A bipinnaria is the first stage in the larval development of most starfish , and is usually followed by a brachiolaria stage. Movement and feeding is accomplished by the bands of cilia. Starfish that brood their young generally lack a bipinnaria stage, with the eggs developing directly into miniature adults. The bipinnaria is free-living, swimming as part of the zooplankton. When it initially forms, the entire body is covered by cilia, but as it grows, these become confined to a narrow band forming a number of loops over the body surface.
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The dipleurula form Fig. The pre-oral lobe which is situated at the anterior to mouth, bears an apical sensory plate and a tuft of cilia. 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 This larval stage is regarded as the next evolutionary step of the Dipleurula larva. The hydrocoel is separated from the rest of the coelom to form the future warer vascular system.
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. 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. The bipinnaria is a feeding larva and leads a free-swimming life.
This type of larva Fig. 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.
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.
There are no calcareous rods, being replaced by spheroids or star-shaped or wheel-like bodies. In certain forms, e. 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. 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. 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 skeletal rods are usually absent; if present, only one in number.
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 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. Top Menu BiologyDiscussion. Characteristics of Protostomes and Deuterostomes. Asterias: An Example of Phylum Echinodermata. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes.
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The dipleurula form Fig. The pre-oral lobe which is situated at the anterior to mouth, bears an apical sensory plate and a tuft of cilia. 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
Medically reviewed and approved by a board-certified member. By BS Media. Be the first to comment! Bipinnaria Larva. Echinoderms are unisexual animals. Sexual dimorphism is absent. Fertilisation takes place in water.