Larval forms of echinoderms
- 1. THE LARVAL FORMS OF ECHINODERM by- Biswadeep dey Year -2020
- 2. Contents: Page no 1. Introduction 1 2. Larva & its forms 2 3. Bipinnaria Larva 3 4. Brancholaria larva 4 5. Ophioplueteus larva 5 6. Ecvhinopleuteus larva 6 7. Auricularia larva 7 8. Doliolaria larva(Holothuroidea) 8 9. Doliolaria larva(crinoidea) 9 10. Homology and phylogeny 10 11. Conclusion 11 12. Significance 12 13. Referance 13
- 3. INTRODUCTION 1 Echinoderms are unisexual animal with no sexual dimorphism. Fertilization external. Echinoderms are deuterosomes and hence cleavage is radial, holoblastic and indeterminate. Development is mostly indirect having larval stage in between.
- 4. LARVA 2 The larvae hatch in water , feed and grow through successive larval stages to become adults. Larvae of echinoderms are bilaterally symmetrical but lose symmetry during metamorphosis. Different classes of Echinoderms show structurally different larval stages. Comparision of the larval stages of different classes can reveal their evolutionary ancestry. LARVAL FORMS OF DIFFERENT CLASSES CLASS LARVAL FORMS Asteroidia - Bipinnaria and Branchiolaria. Ophiuroidea - Ophiopluteus Echinoidea -Echinopluteus Holothuroidea -Auricularia and Doliolaria Crinoidea -Doliolaria
- 5. BIPINNARIA LARVA 3 It is the first larval form of Asteroidea. It is bilaterally symmetrical , free swimming pelagic larva. The pre oral region is elongated , postorial region is broad. It possesses two ciliated bands, the pre oral and post oral bands. The anterior end of the archenteron develop as mouth whereas the blastopore becomes the anus. The pre oral and post oral ciliated bands are continued over a series of the prolongation called arms. The following are the names and the number of arms developing from pre oral and post oral ciliated bands: Postero lateral arm - two Post oral arm - two Postero dorsal arm - two Antero dorsal arm -two Pre oral arm -two Ventero median arm -one Dorso median arm -one The bipinnaria larva is free swimming and free feeding form. After a short period of time, it transforms into branchiolaria larva.
- 6. BRANCHOLARIA LARVA 4 Three additional arms are present on this larval form known as branchiolarian arms. These help the larva to adhere with the substratum. These arms are neither ciliated nor have calcareous rods and the coelomic cavity extends into these arms. The three short arms are at pre oral lobe, one median and two lateral arms. They contain adhesive cells at their tips which act as a sucker. The rest arms degenerate and become long, narrow and slender. METAMORPHOSIS OF BRANCHIOLARIA With the help of adhesive structures, it attaches to some object. Anterior portion acts as stalk for some time while posterior part having gut and coelomic chambers convert into a young starfish. This detaches itself and starts leading a free life.
- 7. OPHIOPLUTEUS LARVA 5 This is the larval form of class Ophuroidea. This is free swimming, bilateral symmetrical form having a single ciliated band. It possesses long arms with ciliated bands at the margin. It has two anterio lateral two post oral, two posterior dorsal and two posterio lateral arms. Out of these posterior lateral arms are the longest and directed forward. It has comparatively smaller, pre oral lobe. The post anal part of the body is quite well developed. Larva consists of coelomic chambers and archenteron. There being no attachment stage. Free swimming larva, metamorphose into tiny serpent star, which sinks to the bottom to begin its adult existence.
- 8. ECHINOPULTEUS LARVA 6 It is a microscopic, free swimming larva of Echinoidea. It resembles the Ophiopluteus larva where the only diffence is that it has more arms. This larva shows ciliated bands which are developed into arms. Fully developed larva consists of six arms supported by calcareous rods and its tips are pigmented. Postero lateral arms are very short and directed outwards or backwards. Locomotion is by ciliated bands, which in some cases become thickened and called Epaulettes. There is no attachment stage. Metamorphosis is extremely rapid taking place in about an hour.
- 9. AURICULARIA LARVA 7 It is the first larval form of Holothuroidea. It is transparent, free swimming , pelagic larva of about 0.5-1 mm in length. Arms are absent. Ciliated bands are well developed. It swims about by a ciliated band which forms pre oral loop and an anal loop. Alimentary canal is developed which opens with mouth and ends with anus. Internally the larva has a curved intestine with sacciform stomach.
- 10. DOLIOLARIA LARVA 8 It is the second larval form of Holothuroidea. It is a transitional stage from Auricularia larva. It is barrel shaped with continuous ciliated band which breaks into three to five flagellated rings. Mouth is shifted to anterior and anus to posterior pole. Metamorphosis is gradual, during which it acquires five tentacles and one to two functional podia. As such it is sometimes called Pentacula. After appearance of more tentacles and podia sea cucumber settles to the sea bottom and leads an adult mode of life. In some cases, there is no Auricularia stage, the embriyo directly develops into doliolaria larva
- 11. DOLIOLARIA LARVA 9 It is t5he larval form of Crinoidea. It is free swimming larva having four to five ciliated bands. It contains an apical tuft of cilia which will be sensory. On the mid ventral line, near apical plate , adhesive pit will be present over the first ciliated band. Between second and third ciliated band lies stomodeum or vestibule. Skeleton also develops at this larval stage. After swimming for some time it will develop a stalk. It is called pentacrinoid larva. Larva now attaches itself and internal organs rotate to 90 degree from the ventral to posterior position. Larva forms a stalk and is now called as Cystidean or Pentacrinoid larva. This after sometime metamorphoses into an adult.
- 12. HOMOLOGY AND PHYLOGENY OF ECHINODERM LARVAE 10 Except for the crinoids, a sedentary group, the larvae of Asteroidea, Holothuroidea, Echinoidea and Ophiroidea exhibit some fundamental resemblances: Having pre oral and post oral loops. Having V-shaped ciliated bands. Presence of gut with its divisions and openings. Coelom enterocoelic. These are some common features indicating that they had a common ancestor.
- 13. CONCLUSION 11 In echinoderms eggs and sperms are released in water and fertilization takes place in water forming zygote. Echinoderms are deuterosomes and hence cleavage is radial, holoblastic and indeterminate. The larvaes are bilaterally symmetrical but lose symmetry during metamorphosis. Different classes of echinoderms shoe structurally different larval stages and their comparision can reveal their evolutionary ancestory.
- 14. SIGNIFICANCE 12 They help in the dispersal of species. They help to study the different group of echinodermata. The larval stages are useful in finding the homologies and affinity of various group.
- 15. REFERENCE: 13 1. Invertebrate zoology by Robert debornis. 2. Zoology for degree students by S.chand. 3. en.m.wikipedia .org 4. https://courses.lumenlearning.com
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