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Chaetognaths, Echinoderms, and Hemichordates

Chaetognaths, Echinoderms, and Hemichordates. Chapter 22. Deuterostomes. Deuterostome characteristics: Radial, indeterminate cleavage Formation of the mouth from a second opening Enterocoelous coelom development Chaetognaths are placed outside both protostome & deuterostome groups.

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Chaetognaths, Echinoderms, and Hemichordates

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  1. Chaetognaths, Echinoderms, and Hemichordates Chapter 22

  2. Deuterostomes • Deuterostome characteristics: • Radial, indeterminate cleavage • Formation of the mouth from a second opening • Enterocoelous coelom development • Chaetognaths are placed outside both protostome & deuterostome groups.

  3. Phylum Chaetognatha • The arrow worms, phylum Chaetognatha, are all marine, planktonic organisms. • Some deuterostome embryological characters. • Molecular works suggests they are protostomes. • Currently not considered to be part of either group.

  4. Clade Ambulacraria • Superphylum Ambulacraria contains two deuterostome phyla • Echinodermata and Hemichordata • Members share a three-part (tripartite) coelom, similar larval forms, and an axial complex (specialized metaneprhidium). • Xenoturbella is the sister taxon to Ambulacraria.

  5. Phylum Echinodermata • Echinoderms include sea stars, brittle stars, sea urchins, crinoids, sea cucumbers. • Entirely marine • Lack ability to osmoregulate. • Almost entirely benthic. • Nonsegmented.

  6. Phylum Echinodermata • Five extant classes of echinoderms are currently recognized.

  7. Phylum Echinodermata – Characteristics • Spiny endoskeleton of plates • Water vascular system • Pedicellariae • Dermal branchiae (skin gills) • Pentaradial symmetry in adults

  8. Phylum Echinodermata - Symmetry • Echinoderms are bilaterally symmetrical as larvae. • This means their ancestors were bilaterally symmetrical.

  9. Phylum Echinodermata - Symmetry • As adults they show secondary radial symmetry – pentaradial (5 parts). • Perhaps an adaptation for sessile living in early echinoderms. • Crinoids

  10. Phylum Echinodermata - Symmetry • Today’s echinoderms are mostly motile. • Many are still radial. • Some have again become superficially bilateral (skeletal & organ systems still pentaradial). • Sea cucumbers. • A few sea urchins. • No well defined head or brain.

  11. Phylum Echinodermata - Deuterostomes • Echinoderms have a true coelom with deuterostome development. • Radial, indeterminate cleavage. • Enterocoelous – the mesoderm lined coelom develops from outpocketing of the primitive gut. • Formation of the mouth at the end of the embryo opposite the blastopore.

  12. Water Vascular System • Echinoderms have a water vascular system derived from part of the coelom. • A system of canals and specialized tube feet that functions in: • Locomotion • Food gathering • Respiration • Excretion

  13. Water Vascular System • The water vascular system opens to the outside through small pores in the madreporite.

  14. Water Vascular System • Canals of the water vascular system lead to the tube feet. • Tube feet may have suckers, allowing the echinoderm to move while remaining firmly attached to the substrate – important in areas with lots of wave action.

  15. Endoskeleton • Echinoderms have an endoskeleton of calcareousossicles often with spines. • Endoskeleton is covered by an epidermis. • Some have a very substantial endoskeleton (sea urchins), others have only a few scattered dermal ossicles (some sea cucumbers).

  16. Eggs (which may be brooded or laid as benthic egg masses) hatch into bilateral, free-swimming larvae. The type of larva is specific to each echinoderm class. Class Asteroidea Bipinnaria Brachiolaria Class Ophiuroidea Ophiopluteus Class Echinoidea Echinopluteus Class Holothuroidea Auricularia Class Crinoidea Doliolaria Development

  17. Development • Metamorphosis involves a reorganization into a radial juvenile. • Left/right becomes oral/aboral.

  18. Class Asteroidea • Class Asteroidea includes sea stars. • Common on rocky shores and coral reefs, some found on sandy substrates.

  19. Class Asteroidea • Sea stars have arms (rays) arranged around a central disc. • The body is flattened, flexible, and covered with a ciliated, pigmented epidermis.

  20. Class Asteroidea • The mouth is on the underside of the sea star. • Ambulacral grooves stretch out from the mouth along each ray. • Tube feet border each groove.

  21. Class Asteroidea • The aboral surface is often rough and spiny. • Around the base of each spine there are pincerlike pedicellariae that keep the surface free of debris and sometimes help with food capture.

  22. Class Asteroidea • Skin gills are soft epidermis covered projections of the coelom that extend between ossicles and serve a respiratory function.

  23. Class Asteroidea • The lower part of the stomach can be everted through the mouth during feeding.

  24. Class Asteroidea • The upper part of the stomach connects to a pair of digestive glands (pyloric ceca) in each arm.

  25. Class Asteroidea - Feeding • Most sea stars are carnivorous; feeding on molluscs, crustaceans, polychaetes, echinoderms, other inverts & sometimes small fish.

  26. Class Asteroidea - Reproduction • Most sea stars have separate sexes with a pair of gonads in each ray. • Fertilization is external.

  27. Class Asteroidea - Regeneration • Echinoderms can regenerate lost parts. • Sea stars can readily replace an arm if it is lost. • This may take several months. • They can also cast off an injured arm.

  28. Class Asteroidea - Regeneration • Some species can even regenerate an entire individual from a broken off arm. • Usually, a small piece of the central disc must be included. • Linckia can regenerate a whole new individual from a broken arm with no central disc attached.

  29. Concentricycloidea • The two species of sea daisies were described for the first time in 1986. • They are tiny (< 1 cm), have no arms and the tube feet are arranged around the periphery of the disc. • Once considered a separate class, they are highly derived sea stars.

  30. Class Ophiuroidea • Brittle stars (Class Ophiuroidea) are the largest group of echinoderms. • Abundant in all benthic marine environments – even the abyssal sea bottom. • Brittle stars have very slender arms.

  31. Class Ophiuroidea • No pedicillariae or skin gills. • Madreporite is on the oral surface. • Tube feet have no suckers, their primary function is to aid in feeding.

  32. Class Ophiuroidea • Brittle stars move using their arms rather than tube feet.

  33. Class Echinoidea • Class Echinoidea includes sea urchins and sand dollars.

  34. Class Echinoidea • The endoskeleton is well developed in echinoids. • Dermal ossicles have become close-fitting plates that form the test. http://www.jaxshells.org/test.htm

  35. Class Echinoidea • Echinoids lack arms, but still show the pentamerous plan in the five ambulacral areas with pores in the test for the tube feet.

  36. Class Echinoidea • Most echinoids are “regular” having a hemispherical shape, radial symmetry, and medium to long spines. • Regular urchins move using their tube feet with some help from spines.

  37. Class Echinoidea • “Irregular” echinoids include the sand dollars and heart urchins that include some species that have become bilateral. • Spines are usually short and are used in locomotion.

  38. Class Echinoidea • Some urchins have very reduced tests, and bright coloration. • The pedicellariae in these species contain painful toxins.

  39. Class Echinoidea • Echinoids live in all seas from the intertidal to the deep sea. • Urchins usually prefer rocky substrate, while sand dollars and heart urchins like to burrow into sandy substrate.

  40. Class Echinoidea • Echinoids have a complex chewing mechanism called Aristotle’s lantern. • Teeth are attached here. • Sea urchins are usually omnivorous feeding mostly on algae.

  41. Class Echinoidea • Sand dollars use their short spines to move sand & its organic contents to the sides, the food particles drop between the spines, and ciliated tracts on the oral side carry the particles to the mouth.

  42. Class Holothuroidea • Sea cucumbers (Class Holothuroidea) are elongated along the oral/aboral axis. • Bilateral • Ossicles are greatly reduced in most species.

  43. Class Holothuroidea • The body wall is usually leathery with tiny ossicles embedded in it, but can be very thin.

  44. Class Holothuroidea • Oral tentacles are modified tube feet located around the mouth. • Food particles are gathered by the oral tentacles. • Tentacles are put into the pharynx one by one so food can be sucked off.

  45. Class Holothuroidea • Sea cucumbers move using ventral tube feet and waves of contraction along the muscular body wall.

  46. Class Holothuroidea • Sea cucumbers have a very unusual defense mechanism: • They are able to cast out part of their viscera. • The lost parts regenerate. • Some have organs of Cuvier that can be expelled in the direction of an enemy. • These tubules become long and sticky, sometimes containing toxins.

  47. Class Crinoidea • Crinoids include sea lilies and feather stars. • At metamorphosis, juveniles become sessile and stalked. • Adults are free-moving in some species. • Long, many branched arms.

  48. Class Crinoidea • Crinoids use their tube feet and mucus nets to feed on small organisms that are passed to their ciliated ambulacral grooves.

  49. Phylogeny • Echinoderms are probably derived from bilateral ancestors. • Pentaradial symmetry may have been an adaptation to a sessile existence. • Some forms then become mobile. • Some mobile forms are secondarily bilateral.

  50. Phylum Hemichordata • Hemichordates (acorn worms) are marine animals that have gill slits and a rudimentary notochord – however, the notochord is not homologous with the notochord in vertebrates.

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