hydra (cnidaria) - show radial symmetry. Biradial symmetry is variant of this and it is found in sea anemones and ctenophores. Although animal seems to be radially symmetrical, it can be split only into two equal halves along 2 per-radial positions - along tenticular plane and along sagittal plane at right angles to it.


Bilaterally symmetrical animals have major axis running from head (anterior) to tail (posterior). They possess a ventral (lower) and dorsal (upper) surface which are different from each other. They contain only two sides which look alike, right and left. Animal can be split into just two identical halves by a plane that passes from anterior to posterior end.

Developmental patterns:

The bilateral metazoans can be split in two great assemblages: Protostomia and Deuterostomia. Platyhelminthes, Mollusca, Annelida, Arthropods and number of minor phyla are categorized as Protostomes while Echinodermata, Chordata and at least two minor phyla are comprised in deuterostomes. Characteristics to place animals in group are largely developmental are given below:


The unicellular zygote starts cell division (cleavage). First, single cell divides forming 2 cells, these redivide further to form four, then eight cells and so on till it gets converted in ball of cell. Cells are called blastomeres.

Planes of first and second cleavage are vertical passing by axis, but at right angles to one another. These 2 cleavages together yield in four blastomeres lying side by side around the axis. Plane of third cleavage is at right angles to first two planes and to axis and is horizontal and therefore parallel to equator of zygote. This results in eight blastomeres. Of eight blastomeres 4 lay on top of other four. Pattern of cleavage and arrangement of blastomeres around the imaginary central axis in the zygote can be one of two kinds radial or spiral.

Radial cleavage generates tiers or layers of cells one on top of another. Radial cleavage is also said to be undefined or regulative as each of blastomeres of early embryo, if separated from other, can control its development and form whole well proportioned embryo.

In Spiral Cleavage, though, the third and fourth cleavage planes are oblique to polar axis and resulting blastomeres don't lie on top of one another but above furrows between cells. The spindles during the third cleavage are arranged in the form of a spiral, therefore, the name spiral cleavage. This type of cleavage is seen in all invertebrate except the echinoderms (i.e. in annelids, molluscs, arthropods nemertenes and polyclad planarians).

Germ layers:

These three embryonic layers from which various organs of the animal are developed are called germ layers. This layer must not be confused with third genii layer that is mesoderm. As animals of these two phyla don't contain mesoderm, they are said to be diploblastic. Two germ layers give rise to several different cell types.

Body cavity and Coelom:

Vacuoles, spaces, lacunae and cavities have been of significance in all organisms, whether it is plant or animal. Every animal have cavities. Cavities carry out different functions in different animals. Spongocoel for instance, a cavity in sponges, is actually a system of water canals. Bilateral animals are categorized according to presence or absence of body cavities. There are two kinds of body cavities in animals pseudocoelom and coelom.


Platyhelminths that don't contain body cavity surrounding gut, have a solid type of body constitution. Mesoderm wholly fills space between body wall and alimentary canal in form of network of cells known as parenchyma. These animals are known as acoelomates. Cavity is in fact persistent blastoporl of embryo and is known as pseudocoel. Internal organs are free within pseudocoel. Body cavity has no lining of peritoneum derived from mesoderm. Animals so created are known as pseudocoelomates


True coelom is body cavity that arises inside embryonic mesoderm so that cavity lies between body wall (integument; ectoderm) and guts (endoderm) and is lined by mesodermal cells. This lining is known as peritoneum in higher animals.

Cephalisation and segmentation:

Bilateral animals when creeping or swimming have the tendency to keep same end of body forward and same surface down towards substratum. In such a case sensory organs and nervous system would also have the tendency to be concentrated at anterior end. This differentiation of head end is called as cephalisation (literally head development).

Segmentation or metamerism is division of body in smaller transverse compartments along anterior-posterior axis. Segmentation is extensive among animals, with true segmentation happening in annelids, arthropods and most chordates although some other groups illustrate superficial segmentation of ectodermal body wall.

Origin and evolution of metazoa:

A series of theories have been put forward to describe origin of multicellular metazoans from unicellular organisms. Of these, three principal theories could be thought.

1. Syncytial theory: Those ancestral metazoans have arisen ciliate by compartmentalization or cellularization.

2. Colonial Theory: Those ancestral metazoans have arisen flagellates by cellular specialisation and interdependency.

3. Polyphyletic Theory: Those metazoans have arisen from more than one group of organisms.

Evolution of Metazoa:

The sponges, coming under phylum Porifera are closest to Protista, and may be regarded even as colony of protists rather than being multicellular. Coelenterates comprising of Cnidaria and Ctenophora, that are both diploblastic and mainly radially symmetrical, can be considered as actually the most primitive of Metazoa. They have evolved from ancesteral planuloid metazoans as the offshoot. Platyhelminthes (flatworms) have evolved from planuloid ancestor. The pseudocoelomate phyla that comprise nematodes and rotifers are thought to have evolved as offshoot from flatworms.

Eucoelomata comprise remainder of Metazoa. Acoeloid flatworm have given rise to 2 major stocks:

i) Molluscan - annelidan, arthropodan stock with schizocoelous coelom.

ii)  Echinoderm, hemichordate, chordate stock with enterocoelic coelom. Coelom provides skeleton in more. Polycheate worms, earthworms etc. In arthropods and molluscs coelom becomes reduced to point of being represented by cavity of the gonads.

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