True coelom appeared first in the course of evolution in

Options

A. Echinodermata
B. Annelida
C. Chordata
D. Aschelminthes

Answer

To answer the question regarding where true coelom first appeared in the course of evolution, we can follow these steps:

### Step 1: Understand the concept of coelom
A coelom is a fluid-filled body cavity that is completely surrounded by mesoderm tissue. It is important for the development of complex organs and systems in animals.

### Step 2: Identify the groups mentioned in the question
The question provides four options:
- A) Echinodermata
- B) Annelida
- C) Chordata
- D) Platyhelminthes

### Step 3: Analyze each group for the presence of true coelom
- **Porifera (sponges)**: They do not have a coelom; they are considered acoelomate.
- **Platyhelminthes (flatworms)**: They are also acoelomate, meaning they lack a true coelom.
- **Nematoda (roundworms)**: They possess a pseudocoelom, which is not a true coelom as it is not completely lined by mesoderm.
- **Annelida (segmented worms)**: This group is the first to exhibit a true coelom. Annelids have a body cavity that is fully lined by mesoderm.
- **Echinodermata (starfish, sea urchins)** and **Chordata (vertebrates)** also have true coeloms, but they evolved after Annelida.

### Step 4: Conclusion
Based on the analysis, the first appearance of a true coelom in the course of evolution is found in the phylum **Annelida**.

### Final Answer
The correct answer is **B) Annelida**.

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