Comment upon the gametic exchange in earthworm during mating. Discuss the physiology of reproduction in earthworms.

(N/A) $\Rightarrow$ The exchange of reproductive cells and the physiological process of reproduction in earthworms can be described as follows:
$(i)$ Earthworms are protandrous,meaning sperm cells mature before egg cells.
$(ii)$ Mating involves the exchange of sperm between two individuals,followed by external fertilization.
$(iii)$ During mating,two earthworms come together in opposite directions,applying their ventral surfaces against each other.
$(iv)$ They exchange sperm packets known as spermatophores.
$(v)$ During this process,the skin surrounding the male genital pore rises to form temporary papillae,which act like a penis to enter the spermathecal pore and keep it open.
$(vi)$ Once one spermatheca is filled,the earthworms shift their positions to align the male genital pore with the other spermatheca. This process typically takes about one hour.
$(vii)$ Sperm cells are stored in the diverticula of the spermathecae,while the ampulla is associated with the secretion of nutrients for the sperm cells.
$(viii)$ Later,both sperm cells and egg cells are deposited into a cocoon,which is secreted by the clitellar glands.
$(ix)$ Fertilization and development occur within the cocoon,which is an external process.