Lecture 12 - Echinodermata

• Deuterostome characteristics:

  • Radial, indeterminate cleavage
  • Formation of the mouth from a second opening
  • Enterocoelous coelom development
• Chaetognaths are placed outside both protostome & deuterostome groups.
• 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).
• Echinoderms - include sea stars, brittle stars, sea urchins, crinoids, sea cucumbers.

  • Entirely marine
  • Almost entirely benthic.
  • Nonsegmented
• What are the five classes under Phylum Echinodermata?

  1. Class Crinoidea
  2. Class Asteroidea
  3. Class Ophiuroidea
  4. Class Echinoidea
  5. Class Holothuroidea
• Phylum Echinodermata - Symmetry

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

  As adults they show secondary radial symmetry – pentaradial (5 parts).

  • Perhaps an adaptation for sessile living in early echinoderms. Crinoids
• Phylum Echinodermata - Symmetry

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

  • Echinoderms have a true coelom with deuterostome development.
  • Enterocoelous – the mesoderm lined coelom develops from outpocketing of the primitive gut.
• 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, and excretion.
• Echinoderm Water Vascular System

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

  • Canals of the water vascular system lead to the tube feet.
• Echinoderm Endoskeleton

  • Echinoderms have an endoskeleton of calcareous ossicles often with spines.
  • Endoskeleton is covered by an epidermis.
• Echinoderm Development

  • Eggs (which may be brooded or laid as benthic egg masses) hatch into bilateral, free-swimming larvae.
• Echinoderm Development

  The type of larva is specific to each echinoderm class.

  • Class Asteroidea - Bipinnaria and Brachiolaria
  • Class Ophiuroidea - Ophiopluteus
  • Class Echinoidea - Echinopluteus
  • Class Holothuroidea - Auricularia
  • Class Crinoidea - Doliolaria
• Class Asteroidea includes sea stars. Common on rocky shores and coral reefs, some found on sandy substrates.
• Class Asteroidea

  • Sea stars have arms (rays) arranged around a central disc.
• Class Asteroidea

  • Ambulacral grooves stretch out from the mouth along each ray.
• Class Asteroidea

  • Around the base of each spine there are pincerlike pedicellariae that keep the surface free of debris and sometimes help with food capture.
• Class Asteroidea

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

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

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

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

  • Linckia can regenerate a whole new individual from a broken arm with no central disc attached.
• Concentricycloidea

  • The two species of sea daisies were described for the first time in 1986.
• Class Ophiuroidea

  • Brittle stars are the largest group of echinoderms.
  • Abundant in all benthic marine environments – even the abyssal sea bottom.
• 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.
• Class Echinoidea - includes sea urchins and sand dollars.
• Class Echinoidea

  • The endoskeleton is well developed in echinoids.
  • Dermal ossicles have become close -fitting plates that form the test.
• 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.
• Class Echinoidea

  • Most echinoids are “regular” having a hemispherical shape, radial symmetry, and medium to long spines.
• Class Echinoidea

  • “Irregular” echinoids include the sand dollars and heart urchins that include some species that have become bilateral.
• Class Echinoidea

  • The pedicellariae in these species contain painful toxins.
• 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.
• 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.
• Class Holothuroidea

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

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

  Sea cucumbers have a very unusual defense mechanism:

  • They are able to cast out part of their viscera.
  • Some have organs of Cuvier that can be expelled in the direction of an enemy.