Question

In the life history and development of sea star, numerous larval forms are formed. Arrange them in the order of their occurrence:
A. Dipleurula
B. Branchiolaria
C. Bipinnaria
Choose the most appropriate answer from the options given below:

• A. A, B, C
• B. A, C, B
• C. B, A, C
• D. C, B, A

Hint:

Remember the alphabetical order trick for the main sea star larvae: \textbf{B}ipinnaria comes before \textbf{Br}achiolaria. The dipleurula is the conceptual starting form for all echinoderm larvae. So, Dipleurula \(\rightarrow\) Bipinnaria \(\rightarrow\) Brachiolaria.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The development of sea stars (Class Asteroidea) involves indirect development with a series of distinct, free-swimming, bilaterally symmetrical larval stages before metamorphosis into a radially symmetrical adult.

Step 2: Detailed Explanation of the Order:
1. Dipleurula (A): The dipleurula is considered the hypothetical ancestral larva for echinoderms. The very first, early larval stage of a sea star that develops from the gastrula is a dipleurula-type larva, which then develops into the bipinnaria. So, this is the starting point.
2. Bipinnaria (C): This is the first well-defined larval stage of the sea star. It develops from the early larva (dipleurula concept) and is characterized by its bilaterally symmetrical body with ciliated bands used for locomotion and feeding. It does not have any attachment arms.
3. Brachiolaria (B): The bipinnaria larva develops into the brachiolaria larva. This is the final larval stage. It is distinguished by the growth of three additional, short, non-ciliated arms (brachiolar arms) at the anterior end, with an adhesive disc. These structures are used for attachment to a substrate before undergoing metamorphosis.

Step 3: Final Answer:
The correct chronological order of larval development is Dipleurula (early larva) \(\rightarrow\) Bipinnaria \(\rightarrow\) Brachiolaria. This corresponds to the sequence A, C, B. Therefore, option (B) is correct.