Which of these pairs of organisms are bilaterally symmetrical and triploblastic ?

Options

A. Ascaris , Planaria
B. Spongilla , Limulus
C. Fasciola , Echinus
D. Hirudinaria , Ctenoplane

Answer

To determine which pairs of organisms are bilaterally symmetrical and triploblastic, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Bilateral Symmetry**:
- Bilateral symmetry means that an organism can be divided into two equal halves along a single plane. This is typically seen in organisms with a distinct head and tail (anterior and posterior).

2. **Understanding Triploblastic**:
- Triploblastic organisms have three germ layers: ectoderm, mesoderm, and endoderm. This is in contrast to diploblastic organisms, which only have two layers (ectoderm and endoderm).

3. **Analyzing Each Option**:
- **Option A: Ascaris and Planaria**
- Ascaris (a roundworm) belongs to the phylum Nematoda and is bilaterally symmetrical and triploblastic.
- Planaria (a flatworm) belongs to the phylum Platyhelminthes and is also bilaterally symmetrical and triploblastic.
- **Conclusion**: This option is correct.

- **Option B: Spondyla and Limulus**
- Spondyla (a type of sponge) is asymmetrical and does not exhibit bilateral symmetry.
- Limulus (horseshoe crab) is bilaterally symmetrical and triploblastic.
- **Conclusion**: This option is incorrect due to Spondyla.

- **Option C: Fasciola and Echinus**
- Fasciola (a liver fluke) is bilaterally symmetrical and triploblastic.
- Echinus (a sea urchin) belongs to the phylum Echinodermata, which exhibits radial symmetry.
- **Conclusion**: This option is incorrect due to Echinus.

- **Option D: Herodinaria and Tinoplana**
- Herodinaria (a type of annelid) is bilaterally symmetrical and triploblastic.
- Tinoplana (a ctenophore) exhibits radial symmetry.
- **Conclusion**: This option is incorrect due to Tinoplana.

4. **Final Answer**:
- The only pair that is both bilaterally symmetrical and triploblastic is **Option A: Ascaris and Planaria**.