Asterias – The Star Fish


Star fish is a triploblastic, radially symmetrical marine echinoderm that crawls on the sea floor and preys upon molluscs. Its skin is very rough and endoskeleton is made of dermal ossicles. On the dorsal side the central disc carries a sieve plate or madreporite between the bases of the two arms through which water enters into the water vascular system. On the other end of the central disc is anus on the dorsal side. Five genital openings are located on the lateral side at the angle of arms. The apex of each arm carries a sensory terminal tentacle.

On the ventral side, mouth is placed in the centre surrounded by 5 jaws or ossicles and 5 bunches of spiny oral papillae or actinosomes. The ambulacralgrooves extend from mouth to the tip of arms and carry ambulacral ossicles fitted together throughout the length. Podia or tube feet emerge through the pores or gaps present between the ossicles. There are 2-3 rows of ambulacral spines on the margins of the ambulacral grooves.

The skin of starfish is made of epidermis and dermis and dermal ossicles. Spines are scattered all over the skin. Also present on the skin are jaw-like pedicellariae which serve to clean the skin of debris. Pedicellariae are made of two opposing ossicles or valves which are moved by abductor and adductor muscles attached at their base like forceps. 

Dermalbranchiae or papulae are membranous structures which are lined by ciliated epithelium on the outer side and coelomic epithelium on the inner side. They are filled with coelomic fluid and carry out respiration and excretion. They emerge through tiny pores and can be retracted inside the body.

DIGESTIVE SYSTEM

Starfish is a carnivore and feeds on bivalves and snails, which it seizes with arms and podia and stretches the shell to open it. As the adductor muscles of the mollusc are tired out a small opening is made, through which the starfish pours its gastric juices that kill and predigest the prey. The stomach of starfish is eversible, and along with the ossicles it chews and swallows the prey slowly, sometimes taking hours in the process. Stomach is divided into cardiac and pyloric portions, the latter carries 5 paired pyloric caeca extending into the arms. These caeca are supplied with sacs or lobules of digestive glands which produce digestive enzymes. Bulk of the digestion and absorption takes place in the pyloric caeca as the intestine is very small in starfish. Intestine also carries 2-3 intestinal caeca but they are primarily excretory in function. Anus opens on the dorsal side on the central disc.

THE AMBULACRAL SYSTEM

This is water vascular system or podial system of the starfish that helps in locomotion as well as respiration. Madreporite on the dorsal surface contains radiating grooves that have hundreds of minute pores through which water enters into the ambulacral system through the stonecanal. The pentagonal ringcanal gives off 5 radialcanals that travel at the base of ambulacral groove in each arm. The radial canal gives off lateral branches called podialcanals, each one of which is connected to the tube foot. Each tube foot is made of an ampulla, a stalk-like podium and a terminal sucker. These podia can attach to any surface and help in slow locomotion and also help in prey capture. There are 1-4 polianvesicles attached on the inner side of ring canal which regulate pressure inside the water vascular system. There are 9 tiedman’sbodies also attached to the ring canal. They are lymphatic bodies that produce amoebocytes and enzymes and they also function like filtering devices. Water vascular system is filled with fluid that is similar to sea water but contains proteins, potassium and coelomocytes.

CIRCULATORY SYSTEM

 True blood vascular system is absent in Asterias. The circulatory system is made of a Haemalsystem containing haemal strands and lacunae, and Perihaemalsystem that is made of coelomic sinuses lined by the coelomic epithelium. Perihaemal system encloses the haemal system which is filled with coelomic fluid having coelomocytes. There is no respiratory pigment in it and the main function of this system is to distribute digested products, enzymes and hormones.

On the oral side there is an oral haemal ring surrounded by perihaemalring sinus. Five radial haemal strands emerge from the ring and travel to the arms. They are surrounded by the radial perihaemal sinuses. 

On the dorsal side there is aboral haemal ring surrounded by the perihaemal ring sinus. From it emerge five paired genital haemal strands that travel in each arm and supply blood to gonads. They are also enclosed inside the perihaemal sinuses.

The oral and aboral haemal rings are connected together by a large axial gland on the dorsal side of which there is a contractile dorsal sac that maintains the flow of blood into the system. From the alimentary canal gastric tufts bring digested food into the haemal circulation. Gastric tufts are not covered with perihaemal system.

EXCRETORY SYSTEM

Excretory organs are lacking in starfish. Excretory material includes ammonium compounds, urea and creatine which are released to the outside either through dermal branchiae or engulfed by amoebocytes. Water vascular system also serves as excretory organ.

NERVOUS SYSTEM

Asterias does not have true nerves, which are made of diffused neurons embedded in skin and organs. There are visceral nerve plexuses embedded in the gut wall and there are nerves on the oral and aboral sides. Aboral nerves are embedded in the body wall. There is an anal nerve ring that gives off five radialnerves. Nerve plexus is embedded in the body wall.

On the oral side ectoneural nerves are superficial and consist of a circumoral nerve ring and five radial nerves. There is nerve plexus embedded in the body wall, which thickens to form a pair of adradialnerves between two radial nerves. The hyponeural nervous system is deep and is made of double circumoral nerve rings and paired Lange’s nerves travelling into the arms at the base of ambulacral grooves.

SENSE ORGANS

Tactilecells are distributed throughout the epidermis, more so on the tube feet. Olfactory cells are distributed around the mouth.

Eye spots are five, one each at the end of each arm on the ventral side. Eye spot consists of numerous eye pits or ocelli, each of which is connected to the sensory cushion of the radial nerve. 

A single ocellus is covered by a transparent cornea, below which there is a biconvex lens. A cone of gelatinous tissue lies below the lens, surrounding which are sensory retinal cells and reddish pigmented cells. Sensory cells are connected to the radial nerve.

REPRODUCTIVE SYSTEM

Sexes are separate but there is no sexual dimorphism. Gonads are 5 pairs located at the base of each arm and open by ciliated ducts on the aboral side in the angle of arms.

Testes are greyish in colour while ovaries are pinkish to orange in colour enclosed in genital sinus of perihaemal system. Sex cells are produced in the axial gland, from where they migrate to the gonads and get maturity. Ova and sperms are released in water and fertilization is external. A starfish female can release 200 million eggs in a season.

LIFE CYCLE OF STARFISH

Male and female starfish release ova and sperms in sea water and fertilisation is external forming zygote. Cleavage is holoblastic, radial and indeterminate after which a morula is formed. Morula gives rise to a hollow blastula with blastocoel. Gastrulation takes place by invagination after which a dipleurula larva is formed. The larva that hatches from the egg is called early bipinnaria but it resembles hypothetical dipleurula and is without arms and with ciliary bands for swimming. Dipleurula transforms into bipinnaria larva which has 5 pairs of ciliated arms and a well-developed gut. It feeds and grows and lives for 6-7 weeks after which it transforms into a sedentary brachiolaria larva. Brachiolaria has three brachiolarian arms with adhesive discs which help it in fixing to the substratum. Other ciliated arms get reduced as they are not required for swimming. Gut is well developed. Tiny starfish develops inside the brachiolaria larva. After maturity brachiolaria bursts releasing the young starfish into water. The tiny starfish feeds and grows to become adult.