The correct answer is option 'B': Paired septa excretory canals.

The septal nephridia are a type of excretory organ found in earthworms. They are located in the septa, which are internal partitions that divide the earthworm's body into segments. These nephridia are responsible for the elimination of metabolic wastes from the worm's body.

Below is a detailed explanation of why the terminal ducts of septal nephridia of earthworm open in paired septa excretory canals:

1. Septal Nephridia:
- The septal nephridia are small, tubular structures found in each segment of the earthworm's body.
- They are responsible for the excretion of metabolic wastes, such as nitrogenous waste (ammonia) and excess water, from the body.

2. Duct System:
- Each septal nephridium consists of a coiled tubule that begins with a funnel-shaped structure called the nephrostome.
- The nephrostome collects waste materials and excess water from the coelomic fluid, which is the fluid-filled cavity within the earthworm's body.
- From the nephrostome, the tubule extends into the body wall, where it is lined with ciliated cells that help in the movement of waste materials.
- The tubule then joins a terminal duct, which carries the waste materials towards the exterior of the worm's body.

3. Paired Septa Excretory Canals:
- The terminal ducts of the septal nephridia open into paired septa excretory canals.
- These canals are located along the ventral side of the septa, which are the internal partitions dividing the segments of the earthworm's body.
- The canals run longitudinally through the body, connecting the terminal ducts of the septal nephridia in each segment.
- The canals eventually merge into larger longitudinal excretory ducts, which further join together to form the main excretory ducts.

4. Function and Excretion:
- The paired septa excretory canals serve as a pathway for the elimination of waste materials from the septal nephridia.
- The terminal ducts of the septal nephridia discharge their contents into these canals, which then carry the waste materials towards the exterior of the worm's body.
- The waste materials, along with excess water, are finally excreted through the excretory pores located on the ventral side of the earthworm's body.

In conclusion, the terminal ducts of the septal nephridia in earthworms open in paired septa excretory canals. These canals serve as a pathway for the elimination of waste materials from the nephridia and play a crucial role in the excretory system of earthworms.