To solve the problem, we will analyze the given dichotomous key step by step to identify the organisms P, Q, R, and S, and then determine which of the provided statements is incorrect.
### Step-by-Step Solution:
1. **Identify Organism P:**
- The key starts with animals having a cellular level of body organization. This refers to **Porifera** (sponges), which have a porous body structure with numerous small openings (ostea) and a large opening (osculum).
- **Conclusion:** Organism P is **Porifera**.
2. **Identify Organism Q:**
- The next step involves animals with tissue/organ/organ system level of body organization. We move to the next part of the key.
- Organism Q is determined by the presence of bilateral symmetry, which is characteristic of many animals, including **Chordates**.
- **Conclusion:** Organism Q is likely a **Chordate**.
3. **Identify Organism R:**
- In the next step, we check for true coelom presence. If true coelom is present, we proceed to check for the presence of a notochord.
- Organism R is identified as possessing a notochord and could be a **Cnidarian** (like jellyfish), which can be represented in the medusa form.
- **Conclusion:** Organism R is represented by the **medusa form**.
4. **Identify Organism S:**
- The last part of the key indicates that if the organism is represented by the polyp form, it is also a **Cnidarian**.
- **Conclusion:** Organism S is represented by the **polyp form**.
### Analyzing the Options:
Now, we will analyze the statements regarding organisms P, Q, R, and S to find the incorrect one.
1. **Statement about Organism P:**
- "Body of organism P could contain numerous small pores called as the ostea and a large opening called as the osculum at the top."
- This statement is **correct** as it describes Porifera accurately.
2. **Statement about Organism Q:**
- "Organism Q could possess gill slit at some stages of their life."
- This statement is **correct** as many chordates exhibit gill slits during embryonic development.
3. **Statement about Organism S:**
- "The body of the organism S could differentiate head, visceral mass, and ventral foot."
- This statement is **incorrect** because it describes characteristics of **Mollusca**, while S is identified as a Cnidarian (polyp form).
4. **Statement about Organism R:**
- "Organism R could possess nematocytes."
- This statement is **correct** as nematocytes are stinging cells found in Cnidarians.
### Conclusion:
The incorrect option regarding organisms P, Q, R, and S is the statement about organism S.
