Diagram of Starfish

Starfish or sea stars are star-shaped echinoderms. About 1,900 species of starfish live on the seabed in all the world's oceans, from warm, tropical zones to frigid, polar regions. They are found from the intertidal zone down to abyssal depths, at 6,000 meter below the surface

Starfish are marine invertebrates. They typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, gray or brown. 

1.0Introduction of Starfish

• Starfish move by using suction from their tube feet to crawl along the ocean floor. 
• They can't swim. Starfish can regenerate lost arms, and in some species, they can even regenerate a new body from a portion of an arm. 
• Starfish don't have brains or blood. Instead, they use a water vascular system to pump nutrients through their bodies using filtered seawater. : Starfish can live up to 35 years. 
• Starfish live in saltwater and can survive in a variety of climates. : Starfish are vulnerable to high temperatures and diseases. 

2.0Classification of Starfish

3.0Morphology of Starfish

Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms.

4.0Body Wall of Starfish

The body wall of a starfish, is made up of multiple layers of tissue: 

• Epidermis: The outer layer of the body wall that contains hair-like projections called cilia, glandular cells, and sensory cells. 
• Dermis: The layer beneath the epidermis that contains connective tissues and a calcareous skeleton. 
• Coelomic myoepithelial layer: A thin layer that contains circular and longitudinal muscles. 

The body wall of a starfish is also made up of calcite ossicles that are connected by muscles and collagenous tissue. The collagenous tissue allows the body wall to vary in stiffness. 

5.0Digestive system of Starfish

The digestive system of a starfish, also known as a sea star, is unique and includes the following features: 

• Two stomachs: Starfish have a cardiac stomach and a pyloric stomach. 
• External digestion: Starfish can extend their cardiac stomach outside of their mouth to partially digest their prey. 
• Digestive enzymes: The stomach secretes a powerful digestive enzyme to break down the prey. 
• Pyloric ceca: The final digestion of nutrients takes place in the pyloric ceca, which are organs located in the arms of the starfish. 
• Recovering hard parts: Once digestion is complete, the starfish pulls its stomach back into its mouth and leaves behind any hard parts of the prey. 
• Anus: The anus is located in the center of the aboral body surface. 

6.0Respiratory System of Starfish

• Starfish, or sea stars, breathe by diffusing oxygen across their bodies through a combination of tube feet and papulae, or skin gills:
• Tube feet
• These small tentacles with suckers on the end are like a starfish's respiratory organs.
• Papulae
• These small projections near the base of the spines on the topside of the starfish are also respiratory organs. 
• Starfish also have a complex water vascular system that helps keep things moving. Water enters the starfish through a network of water vessels into the coelom, or hollow body cavity. Inside the coelom are dermal branchiae, or skin gills, that enable respiration. Water flows around the projections, and diffusion happens. 

7.0Circulatory System of Starfish

• Starfish, also known as sea stars, do not have a circulatory system like humans, but instead have a water vascular system that circulates seawater throughout their bodies: 
• Here are some parts of a starfish's water vascular system: 

Madreporite: A sieve plate made of calcium carbonate that allows water to enter and exit the starfish's body 

• Ring canal: A central ring-shaped canal in the starfish's center disc 
• Radial canals: Canals that branch out from the ring canal and run down each arm 
• Ampulla: A water bulb that attaches to each tube foot and helps move them 
• Tube feet: Hundreds of tube feet on the underside of the starfish that are used for movement and are an important part of the water vascular system 

The water vascular system helps with many functions, including: Locomotion, Adhesion, Food manipulation, Gas exchange, and Moving blood around by changing fluid pressure. 

8.0Excretory  System of Starfish

Excretory material: The excretory material includes ammonium compounds, urea, and creatine. 

• Release of excretory material: The excretory material is released to the outside through dermal branchiae or engulfed by amoebocytes. 
• Coelomocytes: The body fluid contains phagocytic cells called coelomocytes, which are also found within the hemal and water vascular systems. 
• Starfish, also known as sea stars, don't have a distinct excretory organ. Instead, they remove waste ammonia through diffusion through their tube feet and papulae. The water vascular system of a starfish also serves as an excretory system. 

9.0Nervous System of Starfish

The nervous system of starfish, is relatively simple but has some unique features: 

Structure

• A nerve ring around the mouth coordinates motion and eating, and radial nerves extend from the ring along the arms. 

Sensory organs

• Starfish  have sensory cells on their arms and body surface that detect temperature, smell, and contact with other objects. 
• They also have tube feet, spines, and pedicellariae that respond to touch. 

Coordination

• The ring and radial nerves coordinate righting activity, and the nervous system can control whole body coordination in complex behaviors. 

10.0Water Vascular System of Starfish

• The water vascular system of a starfish, is a hydraulic system that allows for locomotion, respiration, and the transportation of food and waste.

• Structure: The system consists of a central ring canal and radial canals that extend along each arm. The madreporite, a structure on top of the body, regulates the amount of water in the system.
• Function: Water circulates through the system, allowing for the exchange of nutrients, gases, and waste. The tube feet, which extend from the canals, can expand and contract based on the volume of water in the system. 
• Movement: Echinoderms move by alternately contracting muscles that force water into the tube feet, causing them to extend and push against the ground. 
• Other names: The water vascular system is also known as the ambulacral system or aquiferous system. 

11.0Reproduction of Starfish

Starfish, reproduce sexually and develop through indirect development: 

Reproduction

• Starfish reproduce sexually by releasing sperm and eggs into the water, where they meet and fertilize. Spawning often occurs during spring or summer, and is synchronized with seasonal changes in water temperature and salinity, and lunar cycles. 

Development

• After fertilization, the eggs develop into larvae that swim in surface waters and feed on plankton. The larvae then undergo metamorphosis, changing from bilateral symmetry to radial symmetry, and eventually turn into adult starfish. 

Brooding

• Some species of starfish brood their eggs, either within their body or on top of them. In brooding species, the eggs develop directly into adults without a larval stage. 
• Brooding is more common in cold environments, where larvae may have difficulty surviving.