Dipleurula Concept and the Echinoderms Theory of Origin of Chordates

DIPLEURULA CONCEPT AND THE ECHINODERMS THEORY OF ORIGIN OF CHORDATES Chordates evolved sometime during the Cambrian period, 500 million years ago during the Cambrian explosion, almost at the same time when invertebrates were beginning to evolve. They may have evolved from some freshwater forms as Chamberlain (1900) pointed out that all modern chordates possess glomerular kidneys that are designed to remove excess water from the body. However, early fossils of chordates have all been recovered from marine sediments and even modern protochordates are all marine forms. Also glomerular kidneys are also found in some marine forms such as myxinoids and sharks. This hints to the Marine origin of chordates. Chordates evolved from some deuterostome ancestor (echinoderms, hemichordates, pogonophorans etc.) as they have similarities in embryonic development, type of coelom and larval stages. Fossils of the earliest vertebrates are known from the Silurian-Devonian period, about 400 million years ago. The following theories have been given to explain the origin of chordates: Theories Dipleurula concept The term dipleurula was coined by Semon (1888) but the proper illustration of this hypothetical form was given by Bather in 1900, which was accepted by most of the zoologists. Majority of echinoderms have indirect development with free swimming and bilaterally symmetrical larval stages. These echinoderms have small eggs and the fertilized eggs develop in seawater. The cleavage is holoblastic, nearly equal, radial and intermediate to form a hollow one layered ciliated blastula. The blastula transforms into a gastrula by invagination. The cilia of the gastrula are restricted to (i) a large pre oral band present around the mouth on the ventral side and (ii)a small adoral band lining the mouth or stomodeum. This larval stage is called Dipleurula larva. This dipleurula larval form is regarded as the hypothetical ancestral form of echinoderms as this larva is universally present in all echinoderms and from it all the larvae of echinoderms have been derived. Dipleurula represents an ancestral form for the primitive deuterostomes. We can see that all well known forms of larvae of echinoderms are derived from the hypothetical dipleurula. Among them fall the Bipinnaria and the Brachiolaria of the sea stars, the Auricularia of the sea rollers and the plutei of the sea hedgehog etc. The Dipleurula concept was propounded by Bather in 1900. The common ancestors didn’t possess all the common characters of the free swimming bilateral larvae of different groups of echinoderms and it might add one or two characters which none of them possess. Hence it appears that the dipleurula concept cannot illustrate the common ancestor of the echinoderms or can explain the characteristic features of the ancestor of echinoderms. Dipleurula is merely a name for features common to present echinoderm larvae. Echinoderm Theory of Origin of Chordates: It is believed that chordates have originated from invertebrates. It is difficult to determine from which invertebrates group the Chordates developed. Chordate ancestors were soft bodied animals. Hence they were not preserved as fossils. There are several theories that have been put forward to explain the origin of chordates. These are directly from some invertebrate group or through the intervention of some Protochordates. Almost every invertebrate phylumCoelenterates, Nemertean, Phoronida, Annelida, Arthropods and Echinoderms has been suggested. But these theories are far from being satisfactory and convincing and have only been of historical value. Only Echinoderm Theory has received some acceptance. Echinoderm theory was given by Johannes Muller(1860) and is based on the comparative studies of larval stages of echinoderms and hemichordates. Garstang and DeBeers proposed the echinoderm larvae gave rise to chordates by neoteny. This theory inferred the origin of chordates, hemichordates and echinoderms from a common ancestor. This theory is based on the following evidence: A) Embryological Evidence: Both echinoderms and Chordates have enterocoelic coelome, mesoderm and deuterostomous mouth. There are resemblances between the bipinnaria larva of echinoderms and the tornaria larva of hemichordates. B) Serological Evidence: A close similarity between the proteins of the body fluid of chordata and echinoderms. Hence the chordates are more related to echinoderms. The radial symmetry of adult echinoderms will disapprove the relationship with the bilaterally symmetrical chordates. The bilateria is divided into two major divisions- Protostomia and Deuterostomia. The division is based on the differences in embryonic and larval development. Prostomia includes from Annelida to Arthropoda while Deuterostomia includes Echinodermata,Pogonophora and Chordates. Deuterostome line of Chordate Evolution: Following common features of Deuterostome suggests strong evidence of a closer evolutionary relationship between the three principal Deuterostome phyla- Echinodermata, Hemichordata and Chordata. (i) Early cleavage of zygote is indeterminate. (ii) Blastopore of gastrula develops into anus. (iii) Coelom ( enterocoelous except vertebrates) is formed by the fusion of pockets developed from the endoderm of developing archenterons of the embryo. (iv) Pelagic larva of Echinoderms and Hemichordates have a close resemblance to vertebrates that do not have a floating larva. (v) Deuterostomes use creatinine as phosphogen whereas invertebrates use arginine. Some Hemichordates as well as echinoids use both.