Stages involved in the development of leech are as follows:
1. Copulation and fertilization:
Copulation takes place during March and April. Two leaches come in contact as their male and female genital pore lies against each other.
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Penis of one leach enters the female genital pore of the other and there is a mutual exchange of seminal fluid containing spermatphores, in both the leaches.
Fertilization of sperm and ova takes place in the vagina and the fertilized ova are discharged into a cocoon for further development.
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2. Cocoon Formation:
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Coccons are formed following copulation during April, May and June. It is secreted by the cliteler glands around the 9, 10 and 11 segments.
Cliteller glands secrete albumin around fertilized ova inside the cocoon. Then the leach slides past the cocoon and withdraws its anterior end completely.
The plugging of cocoon takes place by the secretion of prostomial glands. Complete cocoon is formed in about six hours and becomes hardened after exposure to air.
A well formed cocoon is a light yellow coloured barrel shaped structure about 25 to 30 mm long and 12. To 15 mm in diameter
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Its wall consists of an inner thin tough membranous layer and an outer thick spongy layer. There is a distinct polar plug with a conical projection at each narrow end.
3. Development:
In a cocoon 1 to 24 eggs undergo development. Albumen content of cocoon serves as food for developing embryo.
Cleavage is unequal and development is direct. Young leeches come out from cocoon by dropping off polar plugs. The whole process of development takes about a fortnight for completion.