Basic Concept of Invertebrate Zoology Knowledge: Detailed study of Echinoderm Larva

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Tuesday, 26 March 2013

Description & Types of Echinoderm Larva

Description :-

In Echinoderms the development may be direct or indirect. In direct development is no larva. Direct development is seem in only a few echinoderm living in Arctic & Antarctic waters. In indirect development the life cycle includes one or more larvae. Many types of larvae occur in echinoderms. They are the following-

(1) Dipleurula Larva :-

This larva develops from gastrula. It has the following salient features-

(a) Dipleurula is the fundamental larva of all echinoderms.

(b) It is microscopic.

(d) It is bilaterally symmetrical and oval in shape.

(e) It has a gut formed of the mouth, the oesophagus, the intestine, the stomach & the anus.

(f) It gas two ciliary bands, viz a pre-oral band around the mouth and an adoral band inside.

(g) The pre-oral band is used for locomotion & adoral band is used for the collection of food.

(h) It feed on diatoms.

(i) The dipleurula develops into Bipinnaria larva.

(2) Bipinnaria Larva :-

It is the second larva of Starfish. It develops from dipleurula larva. It has the following salient features-

(a) It is minute, microscopic and swims freely on the water surface.

(b) It is bilaterally symmetrical and has alimentary canal with a mouth of the anterior end and an anus at the posterior end.

(c) The body has a number of outgrowths called arms. The arms are covered by ciliated bands and are used for locomotion. It has two unpaired arms and five pairs of paired arms. They are-

(i) Median dorsal arms

(ii) Median ventral arms

(iii) Pre-oral arms

(iv) Post-oral arms

(v) Antero dorsal arms

(vi) Postero dorsal arms

(vii) Postero lateral arms

(3) Brachiolaria Larva :-

It is the third larva of after a short free swimming life. The Bipinnaria larva. the Brachiolaria larva. The pre-oral region of this larva has three processes called Brachiolar arms. These three arms are tipped with suckers. It swims and feed like a Bipinnaria.

(4) Ophiopluteus :-

(a) It is the larva of Ophiuroidea.

(b) The pre- oral lobe is small.

(d) The arms are supported by Calcareous rods.

(e) The larva has a pair of pre-oral arms, a pair of post oral arms, a pair of postero dorsal arms and a pair of postero lateral arms.

(f) The postero lateral arms are always longer and directed forward, so that the larva appears like a "V" shaped.

(5) Echinopluteus :-

(a) It is the larva of Echinoidea.

(b) It has small pre-oral lobe & a single ciliary band,

(d) The larva is provided with a pair of pre-oral arms, a pair of post-oral arms, a pair of antero lateral arms, a pair of antero dorsal arms & a median posterior arm.

(e) The postero lateral arms are very short & directed backwards.

(6) Auricularia :-

(a) It is the larval form of Holothuroidea.

(b) There is a well developed pre-oral lobe.

(c) Arms are supported not by calcareous rods but the calcareous structures are in the form of wheels, spheres, star shaped bodies etc.

(7) Doliolaria of holothuroidea (Pupa):-

(a) In holothuroidea (sea-cucumber) the auricularia larva develops into doliolaria larva.

(b) The larva is also called a pupa.

(d) It is barrel shaped.

(e) The calcareous skeleton is in the form a spheres.

(f) The ciliated bands are broken into pieces.

(g) Metamorphosis begins during free swimming life. After metamorphosis, the larva sinks into the bottom to become the adult.

(8) Doliolaria of Crinoidea :-

(a) It is the larva of Antedone.

(b) Is is a free-swimming larva.

(d) It is barrel shaped.

(e) It has four or five ciliary bands.

(f) At the anterior end the ectoderm thickens to form an apical plate.

(g) The apical plate bears a tuft of cilia called apical sensor tuft.

(h) An adhesive pit is present between the first and second ciliary bands. It is used for attachment.

(i) A mouth is located between the second and third ciliary bands.

(9) Pentacrinoid larva :-

(a) It is the second larva of antedone.

(b) It develops from Doliolaria larva.

(d) It has a stalk.

(e) the stalk develops from the pre-oral lobe of Doliolaria.

(f) One end of the stalk is attached to the substratum with the help of a disc and the free ends bears a crown.

(g) The crown consists of a central mouth surrounded by a circle of tentacles.

(h) During metamorphosis the crown develops cirri & breaks off from the stalk as a free living antedone.

Metamorphosis :-

During Metamorphosis the bilaterally symmetrical larvae become transformed into radially symmetrical adults.

In all these larval forms during metamorphosis there is alternation of the position of the mouth.

In the Crinoidea, the mouth is shifted to the posterior end. But in all other echinoderms mouth is shifted to the left side with the corresponding shifting of the coelomic cavity.

In Crinoidea and Asteroidea the larva becomes fixed to some substratum by the pre-oral lobe at the time of metamorphosis & a fixation disc is formed for this purpose. In the Crinoidea the fixation persists until the adult form is completely formes. But in Asteroidea fixation is temporary.

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.

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-