Fertilization is external. Cleavage begins about half an hour after fertilization. It is holoblastic, somewhat unequal and determinate.
At 64 cell stage, a single layered flat coeloblastula with a fluid filled, blastocoel is formed. Gastrulation followed by emboly or invagination occurs and a cavity, the archenteron is formed which opens outside through blastopore.
Larval development – tadpole larva :
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After 8 hrs of development embryo elongates and forms a tailed larva. The fully formed larva hatches out to lead free swimming life.
This larva is called tadpole larva, because of its resemblance to a small tadpole larva of frog. It is transparent, minute about 1.2 mm long and highly motile. Body is divided into anterior oval trunk or head and long posterior tail.
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(a) Head or trunk:
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Anteriorly, head possesses three stalked adhesive papillae, two dorso-lateral and one median-ventral.
They serve for attachment of larva when it undergoes metamorphosis. On the dorsal side of the trunk a hollow sensory vesicle or brain encloses the photoreceptor, two ocelli, and a single otocyst which serves to maintain equilibrium, is situated.
Larval nervous system includes a cerebral cone, a visceral ganglion and dorsal tubular nerve cord.
Rudimentary alimentary canal comprising of a narrow branchial siphon, a large sac like pharynx, a short narrow oesophagus, a swollen stomach, a slender intestine and a small rectum. The anus opens into the left side of atrium.
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(b) Tail:
Tail is transparent about 0.9 mm long, laterally compressed and acts as powerful locomotory organ.
It is fringed with a continuous vertical tail fin formed by the test and marked with oblique striations which appears like fin rays.
Notochord forms the supporting axial skeletal rod restricted to the tail. Above the notochord is present hollow nerve chord bearing tail ganglia from which fine nerves arise.
Tail also contains strong muscle bands which show a superficial segmentation similar to vertebrate myotomes.