Which system is absent in ctenophores ?

Options

A. skeletal
B. circulatory
C. respiratory and excretory
D. all of these

Answer

**Step-by-Step Solution:**

1. **Understanding Ctenophores**: Ctenophores, also known as comb jellies, are a primitive phylum of marine invertebrates. They are characterized by their gelatinous bodies and the presence of cilia that they use for locomotion.

2. **Identifying the Systems**: In ctenophores, there are several biological systems that are typically found in more complex organisms. These include the skeletal, circulatory, respiratory, and excretory systems.

3. **Absence of Systems**: The key point is that ctenophores lack a distinct skeletal system, circulatory system, and respiratory system. Instead, they rely on diffusion for gas exchange and nutrient absorption through their body surface.

4. **Excretory Mechanism**: Ctenophores have a very simple excretory mechanism. They possess only one opening that serves to expel waste and excess fluids, indicating that a complex excretory system is absent.

5. **Conclusion**: Therefore, the systems that are absent in ctenophores include the skeletal system, circulatory system, and respiratory system. The correct answer to the question is that all these systems are absent in ctenophores.

**Final Answer**: The skeletal, circulatory, and respiratory systems are absent in ctenophores.

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