Starfish : Diagram and Features

Starfish, also known as sea stars are echinoderms that belong to the class Asteroidea. They are marine invertebrates that are found in seabeds in oceans all over the world.

What is Starfish?

Starfish are star-shaped echinoderms; they are also referred to as sea stars. These marine invertebrates typically have a central disc and five arms; some other species possess a greater number of arms.  Generally, the species are vibrantly coloured in different shades. They have tube feet that operate by a hydraulic system and a mouth towards the centre of the lower surface.

Starfish are opportunistic feeders; different species have special feeding behaviours. They can reproduce sexually and asexually.  Most of them have the capability to regenerate lost or damaged parts of the body and can also shed arms in defense.

Classification

Here is a detailed diagrammatic representation of the genus Asterias (Starfish) along with features explained.

Starfish Diagram

Diagram of Starfish

Features

• There are a total of eight species found in the genus Asterias.
• They have five arms that are broad at the base and taper at the end and curl up a little too.
• The tube feet present on the lower surfaces of their arms are tubular projections that are a part of the vascular system. These projections help in locomotion and feeding.
• The aboral (upper) surface is usually orange to brick red in colour and the oral (lower) surface is of paler shades.
• The madreporite is a pressure-equalising valve like structure that is used to filter water into the vascular systems of the sea stars.
• It is a calcareous opening that looks like a wart or button-like structure on the aboral surface of the organism.
• Ampulla are small sac-like structures that are connected to stone canals. The stone canals together form a circular ring canal.
• The ampulla are also connected to sucker-like podia. As the ampulla contracts, the podia is stretched, water is moved in and the whole process helps in their movement.
• The starfishes can grow up to a maximum diameter of 20 inches but are commonly found in the range of 4 – 12 inches.
• It feeds on molluscs and other benthic invertebrates.

Different parts of a starfish

External Morphology

They show pentaradial symmetry wherein most of the species possess five arms that radiate from a central disk. Towards the end of each of the arm is a microscopic eye called the ocellus which aids them to distinguish between dark and light and detecting movement. Its body comprises ossicles constituting calcium carbonate forming the endoskeleton. These can take different forms externally such as granules and spines. Their top surface is called abactinal or the aboral side while the base side is called the actinal or oral surface.

The madreporite is a faint circle seen somewhat off-centre present on the central disk. It is a porous plate involved in providing excess water for different requirements. Pedicellariae are small valve-like structures seen in some groups. It can be seen all through the body or in tufts at the bottom of every spine which are supposedly functional in feeding or defense or even capturing and killing prey.

Circulatory system of Sea stars

Sea stars do not pump blood through their bodies. They use a seawater and a complex water vascular system for things to move. Its tube are vital in the circulatory system which are also used in movement. On its underside, their have hundreds of tube feet.

Hard body plates

Sea stars lack a backbone; they have a skeleton under their skin. The endoskeleton comprises a complex network of hard bony plates made of calcium carbonate bound by flexible tissues.

Tube feet

Comparatively, they move slowly and are mobile. They locomote using special structures called tube feet that are somewhat like tentacles having small suckers towards the terminal. With tentacles, starfish can have a grip of rock walls etc. in a coordinated manner.

Stomach

They have 2 stomachs – the pyloric and the cardiac stomach. To engulf prey, it pushes the cardiac stomach out of the stomach in the centre of the underside. Then it secretes a digestive enzyme to disintegrate its prey. When the cardiac stomach comes to the back, food gets transferred to the pyloric stomach.

Nerve net

They lack a central nervous system and have a nerve net wherein all nerves are spread over its body. They have sensory parts in the body that detect a stimulus.

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Also Read:

• Animal Kingdom
• Food Vacuole
• Coelom