Question

Septal nephridia of earthworm discharge excretory products into
A. Body cavity
B. Pharynx
C. Outside on the body surface
D. Lumen of intestine

Answer 1

Hint: Septal nephridia occur from the $15^{th}$ segment onwards. They are the largest nephridia in earthworm.

Complete Answer:
Nephridia are the excretory tubular structures of earthworms and other annelids. Nephridia helps to eliminate nitrogenous waste. They are divided into three types according to their location: septal nephridia, pharyngeal nephridia, and integumentary nephridia.

Now, let us find the solution from the given options.
- The body cavity in earthworm is the space between the body wall and alimentary canal. It is divided into segments and covered by coelomic fluid. The coelomic fluid keeps the skin moist and also helps in respiration. Wastes from the coelomic fluid are carried into the septal nephridia through nephrotomes.
- The pharyngeal nephridia are found from $4^{th}$ to $6^{th}$ segments. They are seen as paired bunches on each side of the pharynx and oesophagus. The excretory products from the pharyngeal cavity are carried into the gut/pharynx.
- The integumentary nephridia are present along the entire inner surface of the body wall from the 3rd segment onwards. These nephridia are the smallest of the three nephridia. The excretory products are discharged directly outside the body surface.
- The intestine in earthworm is thin walled and runs from the fifteenth segment to the last segment where it opens to the outside through anus. Septal nephridia discharge waste products into the lumen of the intestine. They are present attached to the septa behind the fifteenth segment. Each septum has about 40-50 septal nephridia attached to its anterior and posterior surfaces.

So, the correct answer is option (D), i.e Lumen of intestine.

Note: Nephridia occur in all the segments of the earthworm except the first two segments. As the septal nephridia are present around the intestine, it would discharge into the intestine.