Larval forms of Echinoderms……..

Larval forms of Echinoderms……..

A. Dipleurula Larva:

1. The dipleurula form is reached during development and is characterised by its bilaterally symmetrical, egg-shaped body.

2. The ventral concave side bears the mouth and is encircled by a circumoral ciliated band.

3. The digestive canal is differentiated into oesophagus, stomach and intestine while anus is disposed ventrally.

4. The pre-oral lobe which is situated at the anterior to mouth, bears an apical sensory plate and a tuft of cilia.

5. 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.

                                        

B. Pentactula Larva:

1. The Pentactula larva has five tentacles around the mouth.

2. The hydrocoel is separated from the rest of the coelom to form the future warer vascular system.

>>> 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.

C. Bipinnaria Larva:

1. It is a bilaterally symmetrical free swimming pelagic larva ; the anterior end of the archenteron develops as mouth and blastopore becomes the anus.

2. It possesses two ciliated bands—the pre-oral and the post-oral.

3. The pre-oral ciliated band surrounds the pre-oral lobe of the larva.

4. The pre-oral lobe is highly developed.

5. The post-oral ciliated band appears to be longitudinally placed and forms a complete ring between the mouth and anus.

               

6. The pre-oral and post-oral ciliated bands are continued over a series of prolongations of the body, called arms.

7. The name and number of the arms developing from pre-oral and post- oral ciliated bands are as follows:

 

8. The pre-oral and ventro-median arms develop from the pre-oral ciliated band and the rest of the arms develop from the post-oral ciliated band.

>>> The bipinnaria is a feeding larva and leads a free-swimming life. After a short period of free-swimming existence it trans­forms into a lecithotrophic brachiolaria larva.

D. Brachiolaria Larva:

1. This type of larva is present in Asteroidea and is regarded as a modified form of bipinnaria larva. It possesses the following special features.

2. There are three additional arms which are not ciliated in their courses except in Bipinnaria papillata. These arms are called the brachiolar arms and are beset with warts to help in temporary adhesion.

3. These arms are devoid of calcareous rods and have prolongations from the coelomic cavity.

>>> The bipinnaria stage is followed by the brachiolaria stage in all Asteroids but direct evidence is only furnished in two cases, e.g., Asterias glacialis and A. vulgaris. In Astropecten the brachiolaria stage is absent and the bipinnaria larva metamorphoses directly into adults.

E. Auricularia Larva:

1. A free-swimming form.

2. Body barrel-shaped and bilaterally symmetrical.

3. The preoral lobe is well formed.

4. A single winding ciliated band, which may be produced into lobes.

5. Gut with mouth, sacciform stomach, hydrocoel and right and left stomocoels and anus.

6. The hydrocoel becomes lobulated forming primary tentacles and communicates with the hydropore by a canal.

7. Calcareous rods replaced by spheroid or star-shaped or wheel-like bodies.

>>> The auricularia larva is transformed into a Doliolaria larva similar to that of Crinoidea.

F. Doliolaria Larva:

1. A free-swimming form.

2. Body barrel-shaped and bilaterally sym­metrical.

3. Preoral lobe well-developed.

4. Wavy, continuous band break into 3-5 flagellated, transverse rings.

5. The auricularia larva transforms into a barrel-like body with five ciliated bands which subsequently break into pieces. This particular stage is also designated as pupa stage.

6. During metamorphosis into an adult form, the ciliated bands disappear and further changes occur.

G. Pluteus Larva:

1. This larval form can be regarded as a modification of the auricularia larva.

2. Like the auricularia larva it has a single ciliated band, but it possesses long arms with ciliated bands at the margin.

3. It has comparatively smaller pre-oral lobe.

4. The post-anal part of the body is quite well-developed.

5. 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.

(1) Ophiopluteus

1.      Free-swimming.

2.    Arms are four pairs, slender and sup­ported by calcareous skeleton.

3.    Posterolateral arms are longest and directed forward, giving the larva V-shaped appearance.

4.    Ciliated bands are present on the edges of arms.

5.     The alimentary canal is divisible into mouth, oesophagus, stomach and intestine opening through the anus.

(2) Echinopluteus—in Echinoidea.

1. Free-swimming.

2. Arms five or six pairs, pigmented and supported by calcareous skeleton.

3. The posterolateral arms are very short and directed outward or backward.

4. The skeletal rods simple or thorny or fenestrated or branched.

5. The zones of the alimentary canal are mouth, oesophagus, stomach and intestine opening through the anus.

H. Antedon or Yolk Larva:

1. This particular larva is also called doliolaria larva or Vitellaria larva.

2. This larval stage is present in Antedon and it has many structural pecularities.

3. It has a barrel-shaped body with slightly flattened ventral side.

4. It is free-swimming and exhibits bilat­eral symmetry.

5. The ciliated bands are in the form of four or five separate transversely placed bands encircling the body. In Antedon bifida, there are four bands. In Antedon adriatica and A. mediterranea there are five bands.

6. 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.

7. The anterior ciliated ring is ventrally incomplete.

8. There is a ciliated depression or lar­val mouth which is ventrally placed between the second and third ciliated rings.

9. A small adhesive pit develops be­tween the first and second ciliated rings by which the larva adheres to the substratum.

10. The internal structures become rotated at an angle of 90° from the ventral to the posterior side.

I. Cystidean or Pentacrinoid Larva:

1. The anterior end of the antedon larva, after attachment, is prolonged into an elongated narrow stalk and the free end becomes broader.

2. The ciliated depression becomes a closed ectodermal vesicle which is gradually shifted to the free end.

3. The floor of the depression is perfo­rated by mouth and with the disappearance of the roof; the mouth and the tentacles become exposed.

4. This particular phase is called Cystidean or Pentacrinoid stage. This stage resembles closely the adult Pentacrinus. The stalk in this form develops from the pre-oral lobe.