Phylum Echinodermata Biology and Asterias Anatomy
English with a size of 5.16 KBPhylum Echinodermata: Characteristics and Classification
Phylum Echinodermata (from Greek echinos = spiny; derma = skin) consists of exclusively marine animals known for their pentamerous radial symmetry and unique water-driven skeletal systems.
General Characters of Echinoderms
- Habitat: Entirely marine; found at all depths from the intertidal zone to the deep sea.
- Symmetry: Adults exhibit pentamerous radial symmetry (body parts arranged in fives), while larvae are bilaterally symmetrical.
- Body Organization: Triploblastic, coelomate, and exhibit an organ-system level of organization.
- Endoskeleton: Composed of calcareous plates called ossicles, which often bear spines (hence the name "spiny-skinned").
- Water Vascular System: A unique system of fluid-filled canals derived from the coelom. It functions in locomotion, respiration, and food capture.
- Locomotion: Carried out by tube feet (podia) operated by hydraulic pressure.
- Digestive System: Usually complete. The mouth is located on the lower (oral) surface and the anus on the upper (aboral) surface.
- Excretion: No specialized excretory organs; nitrogenous waste (ammonia) diffuses through the tube feet and gills.
- Nervous System: Primitive, consisting of a nerve ring and radial nerves. There is no brain.
Classification (Up to Class Level)
| Class | Body Form | Features | Examples |
|---|---|---|---|
| Asteroidea | Star-like | Five arms not sharply marked off from the central disc; move by tube feet. | Asterias (Sea Star) |
| Ophiuroidea | Star-like | Brittle, slender arms sharply marked off from the disc; no anus. | Ophiothrix (Brittle Star) |
| Echinoidea | Spherical/Discoidal | No arms; body enclosed in a shell (test) with movable spines. | Echinus (Sea Urchin) |
| Holothuroidea | Cylindrical | Long, leathery body; no arms or spines; mouth surrounded by tentacles. | Cucumaria (Sea Cucumber) |
| Crinoidea | Cup-like | Attached to substratum by a stalk; arms branched and feathery. | Antedon (Sea Lily) |
Type Study: Asterias (Sea Star)
Habitat and Habits
- Habitat: Found in rocky or sandy coastal waters (benthic).
- Habits: Sluggish, bottom-dwelling animals. They are carnivorous, feeding mainly on mussels and oysters.
- Special Features: They possess great powers of regeneration and can shed an arm voluntarily to escape predators (autotomy).
External Morphology
- Shape: A central disc with five radiating arms.
- Aboral Surface (Upper): Darker in color. Bears the anus (central) and the madreporite (a sieve-like plate located between two arms, known as the bivium).
- Oral Surface (Lower): Paler in color. Bears the central mouth and five ambulacral grooves running along each arm, which house the tube feet.
- Pedicellariae: Minute, pincer-like structures on the surface used to clean the body and capture tiny prey.
Water Vascular System (Ambulacral System)
This is the most characteristic system of Asterias. It works on hydraulic pressure to facilitate movement.
- Madreporite: A perforated plate that allows seawater to enter the system.
- Stone Canal: An S-shaped tube connecting the madreporite to the ring canal.
- Ring Canal: A circular vessel located around the mouth.
- Radial Canals: Five canals that run from the ring canal into each arm.
- Lateral Canals: Short branches that connect the radial canals to the tube feet.
- Tube Feet: Each foot consists of a bulb-like ampulla, a middle podium, and a terminal sucker. When the ampulla contracts, water is forced into the podium, extending the foot.
Circulatory System (Haemal System)
Asterias does not have a true closed circulatory system with a heart. Instead, it has a Haemal System consisting of a set of intercommunicating channels filled with coelomic fluid:
- Oral Haemal Ring: A ring around the mouth.
- Radial Haemal Strands: Extend from the ring into each arm.
- Aboral Haemal Ring: Located on the upper surface.
Because the Water Vascular System and the Coelom handle most of the transport of nutrients and gases, the Haemal system is relatively poorly developed.