Bipinnaria larva : It is the larva form seen in the life history of Star fish. The fertilised egg is homolecithal. The gastrula elongates in length and it gives rise to Bipinnaria larva. It is a bilaterally symmetrical free swimming pelagic larva.
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Dipleurula Larva: 1. The dipleurula form Fig. 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 The Pentactula larva Fig. The hydrocoel is separated from the rest of the coelom to form the future warer vascular system.
Bipinnaria Larva: 1. This type of larva Fig. It possesses two ciliated bands—the pre-oral and the post-oral. The pre-oral ciliated band surrounds the pre-oral lobe of the larva. The pre-oral lobe is highly developed. The post-oral ciliated band appears to be longitudinally placed and forms a complete ring between the mouth and anus. The pre-oral and post-oral ciliated bands are continued over a series of prolongations of the body, called arms.
The name and number of the arms developing from pre-oral and post- oral ciliated bands are as follows: 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 arms are provided with muscles and are contractile in nature. The antero-lateral arms are absent. These two ciliated bands are regarded to have arisen from a single ciliated band as in auricularia which becomes subsequently divided. In Asterina gibbosa, the typical bipinnaria larva is slightly modified and it moves by the action of the cilia present in the larval organ. In the genus Luidia, the bipinnaria larva is peculiar in having a slender long anterior part which terminates into two wide arms.
This larval form is named by Sars as Bipinnaria asterigera. Brachiolaria Larva: 1. It possesses the following special features. 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. 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. In Astropecten the brachiolaria stage is absent and the bipinnaria larva metamorphoses directly into adults.
Auricularia Larva: 1. The pre-oral lobe is very well-formed. There are no calcareous rods, being replaced by spheroids or star-shaped or wheel-like bodies. In certain forms, e. Doliolaria Larva: 1. The larval form is observed in Holothuroidea.
The auricularia larva transforms into a barrel-like body with five ciliated bands Fig. This particular stage is also designated as pupa stage. During metamorphosis into an adult form, the ciliated bands disappear and further changes occur. This type of larva is also found in crinoids and a few Ophiuroids which possesses ciliated band but no arms. In Cucumaria frondosa, C.
In Holothuria floridana, there is no larval form and the embryo develops directly into a young Holothuroid. Pluteus Larva: 1. This larval form Fig. 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 pluteus larvae are of two kinds: 1 Ophiopluteus—in Ophiuroidea.
Both the larval forms possess the post-oral arms, antero-lateral arms, postero-lateral arms and postero-dorsal arms.
But they differ in detail which are summarised in Table The arms are small in Ophiopluteus metschnikoffi and O. In Ophionotus hexactis the ophiopluteus lacks arms. The skeletal rods are usually absent; if present, only one in number. Antedon or Yolk Larva: 1. This particular larva is also called doliolaria larva or Vitellaria larva Fig. This larval stage is present in Antedon and it has many structural pecularities. It has a barrel-shaped body with slightly flattened ventral side.
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. 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. Cystidean or Pentacrinoid Larva: 1. This larval stage is also present in Crinoids. It is the second larval stage of crinoids. The anterior end of the antedon larva, after attachment, is prolonged into an elongated narrow stalk and the free end becomes broader Fig.
The ciliated depression becomes a closed ectodermal vesicle which is gradually shifted to the free end. 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. This stage is quite similar to that of Asteroidea excepting that it lacks circumoral vessel.
Brachiolaria have bilateral symmetry , unlike the adult starfish, which have a pentaradial symmetry. Starfish of the order Paxillosida Astropecten and Asterina have no brachiolaria stage, with the bipinnaria developing directly into an adult. The brachiolaria develops from the bipinnaria larva when the latter grows three short arms at the underside of its anterior end. These arms each bear sticky cells at the tip, and they surround an adhesive sucker.
Dipleurula Larva: 1. The dipleurula form Fig. 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.
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