Assertion Branchiostoma and Balanoglossus are bilaterally symmetrical and triplobastic animals
Reason They are exlusively marine and possess notochord

Answer

To solve the question regarding the assertion and reason about Branchiostoma and Balanoglossus, we can break it down into clear steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Branchiostoma and Balanoglossus are bilaterally symmetrical and triploblastic animals."
- Both Branchiostoma (also known as lancelets) and Balanoglossus (acorn worms) exhibit bilateral symmetry, meaning their body can be divided into two mirror-image halves.
- They are also triploblastic, which means they have three primary germ layers: ectoderm, mesoderm, and endoderm.

2. **Evaluating the Reason**:
- The reason provided is "They are exclusively marine and possess notochord."
- While it is true that both Branchiostoma and Balanoglossus are marine organisms, the statement about the notochord is only partially correct.
- Branchiostoma possesses a notochord that persists throughout its life, which is a characteristic of chordates.
- However, Balanoglossus does not possess a true notochord; instead, it has a structure called a stomochord, which is not considered a true notochord.

3. **Conclusion**:
- The assertion is correct because both Branchiostoma and Balanoglossus are indeed bilaterally symmetrical and triploblastic.
- The reason is incorrect because while they are marine, Balanoglossus does not possess a true notochord.
- Therefore, the assertion is true, but the reason is false.

### Final Answer:
- **Assertion**: True
- **Reason**: False