Phylum Echinodermata

• Echinodermata live in marine habitats and include sea stars, brittle stars, feather stars, sea urchins, sand dollars sea cucumbers
• Body structure 

  • Body is unsegmented with no head
  • Most adult forms have penta-radial symmetry
  • Larval forms (free-swimming) have bilateral symmetry, suggesting that the ancestor of echinoderms was a bilateral animal
  • Adult body is round, cylindrical or star-shaped with 5 (or multiples of 5) parts around a central axis
• Sea star

  • Surface covered by spines that help defend against predators
  • Surface covered by small gills that provide gas exchange
• Digestive system

  1. Mouth on bottom of central disk

  2. Digestive tract runs to anus on top of disk
• Central disk

  • Has a nerve ring
  • Has nerve cords radiating from the ring into the arms
• Digestive glands

  • Secrete digestive juices
  • Aid in the absorption and storage of nutrients
• Water vascular system

  1. Ring canal in central disk

  2. 5 radial canals, each running in a groove down an arm
• Tube feet

  • Hundreds of hollow, muscular tube feet filled with fluid branch from each radial canal
  • Each tube has a bulb-like ampulla and suckered podium
• Thin epidermis covers a hard mesodermal endoskeleton of small calcareous plates called ossicles, joined together by connective tissue

  • Spines project from the ossicles
  • Calcareous endoskeleton provides protection against mechanical injury and attack by predators
• Water vascular system consists of a network of hydraulic canals branching into tiny hollow muscular extensions called tube feet that function in locomotion and feeding
• Canal consists of central ring canal in the central disk and radial canals that extend along the entire length of each arm
• Tiny tube feet project out from ambulacra grooves which extend from the mouth to the tip of each arm
• Each tube foot consists of a muscular ampulla (like bulb on a medicine dropper) and a podium
• A sea star adheres to rocks or creeps along by extending, gripping, releasing, extending, and gripping again

  • Gripping action of the tube feet is a combination of suction force (suction cup at base) and chemical adhesion (secretion of adhesive mucus)
• Water flows in or out of water vascular system through porous circular calcareous plates called madreporite on the aboral surface of the sea star
• Movement of foot

  • Once water enters the central ring canal, it is distributed to the radial canals in the arms and water is channelled into the muscular ampullae
  • Contraction of ampulla forces fluid into the attached podium, causing the podium to extend
  • In addition to a suction force by the suction force by the suction cup at the base of the podium, adhesive chemicals are secreted from the base of the podium to attach it to the substrate surface
  • To detach the podium, de-adhesive chemicals and cause muscles to contract, forcing water back into the ampulla and shortening the podium
• Sea stars are generally active predators and most of them feed on bivalves
• Mouth is found on the oral (ventral) surface and the anus on the aboral (dorsal) surface
• Short digestive tract runs through the central disk from the mouth to the anus
• How sea stars feed

  1. Sea stars use their tube feet to grasp prey like clams and oysters

  2. Arms of sea star embrace the closed bivalve, attaching their tube feet tightly on the shells and exert a steady pull

  3. Adductor muscles of the bivalve fatigue and relax, forming a small gap between the two valves

  4. Sea star everts its stomach through the mouth and into the bivalve shells

  5. Stomach secretes digestive enzymes to partially digest bivalve's soft body

  6. Liquefied food enters into the gut as the stomach is taken back into the body

  7. In the gut, the food is completed digested by digestive enzymes secreted by the digestive glands found in the arms

  8. Digestive glands help absorb and store the nutrients
• Types of Echinoderms – Brittle Stars

  • Most diverse and abundant echinoderms
  • Distinct central disk with long, highly flexible arms
  • Move by lashing their arms in a snake-like manner
  • Scavengers on the seafloor
  • Tube feet do not have suckers and aid in feeding and gripping substances, limited use in locomotion
• Types of Echinoderms – Feather Stars

  • Long, many-branched arms
  • Adults are free-moving even though they may remain in the same spot for a long time
  • Move using their cirri found on their ventral surface to crawl or by sweeping their long feathery arms to swim
  • Use their arms for suspension-feeding
  • Arms encircle the mouth, which is found on the upper surface away from the substrate
• Types of Echinoderms – Sea Urchins

  • No arms, but have five rows of tube feet that function in slow movement
  • Calcium carbonate ossicles become close-fitting plates to form an endoskeletal test
  • Rounded in sea urchins and flattened in sand dollars
  • Spines protrude out from the test and spines are pivoted by muscles
  • Spines (may be long or short) provide protection and are used in movement
  • Mouth of sea urchin is ringed by a highly complex, jaw-like structure with teeth called Aristotle’s lantern - used for tearing and chewing algae
• Sea Cucumbers

  • Lack spines
  • Endoskeleton is greatly reduced
  • Hard ossicles have been reduced to microscopic plates embedded in a soft body
  • Elongated in the oral-aboral axis and their body is worm-like
  • Tube feet are usually well-developed in only 3 out of 5 rows which are in contact with substrate since they typically lie on only one side of their body
  • Tube feet on dorsal side are reduced and might serve a sensory role
  • Tube feet around the mouth modified to be retractile oral tentacles which are used for gathering organic materials from sediments during feeding
• Defence mechanism in sea cucumbers

  Expel a sticky mass of specialised threads or internal organs out through the anus to entangle and distract predators
• Sea cucumbers can regenerate missing organs it has discharged