The digested food in small intestine is absorbed into blood vessels in the walls of small intestine through millions of tiny finger-like outgrowths called W. The undigested and unabsorbed food from the small intestine enters into another organ X. The organ X absorbs most of the water from the undigested waste food material making ir semi-solid. This semi-solid waste is stored in Part Y of organ X for some time. It is passed out from the body through part Z in the form of faeces.
What is Y?

Explanation

To solve the question step by step, let's break down the information provided:

1. **Identify the function of the small intestine**: The small intestine is responsible for the digestion and absorption of nutrients. It has millions of tiny finger-like outgrowths called **villi** (W) that help increase the surface area for absorption.

2. **Understanding the flow of undigested food**: After the digestion process, any undigested and unabsorbed food moves from the small intestine into another organ, which is the **large intestine** (X).

3. **Function of the large intestine**: The large intestine absorbs most of the water from the undigested waste food material, converting it into a semi-solid form.

4. **Identifying part Y**: The semi-solid waste that is formed in the large intestine is stored in a specific part of the organ. This part is known as the **rectum** (Y).

5. **Final passage of waste**: The semi-solid waste is eventually passed out of the body through another part, which is the **anus** (Z), in the form of feces.

Thus, the answer to the question "What is Y?" is **rectum**.

### Summary of Steps:
1. Identify the role of the small intestine and its structures (villi).
2. Recognize the transition of undigested food to the large intestine.
3. Understand the function of the large intestine in water absorption.
4. Identify the storage part of the waste in the large intestine (rectum).
5. Recognize the exit point for waste (anus).