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.
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.
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.
Difference between Bilateria and Radiata
|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
Theories to explain the origin of Bilateria from Radiata
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.
- What are bilaterians? Give out examples.
- Give examples of Radially symmetrical animals.
- Elucidate on the theories which explain the origin of Bilateria from Radiata.
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