Echinoderm Larva

Introduction :-

Larva is the sexually immature, pre-adult, free living, stage of development, developing indirectly by metamorphosis. There are so many larval forms in each and every phyla. All these larval forms pay great attention to the zoologist, due to various reasons. Echinoderm Larva, among all reasons.

The main important reason is its bilateral symmetry and another reason is precursor of chordate ancestry. Although echinoderm larva shows bilateral symmetry. However larval morphology and adult morphology has no resemblance. It demanded another or phylo genetic importance.

Brief diary on pre Larval events :- 

(i) Echinoderm, the only sole marine invertebrate are uni-sexual. Eggs and sperms fertilized externally and Zygote undergoes holloblastic clevage to produce hollow one larval blastula.

(ii) By invagination, a gastrula is formed with outer ectoderm and inner end mesoderm.

(iii) Mesenchome layer and networks are formed between ectoderm and endoderm.

(iv) By gradual development blastopore develops in the anus and a new opening on ventral side forms larva mouth.

(v) The larval form is the focus of our discussion which gradually modified into adult.

Basic change of Symmetry from Larva to adult :-

The main astonishing change of the development is the bilateral symmetrical larva changes into radial symmetry of adult.

Developmental fate of Larval Mouth & Anas :- 

(i) In Asteroidea and Echinoidea new mouth and anus are formed in adult, because larval mouth and anus are closed.

(ii) In Holothuroidea larval mouth and anus persist in adult.

(iii) In Ophiuroidea, larval mouth persists, but anus is lacking in adult.

(iv) In Crinoidea, larval mouth and anus are absent in adult.

Basic Larval Forms :- 

There are two basic larval forms in the echinoderm larva. There are Dipleurula larva and Pentactula larva. From these two basic forms various diverse larval forms appears in course of evolution.

(A) Dipleurula Larva :- 

This larva concept was first proposed by Bather (1990). It is regarded by many, as the hypothelical present. The main important features are-

(i) Symmetry - Bilateral

(ii) Shape - Egg shaped

(iii) Ciliated band - A circumoral ciliated band encircles the mouth on ventral side.

(iv) Disposition of Anus - Ventral

(v) Note - The major changes involve in other echinoderm larva are due to differential disposition of ciliated bands.

(B) Pentactula Larva :- 

This concept is supported by Semon(1888), Burry(1895), Hyman(1955) & many others. They presumed the pentactula larva is the next larval stage of Dipleurula larva. The main features are-

(i) Symmetry - Bilateral

(ii) Shape - Belt shaped

(iii) Tentacles - Five around Mouth

(iv) Hydrocoel - Separated from Coelom