Internal Anatomy of a Sea Star

Dissection Overview

• Removal of body wall from three arms and central disc.
• Madreporite:
  • Small white structure left intact during dissection.
  • It is a porous surface critical for water entry.

Internal Structures

Pyloric Cea

• Brownish, spongy structures on top of each arm.
• Function as digestive glands, analogous to a liver.
• Secrete enzymes to aid the stomach in digestion.

Gonads

• Located beneath the pyloric cea.
• Round structures responsible for reproduction.
• Produce sex cells.

Ambulacral Ridge

• Bony ridge structure.
• Flanked by small, round, jelly-like structures called ampullae.
• Part of the water vascular system.

Water Vascular System

• Madreporite: Entry point for water.
• Connects to a series of canals:
  • Ring Canal: Encircles the central disc internally.
  • Radial Canal: Runs beneath the ambulacral ridge.
• Ampullae:
  • Fill with water from the radial canal.
  • When squeezed, they extend/retract tube feet.
• Operates as a hydraulic system aiding movement.

Skeletal System

• Dermal Ossicles: Rib-like structures along the arms.
• Form part of the skeletal framework, connected to external spines.

Digestive System

• Pyloric Stomach: Spongy mass, part of a two-chambered stomach system.
• Cardiac Stomach: Beneath pyloric stomach.
  • Eversible, can be extended out of the body.
  • Used to digest prey externally before taking back the digested material.
• Complete digestive system with an anus for waste excretion through the central disc.

Feeding Mechanism

• Stomach can be everted to feed on prey like bivalves.
• Digestive glands secrete enzymes to start digestion externally.
• Digestion is completed internally once the stomach retracts.

Summary

• The sea star's anatomy includes specialized structures for digestion, movement, and support.
• The integration of these systems allows for effective feeding, mobility, and reproduction.