Larval forms in echinodermata

larval forms in different classes of phylum Echinodermata

1. Larval forms in E chinodermata Presented by- Aanchal Manchanda M Sc. Zoology

2. 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 larva stages. In different classes of echinoderms, different types of larvae complete the development . The larval form is bilaterally symmetrical. It undergoes metamorphosis and radial symmetri­cal adult is developed.

3. Class of Phylum Echinodermata Larval form 1. Asteriodea 2. Ophiuroidea 3. Echinoidea 4. Holothuroidea 5. Crinoidea Bipinnaria & Brachiolaria Ophiopluteus Echinopluteus Auricularia Dololaria & Pentacrinoid  

4. Dipleurula larva The dipleurula form is reached during development and is characterized by its bilaterally symmetrical, egg-shaped body. The ventral concave side bears the mouth and is encircled by a circumoral ciliated band. The anus is disposed ventrally.   The digestive canal is differentiated into oesophagus, stomach and intestine. The pre-oral lobe which is situated at the anterior to mouth, bears an apical sensory plate and a tuft of cilia. The ciliation on the surface of the body becomes reduced to a ciliard band. This Dipleurula form is regarded by many as the hypothetical ancestral form of Echinoderm, as this form is universally present. The Dipleurula concept was first propounded by Bather (1900). The major changes involved in other Echinoderm larvae are due to differential disposition of the ciliated bands.

5. Pentacula larva   This larval stage is regarded as the next evolutionary step of the Dipleurula larva. This concept has been supported by Semon (1888), Burry (1895), Hyman (1955) and many others. The Pentactula larva has five tentacles around the mouth. The hydrocoel is separated from the rest of the coelom to form the future water vascular system.

6. Bipinnaria larva: It is the larva form seen in the life history of Star fish . The fertilised egg is homolecithal. ft undergoes holoblastic cleavage and develops into blastula and gastrula stages. The gastrula elongates in length and it gives rise to Bipinnaria larva .   It is a bilaterally symmetrical free swimming pelagic larva. The pre- oral region is elongated. Post-oral region is broad. The anterior end forms pre-oral lobe .

7. The ciliated band at the pre-oral lobe forms into 2 separate bands, Pre-oral band of cilia, and post oral band of cilia. These 2 bands of cilia are drawn into many arms. They are nothing to do with the arms of the star fish. They are,  Ventro-median arm. A pair of pre-oral arm. Median dorsal arm. A pair of antero-dorsai arm. A pair of posterio-dorsal arm. A pair of posterio-lateral arm. A pair of post oral arm. The digestive system is developed with mouth and anus. This larva resembles Tomaria larva of Balanoglossus.  This larva slowly grow s into the next larval form called Brachiolaria larva.  

8. Brachiolaria larva Bipinnaria larva swims for few weeks in the sea water.lt finally transforms into next larval stage called Brachiolaria larva .   It is bilaterally symmetrical larva. It is pelagic larval form, it shows 3 brachiolar arms with suckers. They are one median and two lateral in position. At the tip of brachiolar arms adhesive structures will make their appearance and they are for attachment.

9. The larva shows all the arms that are seen in the Bipinnaria, but these arms are very long and hanging. These ciliated arms will be helpful for swimming in the water. The digestive system is completely developed with definite stomach and intestine. This larva after swimming for few days settles on a solid object and gets attached to it by its adhesive arms. Posterior end of the larva enlarges and lifts to the right-side. From this rudiments of 5 arms will arise. Thus slowly the larva metamorphosis into an adult.

10. Auricularia larva In Holothuroidea this larval form is seen .   It is a free swimming pelagic larva. Arms are absent. Alimentary canal is developed. It opens with mouth and ends with anus. Intestine is curved. In Japan and Bermuda very big auricularia larval forms are developed. They are 15 mm in length. Usually this larva is 1 mm in length. Ciliated bands are well-developed. Ciliated band continues through oral loop and anal loop.

11. Pluteus larva This larval form (Fig. 21.39H, I) can be regarded as a modification of the auricularia larva of Holothuroid. Like the auricularia larva it has a single ciliated band, but it possesses long arms with ciliated bands at the margin. It has comparatively smaller pre-oral lobe. The post-anal part of the body is quite well-developed. The arms are also supported by calcar­eous rods. The pluteus larvae are of two kinds: (1) Ophiopluteus—in Ophiuroidea. (2) Echinopluteus—in Echinoidea. Both the larval forms possess the post-oral arms, antero-lateral arms, postero-lateral arms and postero-dorsal arms. The typical ophiopluteus may be ab­sent in certain forms. The larva may be elongated and with­out ciliated bands. The skeletal rods are usually absent; if present, only one in number. The presence of pluteus larva in both classes is an example of parallel evolu­tion and the similarity between the two larval forms e.g. ophiopluteus and echinopluteus, was once considered a closely related group but phylogenetic analyses do not indicate this (Anderson, 1998).

12. Ophiopluteus larva This larva is seen in the life history of ophiuroidea(Brittle star ). It shows many long arms . It is bilaterally symmetrical . It is transparent . It is Pelagic . The arms are supported by calcareous rods. The arms are directed upwards . Pre-oral loop is reduced . Ciliated band is undivided . The postero-lateral arms are very long and they are directed forwards . The digestive system is developed. It opens with mouth and ends with anus. This larva swims for some time before undergoing metamorphosis.

13. Echinopluteus larva It is seen in the life history of Echinoidea .   It is a microscopic larva. It swims in water. This larva shows ciliated bands which are developed into arms. Fully developed echinopluteus larva 4 or 5 pairs of arms are present. Usually 6 pairs of arms should be resulted. The arms are supported by (CaCO 3 ) Calcareous rods. The digestive system is developed which shows mouth and anus. It develops hydrocoel and vestibule. These parts grow on the oral side of the animal. From the hydrocoel five radial canals will develop . This larva undergoes rapid metamorphosis and develops into an adult .

14. Doliolaria larva In crinoidea group of animals the larval form is Doliolaria larva .   It is a free swimming larval form. It contains an apical tuft of cilia which will be sensory. On the mid ventral line near apical plate adhesive pit will be present. The body shows 4 or 5 ciliated bands In between 3rd and 2nd ciliated bands vestibule is present . After swimming for some time it will develop a stalk. It is called Pantacrinoid larva. It will attach to the substartum. The internal organs will rotate at 90°. It develops into an adult .

15. Antedon or yolk larva This particular larva is also called doliolaria larva or Vitellaria larva. This larval stage is present in Antedon and it has many structural pecularities. It has a barrel-shaped body with slightly flattened ventral side. It is free-swimming and exhibits bilat­eral symmetry. The ciliated bands are in the form of four or five separate transversely placed bands encircling the body. A tuft of cilia with stiff sensory hair springs from a thickened ectodermal patch, called apical neural plate, which is comparable to that of Tonaria larva of Balanoglossus. The anterior ciliated ring is ventrally incomplete. There is a ciliated depression or lar­val mouth which is ventrally placed between the second and third ciliated rings. A small adhesive pit develops be­tween the first and second ciliated rings by which the larva adheres to the substratum. The internal structures become rotated at an angle of 90° from the ventral to the posterior side.

16. Significance of these larval forms The larval forms of all classes in Echinodermata will show general resemblance . The crinoidea larva differs from this pattern. In general all the larvae show that they might have come from 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 .