Larval form in Echinodermata.

Larval Forms in Echinodermata:

Echinoderms are unisexual animals. Sexual dimorphism is absent. Fertilisation takes place in water. The development may be direct or indirect. If the development is indirect it includes larval stages.No other group of animals has such complicated metamorphosis in the course of development. Development may be direct or indirect. In direct one, the larval stages are missing while in indirect one, various types of free swimming larvae are formed. In each class, a few members, are viviparous, that is, they brood their young in a sort of brood pouch on the surface of their body. The development of larva takes place in a typical deuterostomes fashion. In most cases the characteristic free swimming larvae developed externally which are of phylogenetic significance.

Echinodermata lava is strikingly bilaterally symmetrical in marked contrast to radially symmetrical adult. Based upon the nature and position of the arms or their absence, larvae of different classes of Echinodermata may be distinguished. After a free-swimming planktonic existence, the bilateral larva undergoes a metamorphosis, in which the radial symmetry of the adult is developed. The larvae of echinoderms pass through a number of stages and these have specific names derived from the taxonomic names of the adult or from their appearance. For example. a sea urchin has an 'echinopluteus' larva while a brittle star has a 'bipinnaria' larva.They are classified into three classes.

[I] Class 1.Asteroidea.

Bipinnaria larva

Two types of development occur in asteroids. The direct type has large, yolky eggs and a free-swimming larval stage. The indirect stage has homolecithal eggs with little yolk and a free-swimming larval stage. After hatching the larva develops cilia and begins a free swimming life. The larva feeds on diatoms as an alimentary canal is formed. The presence of the powerful ciliary band on the stomodaeal walls help in feeding. Two lateral longitudinal locomotory ciliated bands develop which connect in front of the mouth, forming a preoral loop and in front of the anus, to form a preoral loop. Preoral loop later separates or in some cases develops independently into an anterior ciliated ring around the body. Three lobe or projections are also developed on each side of the body bordered by ciliary bands. This larva is known as bipinnaria and develops in 2 to 7 days.

Brachiolaria larva.

Brachiolaria larva transforms into brachiolaria larva which develops three short arms at the preoral lobe, known as bronchiolar arms. They contain coelomic extensions and adhesive cells at their tips. An adhesive glandular area at their base acts as a sucker. The appearance of the sucker marks the beginning of metamorphosis.

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II] Class II. Ophiuroidea

Ophiopluters larva.

Pluteus is the free-swimming larva in brittle stars which is known as ophiopluteus. It is similar to echinopluteus of echinoids with the only difference that the former has fewer arms than the later. The posterolateral arms are the longest and directed forward. After gastrulation, the arms develop gradually. Posterolateral arms are formed first. After 4, 10 and 18 days, anterolateral, postoral and posterodorsal arms develop, respectively. Ciliated bands accompany the arms edges. Internally the larva contains coelomic chambers and archenteron. Internal development proceeds in the same way as in other classes. While free swimming metamorphosis of the larva starts, there being no attachment stage. Tiny serpent star sinks to the bottom to begin its adult existence.

[III] Class 3. Echinoidea

Echinopluteus larva.

Larva is formed after gastrulation. Gastrula becomes conical, one side of which flatters to form the oral surface. Stomodaeal invagination communicates with archenteron and the gut is differentiated into mouth, oesophagus, stomach and intestine. Blastopore remains as a larval anus. Larva begins to form projections which develop into arms. There are six arms namely, preoral, anterolateral, anterodorsal, postoral, posterodorsal and posterolateral.

[IV]Class IV.Holothuroidea

Auricularialarva.

After gastrulation and formation of coelomic sacs and gut, the embryo becomes a free-swimming larva called auricularia larva, within three days. It is transparent, pelagic about 0.5 to 1 mm in length. It swims about by a ciliated band which forms a preoral loop and an anal loop. Internally, the larva has a curved gut with sacciform stomach, hydrocoele and right and left stomatocoels. Some giant auricularians of unknown adults reported from Bermuda, Japan and Canary Islands measure about 15mm in length and possess a frilly flagellated band.

Doliolaria larva.

It is a transitional stage from auricularia and appears barrel-shaped and alike radiolaria of crinoids. The continuous ciliated band breaks in 3 to 5 flagellated rings. Mouth is shifted to anterior and anus to the posterior pole. Metamorphosis is gradual during which it acquires 5 tentacles and 1 to 2functional podia. As such it is sometimes known as tentacula. After the appearance of more podia and tentacles, sea cucumber settles to the sea bottom and leads an adult mode of life.

[V] Class V. Crinoidea

Doliolaria larva.

It hatches into a free-swimming larva. The body has 4 to 5 ciliated bands with an apical sensory plate at the anterior end provided with a bunch of cilia. There is an adhesive pit over the first ciliated band, near the apical plate in the mid-ventral line. Between second and third ciliated band lies the stomodaeum or vestibule. The skeleton also develops at this larval stage. After the differentiation into prospective organs, larva attaches itself and internal organs rotate at an angle of 90 degrees from ventral to the posterior position. Larval forms a stalk and is now referred as cystidean or pentacrinoid larva which, after sometimes metamorphoses into an adult.

Significances of an Echinodern larva.

The larval forms of all classes in Echinodermata will show the general resemblance. The crinoidea larva differs from this pattern. In general, all the larvae show that they might have come from the same ancestor. Hence, the common ancestor is coelomate, bilaterally symmetrical and free swimming.

These larvae also show resemblance with Toronaria of Balanoglossus. Thus, the study of Echinoderm larva has a phylogenetic significance

Reference:

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

http://www.slideshare.net/himu0007/phylum-echinodermata-clear-concept

http://www.biozoomer.com/2016/01/echinodermata-larvae.html

https://en.wikipedia.org/wiki/Echinoderm