The organ in an earthworm that is analogous to the kidney in humans is the nephridium.

Nephridium:
- The nephridium is an excretory organ found in earthworms.
- Earthworms have multiple pairs of nephridia, which are distributed along the length of their body segments.
- Each nephridium consists of a coiled tubule that opens into the body cavity through a nephrostome, and then connects to an excretory pore on the body surface.
- The nephridia are responsible for excreting metabolic wastes and maintaining the osmotic balance in the earthworm's body.

Function:
- The main function of the nephridium in earthworms is excretion.
- It filters metabolic waste products, such as ammonia, from the coelomic fluid (the fluid that fills the body cavity of the earthworm).
- The nephridia reabsorb useful substances, such as water, glucose, and amino acids, from the filtrate back into the body.
- The remaining waste products are then eliminated from the body through the excretory pores.

Analogous to the Kidney:
- The kidney in humans is the primary excretory organ responsible for filtering metabolic waste products from the blood and maintaining fluid and electrolyte balance.
- Like the nephridia in earthworms, the kidneys have a complex network of tubules that filter the blood and reabsorb useful substances.
- The waste products, along with excess water and electrolytes, are then eliminated from the body as urine.

Similarities:
- Both the nephridia in earthworms and the kidneys in humans are excretory organs that play a vital role in maintaining the internal environment of the organism.
- They both filter metabolic waste products and maintain fluid and electrolyte balance.
- Both organs have a tubular structure and function by filtration, reabsorption, and secretion processes.
- The excretory pores in earthworms and the urethra in humans are responsible for eliminating the waste products from the body.

In conclusion, the organ in an earthworm that is analogous to the kidney in humans is the nephridium. Both organs have similar functions and structures, and they play a crucial role in excretion and maintaining the internal environment of the organism.