What are metazoan animals | Define metazoa | Body symmetry in metazoa


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What are metazoan animals | Define metazoa | Body symmetry in metazoa


Metazoans are defined as the motile, multicellular, heterotrophic organisms that develop from embryos. However, metazoans such as sponges and some cnidarians are sessile, although they develop from motile larvae. The differentiation of cells is a distinctive feature of metazoans which are generally differentiated into tissues and organs (except the poriferans which have a cellular grade of organization and cnidarians which have simple tissue grade organization). The metazoans are mainly diploid organisms in which the haploid gametes are formed by meiosis within multicellular sex organs of mainly.


Body symmetry:

Arrangement of different parts of the body or organs of an organism on either side of an imaginary line, or around a common axis, or radially around a point so that the opposite parts of the body are the mirror images of one another is called symmetry. Thus, division of the body into equal parts by lines or planes can also be termed as symmetry.

With regard to general body patterns, animals may basically possess five types of symmetry:

1. Asymmetry:

An animal which cannot be divided into similar halves by sections passing through any plane or axis (sagittal or transverse) is said to be asymmetrical. These animals appear to be irregular in shape and lack symmetry of any kind e.g. Amoeba and some sponges among the metazoans.


2. Radial symmetry:

Radial symmetry occurs when a body is constructed around a central (oral-aboral) axis; i.e. one side of the body has mouth, known as the oral surface; while the opposite side is aboral surface. In this case, the body parts are arranged around a central axis or shaft (like the spokes of a wheel) in such a way that, any vertical cut through the axis would divide the whole animal into two identical halves or antimeres. All planes that passes through the centre are called radial planes. The common jelly fish and Hydra (cnidaria) exhibit radial symmetry. The starfishes have modified form of radial symmetry as they can be divided along five planes and each gives two distinct halves. This is known as pentamerous radial symmetry and is a character of the echinoderms.


3. Biradial symmetry:

Biradial symmetry is a variation of radial symmetry. In this case, there are only two planes of symmetry: one through the longitudinal and sagittal axis and the other through the longitudinal and at right angles to the sagittal axis that divides the animal into similar halves. Thus in this case each half of the body is identical to the opposite side e.g. sea anemone, and comb-jellies (ctenophores). Both radial as well as biradial symmetry is best suited for floating life. Radial and biradial animals are usually sessile, free floating or weak swimmers. These animals are also called as Radiata.


4. Spherical symmetry:

Spherical symmetry is mainly observed in those animals whose body is in the form of a sphere. Animals exhibiting spherical symmetry can be divided into identical halves along a number of different planes which passes through the centre, or in other words every plane that passes through the centre will yield two halves which are mirror images of each other. This type of symmetry is also  suited for free floating animals or also for animals showing rolling movements and is chiefly found in some protozoa. Example Actinophrys and colonial Volvox.


5. Bilateral symmetry:

Almost in all higher animals, the longitudinal axis of the body usually runs from the anterior (head) to the posterior (tail) end and the body parts in them are arranged similarly on either side of this axis so that the right and left halves are mirror images of each other. They also have a ventral (lower) and dorsal (upper) surface that are different from each other. Thus, they have a single plane (the median longitudinal plane) through which the body can be divided into two similar halves. Bilateral symmetry is a characteristic feature of active, crawling or swimming animals e.g. all higher invertebrates and vertebrates. 

The first phylum of animal kingdom that exhibit bilateral symmetry is phylum Platyhelminthes. Almost all animals except for sponges, ctenophores and cnidarians show bilateral symmetry. Adult echinoderms, though are radially symmetrical (pentamerous) but their larvae are mostly bilateral; because they evolve from bilaterally symmetrical ancestors. Bilateral animals are also called as Bilateria.

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