Triploblastic, acoelomate and bilateral symmetry with most of parasitic forms. The above description is characteristic of the phylum :-

Options

A. Protozoa
B. Annelida
C. Nematoda
D. Platyhelminthes

Answer

To solve the question regarding the characteristics of a specific phylum based on the description provided, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Description**: The question describes an organism that is triploblastic, acoelomate, and exhibits bilateral symmetry, with a majority of its forms being parasitic.

2. **Defining Key Terms**:
- **Triploblastic**: Organisms that have three germ layers - ectoderm, mesoderm, and endoderm.
- **Acoelomate**: Organisms that lack a true coelom (body cavity).
- **Bilateral Symmetry**: Organisms that can be divided into two mirror-image halves along one plane.

3. **Analyzing Options**:
- **Protozoans**: Generally unicellular and do not fit the description as they lack the mentioned body plan.
- **Annelids**: These are coelomate organisms, meaning they have a body cavity, thus they do not fit the description of being acoelomate.
- **Nematodes**: These are pseudocoelomates, which means they have a body cavity but not a true coelom, so they also do not fit the description.
- **Platyhelminthes**: This phylum includes flatworms, which are triploblastic, acoelomate, and exhibit bilateral symmetry. Many members of this phylum are parasitic (e.g., tapeworms).

4. **Conclusion**: Based on the analysis, the phylum that matches the description of being triploblastic, acoelomate, and exhibiting bilateral symmetry with most forms being parasitic is **Platyhelminthes**.

### Final Answer:
The phylum characterized by being triploblastic, acoelomate, and having bilateral symmetry with most forms being parasitic is **Platyhelminthes**.

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