Importance & Significance of Echinoderm Larva

Importance :-

(i) Echinoderm Larva v/s Origin of Chordate :-

It is an establishment fact that Echinoderm is very near to Chordate. Many Scientists are attempting to prove that the Chordates have been originated from echinoderms. So, the echinoderms Hemichordates, Tumicules and the higher chordates are together placed under a group Deutero stomial, due to secondary development of oral aperture. In this view of similarities some important views are described here-

(a) Similarity about Morphological & Anatomical :-

(i) The bilateral symmetry of Aurecularia of Echinoderm is similar to the Tornaria of Balanoglossus of Hemichordate in respect to their cleavage pattern, looping bonds enterocoelic origin of coelom and body cavity.

(ii) According to Willy, Echinoderms are descended from bilateral symmetrical, pelagic. The general likeness between Auricularia and Tornaria is so great that it can only be accounted for on the ground of genetic affinity.

Remark :-

H.B. Fell (1948) showed that parallalism in development has often take place among the different sub classes of Echinoderms and concluded that the similarity of the free swimming larva of certain Echinoderm and Balanoglossus supplies trust worthy evidence of common ancestry.

(b) Similarity between Bipinaria & Tornaria :-( according to Metschnikoff-1869)

(i) Free swimming and bilateral symmetrical larval form in both.

(ii) Transparent with similar ciliated bands.

(iii) Similar location of mouth and anus.

(iv) The madriporic vesicle in Bipinnaria are thought. its homologous with heart vesicle of Balanoglossus.

Bather's view(1900) :-

Bather said that common ancestry of Hemichordates and Echinoderm from Dipturula.

Taxonomic Affinities :-

Closely looking as the it is seen that the larval similarities do not indicate the taxonomic affinities. Among Eleutherozoa, two well developed larval forms occurs-

(a) Pluteus (Ophiroidea & Echinoidea ) with two arms & bilateral symmetry.

(b) Auricularia group (Asteroidea & Holothuroidea) with ciliated bands.

On the basis of larval taxonomy Ophiroidea should be placed near Echinoidea & Asteroidea near to Holothuroidea. But this is not in common arrangement from the Plaentological & morphological studies.

 Significance :-

All the larval of echinoderms have a bilateral symmetry.  Hence it is believed that the ancestor of echinoderms was a bilaterally symmetrical animal. According to Bather(1900), this ancestor was called dipleurula. But according to Semon(1988) this ancestor was called Pentaetulla. The pentaetulla ancestor was universally accepted.

The radial symmetry of exhibited by Coelenterate and Porifera is primary. The radial symmetry in Echinoderm is superficial, concealing the true bilateral symmetry.

The adult Echinoderms are more primitive than larvae because they possess the features of lower animals like Porifera and Coelenterala. The primitive characters are radial symmetry, absence of head, lack of anterior and posterior ends. Hence during Metamorphosis the advanced larva becomes a primitive adult. Hence the Metamorphosis is Retrogressive.