Arrangement of body parts in a balanced geometrical design, divisible into equal parts by planes of division is called symmetry. The idea of symmetry is mainly derived from Ernst Haeckel.
An animal is said to be symmetrical only when a plane passing through its center will divide it into similar halves. When an animal cannot be divided into similar halves it is said to be asymmetrical.
Asymmetrical animals such as Amoeba or sponges possess irregular body shapes and hence have no symmetry but higher metazoans possess some kind of symmetry depending on their habits and habitats that balances their activities. All animals are either symmetric or asymmetric. An axis is an imaginary line passing through the center of body. Either end of the axis is termed as pole. Thus each axis has two poles. A plane of symmetry is a straight line that divides an animal into two equal halves.
1. Spherical symmetry: It is found in the animals whose body is ball-like and all planes passing through the center of body will cut the animal into equal halves. This type of symmetry is suited for rolling movement, for floating in water or in sedentary habits in which case food is available in all directions. Body organs like cilia or tentacles are located all around the body in a radiating manner. For example, Volvox, Actinophrys (Heliozoa) and Thalassicola (Radiolaria)
2. Radial symmetry: This type of symmetry is found in Coelenterates and Echinoderms in which Body parts are arranged along the main longitudinal axis of body. It is best suited for sessile existence where food is planktonic and available in abundance in all directions. Food capturing organs are therefore arranged radially and the animal does not have to move in search of food. Some of the Echinoderms, like star fishes, have given up their sessile existence to become hunters in pursuit of larger prey but not their ancestral radial symmetry.
3. Biradial symmetry: Biradial symmetry is a mixture of bilateral and radial symmetry. This is found in Ctenophores which are not sedentary but floating animals and show a mixture of bilateral and radial symmetries. Animals such as Pleurobrachia have oval body on which eight comb plates are radially arranged like bands and are used for swimming, whereas mouth, anal pore and statocysts are placed on the anterio-posterior axis.
They also have a pair of retractile tentacles that bear colloblasts which secrete sticky substance that helps in capturing planktonic food on which they feed. Tentacles demonstrate bilateral symmetry whereas comb plates show radial symmetry and the animal takes advantage of both symmetries for food hunting and active swimming.
4. Bilateral symmetry: This type of symmetry is found in most of the higher animals above Platyhelminthes and is best suited in animals which move in a definite direction, due to which the sense organs and nervous system concentrate on the anterior side and locomotory organs become paired for balanced propulsion of body.
A single line passing through the longitudinal axis will divide the body into two equal halves in such a way that one half is a mirror image of the other. Flat worms were the first bilaterally symmetrical animals and other higher groups such as annelids, arthropods, some mollusks and chordates are all bilaterally symmetrical.
Eumetazoa is divided into two groups by Hatschek. These two groups Radiata and Bilateria are divided depending on the symmetry they possess. Radiata includes Coelenterates and Ctenophores and bilateria includes all phyla starting from Helminths to chordates.
Bilaterial animals: Bilaterians are bilaterally symmetrical animals. These are the animals that can only be cut in one plane to create a single mirror image. They have top (dorsal), bottom (ventral), head (anterior), tail (posterior), right, and left sides. Cephalization is another important feature of Bilaterians. Cephalization is the concentration of nervous tissue in the head region.
Bilaterians have bodies that develop from three different germ layers namely the endoderm, mesoderm, and ectoderm. They are called triploblastic. Except for a few highly reduced forms, the Bilaterians have complete digestive tracts with separate mouth and anus. Most Bilateria also have a type of internal body cavity, called a coelom.
Most of the phyla are bilaterians with exceptions of Sponges of Parazoa and Cnidarians. Also most notable exception is echinoderms, which are radially symmetrical as adults, but are bilaterally symmetrical as larvae.
Radial animals: Radiata are radially symmetrical animals. These are the animals that can be divided multiple times through a central axis creating multiple mirror images. They have a top and a bottom but no left nor right, no head nor tail. The best example is Phylum Cnidaria which includes jelly fish and sea anemones. Radiata have bodies that develop from two different germ layers, called the ectoderm and endoderm hence they are diploblastic.
| Radiata | Bilateria |
|---|---|
| Body radially or biradially symmetrical | Body is bilaterally symmetrical |
| Sometimes bilateral symmetry is the adaptation in some animals | Sometimes radial symmetry is the secondary adaptation in some animals |
| Organ systems are not well marked | Organ systems are well marked |
| Mesoderm is not developed so the animals of the grade radiata are diploblastic in nature | Mesoderm is well developed so the animals of the grade bilateria are triploblastic in nature |
| Coelo cavity is invariably absent | Coelom can be pseudocoelom or true coelom or may be absent |
| Tentacles with nematocycts are present | Tentacles if present have no nematocycts |
| Comb plates (locomotory organs) are present | Comb plates are absent |
| Principal external opening of the digestive cavity is mouth | External opening of the digestive cavity are mouth and anus |
It is universally believed that the first metazoans were radially symmetrical and bilateral symmetry evolved later owing to the creeping habit acquired by the animals to feed on detritus on the bottom. The following are the theories put forth in support of evolution of Bilateria from Radiata.
Ctenophore-polyclad theory:
Proposed by Kovalevsky and Arnold Lang. It emphasizes that polyclads evolved from ctenophore-like ancestor. Modern polyclads, such as Leptoplana and Notoplana are marine, free-living, bottom dwelling turbellarians that belong to Order Polycladida or Phylum Platyhelminthes. They creep on the bottom and use their ventral mouth to feed on detritus. On the other hand ctenophores are freely floating animals exhibiting radial as well as bilateral symmetry aka biradial symmetry.
Ciliary bands are radially placed on the body while a pair of antennae is bilateral. A ctenophore-like ancestor could have given rise to bilaterally symmetrical animals by acquiring bottom crawling mode of life. Some crawling ctenophores existing today are Ctenoplana and Coeloplana.
Ctenophore-trochophore theory:
This theory takes into consideration the larval stages of Coelenterates, Ctenophores, Helminthes and Annelids and tries to establish evolutionary relationship among them.
Planula larva of coelenterates has elongated and cylindrical body that is ciliated all over. Cydippid larva of ctenophores is also ovoid in shape but has longitudinal ciliary bands arranged radially around the body. Muller’s larva of Polycladida also has ciliary bands on swimming arms and apical tuft of cilia on the anterior side. Mouth is ventral in this larva and there is no anus.
The trochophore larva of Polychaeta resembles Muller’s larva in having ciliary bands and apical tuft of cilia and ventral mouth. Since Cydippid larva of ctenophore, Muller’s larva of polyclads and trochophore larva of polychaetes all resemble one another in structure and ciliary band, this theory considers larvae of acoelomate bilateria (flat worms) as early stages of trochophore.
Planuloid-Acoeloid theory:
Proposed by Ludwig von Graff and the elaborated by Hyman. The theory postulates that the primitive acoelomate bilateria evolved from some planuloid ancestor which was very similar to the planula larva of coelenterates. The planuloid ancestor must have been free-living, radially symmetrical, ciliated and with a diffused nerve net.
Such planuloid larvae must have developed into a gastrula-like ancestor by the formation of mouth and archenteron and adopted a bottom creeping mode of living rather than free swimming habit of planula. Creeping habit produced cephalization of nervous system towards the anterior side and since the food was available at the bottom, the anterior mouth moved to the ventral side and the body became dorso-ventrally flattened, as is the case in turbellarian Helminthes of today.
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