Dipleurula Concept & Echinoderm Theory of Origin of Chordates

Dr. Tapashi Gupta

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 the primitive deuterostomes. We can see that all well known forms of larvae of echinodermata 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 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 have been put forwarded to explain the origin of chordates. These are directly from some invertebrate group or through the intervention of some Protochordates. Almost every invertebrate phylum- Coelenterates, Nemertean, Phoronida, Annelida, Arthropods and Echinoderms has been suggested. But these theories are far from being satisfactory and convincing and have been only 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 infers origin of chordates, hemichordates and echinoderms from a common ancestors. This theory is based on the following evidence:

A) Embryological Evidence: Both echinoderms and Chordates have enterocoelic coelome, mesoderm and deuterostomous mouth. There is 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 disapproved the relationship with the bilaterally symmetrical chordates. The bilateria is divided into two major divisions- Prostomia 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 archenteron of the embryo.

(iv) Pelagic larva of Echinoderms and Hemichordates have a close resemblance vertebrate does not have a floating larva.

(v) Deuterostomes use creatinine as phosphogen whereas invertebrates use arginine. Some Hemichordates as well as echinoids use both.

1. Echinoderm Ancestry:- The hemichordate larva (tornaria) is strictly similar to the larva (bipinnaria or dipleurula) of echinoderms. Both are small, transparent, free swimming, bilaterally symmetrical. Both have similar ciliated band in loops, a dorsal pore, sensory cilia at the anterior end and a complete digestive system of ventral mouth and posterior anus. This striking larval resemblance Johannes Muller and Bateson to suggest a common ancestry for the echinoderms and the hemichordates.

Dissimilarity and Doubts:

Presence of apical plate with eyespots in tornaria larva builds doubts the common ancestry of echinoderms and hemichordates. Garstang and DeBeers proposed the Neotenous larva theory suggesting that probably the Auricularia larva of echinoderms became sexually mature and later this neotenic larva gave rise to chordates. Cambrian and Ordovician fossil records of carapoid echinoderm lead Torsten and Gisten to assume that carapoid echinoderms might have evolved from tornaria like creatures which have began to settle down to lead sedentary life. The water vascular system might have developed out of ciliated grooves of these creatures. Besides this, it was also claimed that in the lower Silurian period, one carapoid echinoderm had the calyx perforated by a series of 16 small apertures. These apertures can be compared with the gill slits of Branchiostoma. Some isolated biochemical studies ( Needham,1932 and Whelmi,1942) put some weight on the concept of the diversion of Chordates from echinodermates. Most of the non chordates use arginine phosphate for the transfer of energy but Ophiuronoids, Cephalochordates, ascidian and vertebrates use creatinine phosphate. On the other hand, Hemichordata and Echinoderms use both arginine and creatinine phosphate as phosphate carrier. The descent of chordata from echinodermates by the direct transformation of any echinoderms or its neotenous larva into a chordate is no longer accepted now-a days. Instead they had a common an immediate ancestor.

2. Hemichordate Ancestry:- There is a strong suggestive evidence that the early evolutionary stages of Deuterostomia was sessile or sedentary. The Pharynx perforated by gill slits is likely an adaptation to sedentary habit. No doubt, hemichordates are sedentary and have pharyngeal gill slits and a hollow dorsal nerve cord. Nevertheless, the presence of a true notochord is doubtful and their adult body plan is quite different from vertebrates. Therefore, the prospect of some hemichordates as a likely ancestor of vertebrates seems to be impossible.

3. Urochordate Ancestry:- The Urochordate or Ascidian theory of ' Vertebrate Origin' was advocated W. Garstang in 1928 and later elaborated by N.J. Berill in 1955 in his book “Origin of Vertebrates”. The adult tunicates reflect the primitive sessile, marine and filter feeding condition of the ancestral vertebrates. But their body plan are so divergent that it is impossible to imagine a direct evolutionary transformation of an adult ascidian into a vertebrate. On the other hand, the ascidian larva are tadpole like, elongated, bilaterally symmetrical and free swimming creatures with pharyngeal gill slits, notochord dorsal, hollow nerve cord, and a muscular post anal tail. They represent only slightly modified living creatures of the ancestral vertebrates that gave rise to vertebrate line of evolution. According to this theory, some of these larva failed to metamorphose into adults, but became neotenous and later evolved into the cephalochordates and vertebrates. The sessile nature of the primitive chordate ancestry, hemichordates, primitive pterobranch and echinoderms is considered by the workers resulting from common ancestry. However, the Ascidian theory of Chordate origin does not seem to be perfect. The principal drawback is that the theory considers sessile urochordate to be ancestral to chordates. Whereas, they are highly specialised because sessility is a specialised condition wherever it occurs in the Animal kingdom.