Question

Sexual dimorphism is seen in:

• A. Cnidaria
• B. Porifera
• C. Nemathelminthes
• D. Platyhelminthes

Hint:

Nematodes (*Ascaris*) = Dioecious with marked sexual dimorphism.
Female *Ascaris* is longer with a straight tail; Male *Ascaris* is shorter with a curved tail bearing spicules.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the phylum among the given options that exhibits distinct, well-defined sexual dimorphism (morphological differences between male and female individuals).

Step 2: Key Formula or Approach:
Sexual dimorphism refers to the condition where males and females of the same species exhibit obvious phenotypic differences in secondary sexual characters, body size, shape, or atomy.

Step 3: Detailed Explation:

• Phylum Nemathelminthes (Aschelminthes Roundworms): Roundworms are predomintly dioecious (unisexual) with prominent sexual dimorphism.

• Example in *Ascaris lumbricoides*:
• Female: Considerably longer and thicker, with a straight posterior tail end.

• Male: Smaller and shorter, with a distinctly curved posterior tail bearing penial setae (spicules) for copulation.

• Comparison with Other Phyla:
• Porifera (Sponges): Mostly hermaphroditic or monoecious; lack distinct male/female body forms.

• Cnidaria: Show metagenesis/polymorphism (polyp and medusa phases), but not true sexual dimorphism.

• Platyhelminthes (Flatworms): Predomintly monoecious/hermaphroditic (e.g., *Plaria*, *Tapeworm*, *Liver fluke*), except for *Schistosoma*.

Step 4: Fil Answer:
Distinct sexual dimorphism is characteristic of phylum Nemathelminthes.