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Origin of bilateria from radiata

- Radiata and Bilateria differ in body axes, symmetry, and germ layers. Radiates have radial symmetry and are diploblastic, while bilaterians have bilateral symmetry and are triploblastic. - There are three main theories that attempt to explain the evolution of bilateral symmetry from radial symmetry: 1) the ctenophore-polyclad theory, 2) the ctenophore-trochophore theory, and 3) the planuloid-acoeloid theory. - The planuloid-acoeloid theory proposes that primitive acoelomate bilaterians evolved from a planuloid ancestor similar to coelenterate planula larvae, which adapted from free

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Origin of Bilateria from Radiata Shreeram Ghimire Amrit Campus Department of Zoology ghimireshreeram25@yahoo.com
• Symmetry means an arrangement of body parts into geometric designs. It refers to the division of body into equal parts by lines or planes. Based on that symmetry, Hatschek (1881-1891) divided metazoans into two groups: Radiata and Bilateria. • Radial symmetry: The body is in the form of flat or tall cylinder. All the lines passing through longitudinal axis divide body into equal halves in any plane. Some Poriferans, Coelenterates, Ctenophorans and some Echinoderms have such body symmetry. These phylum are grouped together to give a name called radiata. It is best suited for sessile animals. • Bilateral symmetry: The body can be divided into equal right and left halves. First phylum of animal kingdom bearing such symmetry is Platyhelminthes. All other higher phylum from Platyhelminths bear bilateral symmetry and are grouped into bilateria. • Radiata and Bilateria differs in three aspects: body axes, symmetry and germ layers. Radiates are diploblastic and bilateria are triploblastic. Radiates have two main axes: anterior-posterior and the dorsal-ventral axes. But bilateria have only one axis: the oral-aboral axis.
•It is universally believed that the bilateral symmetry developed in the radial ancestor when it started creeping. •Three different theories are in practice explain such evolution of bilateral symmetry from radial symmetry: •1.Ctenophore-polyclad theory •2.Ctenophore-trochophore theory •3.Planuloid-acoeloid theory
Ctenophore-polyclad theory • This theory was proposed by Kowalevsky (1880) and Arnold Lang (1881- 84). • According to this theory, ctenophores like Ctenoplana and Coeloplana acted as intermediate forms between radiate coelenterate ancestors and the bilateral polyclad flatworms. The theory is supported by following facts about Ctenoplana and Coeloplana: • Oval flattened shape with dorso-ventral differentiation • Creeping upon the entire ventral surface • Presence of two dorsal tentacles • Centrally located ventral mount • Radially arranged ciliary bands
Cydippid Ctenophore
Ctenophore- trochophore theory • The theory is based on the resemblances between ctenophores and trochophore larvae. Evolutionary relationship between larval stages of coelenterates, ctenophores, helminthes and annelids is taken as basis of this theory. • Planula larva of coelenterates has ciliated, elongated and cylindrical body. • Ctenophores larvae, Cydippid is ovoid in shape and has radially arranged ciliary bands. • Muller’s Larva has ciliary body and apical tuft of cilia on the anterior side. • Trochophore larva of Polychaeta resembles Muller’s larva in having ciliary bands, apical tuft and ventral mouth. • Based on resemblances like presence of ciliary bands and other structures, the theory considers larvae of acoelomate bilateria as early stages of trochophore larvae (Annelid larvae having bilateria body plan).
Planuloid-acoeloid theory • The theory was proposed by Ludwig von Graff (1882) and later reviewed by Hyman (1951). • According to this theory, primitive acoelomate bilateria (Turbellarians) evolved from some planuloid ancestor similar to planula larvae of coelenterates. • It is assumed that the planuloid ancestor planuloid ancestor must be free- living, radially symmetrical and ciliated. Later on, mouth and archenteron developed on the larvae and adopted to bottom creeping from free swimming habit. • Creeping habit produced cephalization of nervous system towards the anterior side. Availability of food at the bottom made the anterior mouth moving to the ventral side and body became dorso-ventrally flattened, same as in the Turbellarians helminths body.
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