Hatschek (1881-1891) divided Eumetazoa into two groups, namely, Radiata and Bilateria depending on the symmetry they possess, the former includes coelenterates and ctenophores and the latter includes all phyla from helminths to chordates.
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.
Theories to explain the origin of Bilateria from Radiata
1. Ctenophore-polyclad theory
This theory was proposed by Kowalevsky (1880) and Arnold Lang (1881-84) and emphasises 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 that is also termed as 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. There are some crawling ctenophores existing today, e.g. Ctenoplana and Coeloplana.
2. 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 (Helminthes: Turbellaria) 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.
3. Planuloid-Acoeloid theory
This theory was first proposed by Ludwig von Graff (1882) and later elaborated by L. H. Hyman (1951). The theory postulates that the primitive acoelomate bilateria (helminths) 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 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 cephalisation 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 helminths of today.