Select the correct order of phyla on the basis of the number of species.

Options

A. Mollusca `gt` Arthropoda `gt` Chordata
B. Arthropoda `gt` Mollusca `gt` Chordata `gt` Aschelminthes
C. Arthropoda `gt` Chordata `gt` Mollusca
D. Arthropoda `gt` Aschelminthes `gt` Chordata

Answer

To solve the question of selecting the correct order of phyla based on the number of species, we need to analyze the different phyla mentioned and their respective species counts. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Phyla
The phyla mentioned in the options are:
1. Arthropoda
2. Mollusca
3. Chordata
4. Aschelminthes (also known as Nematoda)

### Step 2: Understand the Characteristics and Species Count
- **Arthropoda**: This is the largest phylum in the animal kingdom, comprising a vast number of species, primarily insects. It is estimated that about 75% of all known animal species belong to this phylum.

- **Mollusca**: This is the second largest phylum, but it has significantly fewer species compared to Arthropoda. Examples include snails, octopuses, and clams.

- **Chordata**: This phylum includes vertebrates and some invertebrates. While it is diverse, it has fewer species than Arthropoda and is less populous than Mollusca.

- **Aschelminthes**: This phylum includes roundworms and is known for having a considerable number of species, but still fewer than Arthropoda.

### Step 3: Compare the Number of Species
Based on the analysis:
- **Arthropoda** has the highest number of species.
- **Mollusca** comes next but has fewer species than Arthropoda.
- **Aschelminthes** has a significant number of species but is still less than Arthropoda.
- **Chordata** has the fewest species among these phyla.

### Step 4: Determine the Correct Order
From the analysis, the correct order of phyla based on the number of species is:
1. **Arthropoda** (most species)
2. **Mollusca** (second most)
3. **Aschelminthes** (third)
4. **Chordata** (least)

### Conclusion
The correct order of phyla based on the number of species is:
**Arthropoda > Mollusca > Aschelminthes > Chordata**