Which of the following is not a trend seen in the evolution of animals ?

Options

A. Ancestral forms with two openings in digestive tract `to`derived forms with one opening in digestive tract
B. Ancestral forms with open circulatory system `to` derived forms with closed circulatory system
C. Ancestral forms with one or two embryonic germ layers `to` derived forms with three distinct germ layers
D. Ancestral forms with radial symmetry `to` derived forms with bilateral symmetry

Explanation

To solve the question "Which of the following is not a trend seen in the evolution of animals?", we need to analyze each of the provided options to determine which one does not align with the known evolutionary trends in animal biology.

### Step-by-Step Solution:

1. **Understand the Options**: We have four options to consider. Each option describes a relationship between ancestral forms of animals and their derived forms.

2. **Analyze Option 1**:
- **Statement**: Ancestral forms with two openings in the digestive tract and derived forms with one opening in the digestive tract.
- **Evaluation**: This statement is incorrect. Primitive animals (like some cnidarians) typically have one opening (gastrovascular cavity), while more evolved animals (like nematodes) possess two openings (mouth and anus). Therefore, this is not a trend seen in evolution.

3. **Analyze Option 2**:
- **Statement**: Ancestral forms with an open circulatory system and derived forms with a closed circulatory system.
- **Evaluation**: This statement is true. Ancestral forms such as prawns have an open circulatory system, while higher animals have a closed circulatory system. This reflects a true evolutionary trend.

4. **Analyze Option 3**:
- **Statement**: Ancestral forms with one or two embryonic germ layers and derived forms with three distinct germ layers.
- **Evaluation**: This statement is also true. Ancestral forms like porifera are diploblastic (two germ layers), while more complex animals (like humans) are triploblastic (three germ layers). This is a recognized trend in evolution.

5. **Analyze Option 4**:
- **Statement**: Ancestral forms with radial symmetry and derived forms with bilateral symmetry.
- **Evaluation**: This statement is true as well. Radial symmetry is seen in simpler organisms (like porifera), while bilateral symmetry is characteristic of more complex organisms. This is indeed a trend in evolution.

6. **Conclusion**:
- After analyzing all options, we find that the first option is the only one that does not represent a trend in the evolution of animals. Thus, the correct answer to the question is: **Ancestral forms with two openings in the digestive tract and derived forms with one opening in the digestive tract**.

### Final Answer:
The option that is not a trend seen in the evolution of animals is: **Ancestral forms with two openings in the digestive tract and derived forms with one opening in the digestive tract**.