In breeding season (spring) mature gametes are shed freely in surrounding water where external fertilization takes place.
Both male and female produce enormous quantity of gametes. After fertilization zygote soon forms a delicate fertilization membrane around it.
Embryogeny :
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Development is indirect, Zygote is microlecithal. Cleavage is rapid, holoblastic, equal, radial and indeterminate.
On the second day of cleavage the blastula or coeloblastic stage is reached. It is ciliated and swims freely in water.
Blastocoel is filled with blastocoelic fluid. During gastrulation the advancing end of archenteron budds off cells into the blastocoel which form the mesenchyme or mesoderm. Coelom is enterocoelic.
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Larval development :
Three larval stages come one by one in the life history of a star fish.
1. Dipleurula larva or early bipinnaria:
Early bipinnaria is first larval stage of all echinoderms. The origin of all modern echinoderms is believed from the dipleurula like ancestors.
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An anterior mid-ventral ectodermal invagination called stomodaeum forms larval mouth and blastopore becomes larval anus. Archenteron
2.Bipinnaria larva:
In dipleura larva, a large pre-oral lobe bordered by pre-oral loop of cilia and three lateral lobes bordered by a post oral loop of cilia formed. Both the loops of cilia arise by the splitting of the peri oral band of cilia.
This larva is called bipinnaria larva. It swims and feeds freely and after some weeks transforms into the next larval stage, the brachiolaria larva.
3. Brachiolaria larva:
The lobes of the bipinnaria develop into long slender, ciliated contractile structures called larval arms. Three of them bear sucker or adhesive disc and are called brachiolarian arms or fixing processes.
This larva is called brachiolaria. It is also bilateral, symmetrical and swims and feeds actively. It gradually metamorphoses into a small sea star.
Metamorphosis :
In about six or seven weeks of time, the brachiolaria settles on some solid surface with the help of adhesive disc.
Pre oral lobe (representing the anterior end of larva) forms a sort of stalk for adhesion. Stalk later degenerates and becomes completely absorbed. Adult body develops from the rounded posterior end of the larva.
Regeneration and autotomy :
Star fish have great power of regeneration. Whenever an arm is held, injured or unduly stimulated, the sea star readily breaks off its own arm.
This process is termed autotomy. This ability is found in many other invertebrates also. Autotomy serves as a means of protection to the animal.