Assertion : External parasitism is generally marked by much more exterme specialisation than internal parasitism.
Reason : The structure of an internal parasite is usually very complex possessing suckers, reproductive organs, etc.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion:** The assertion states that "External parasitism is generally marked by much more extreme specialization than internal parasitism." To analyze this, we need to define what external (ectoparasites) and internal (endoparasites) parasitism are. Ectoparasites live on the surface of the host (e.g., lice), while endoparasites live inside the host's body (e.g., tapeworms).

2. **Evaluating the Assertion:** Ectoparasites do not require extreme specialization because they are in a relatively stable environment outside the host. They primarily need adaptations to attach to the host and feed. In contrast, endoparasites must adapt to survive in the host's internal environment, which is more variable and hostile (e.g., acidic stomach, digestive enzymes). Therefore, the assertion is incorrect.

3. **Understanding the Reason:** The reason states that "The structure of an internal parasite is usually very complex, possessing suckers, reproductive organs, etc." This implies that internal parasites have a complicated structure due to their adaptations.

4. **Evaluating the Reason:** While it is true that many endoparasites have suckers and reproductive organs, the overall structure of internal parasites is not complex. Over evolutionary time, many endoparasites have lost unnecessary structures (like digestive systems) because they obtain nutrients directly from the host. Thus, their structure is often simpler, not more complex. Therefore, the reason is also incorrect.

5. **Conclusion:** Both the assertion and the reason are incorrect. The correct answer is that both statements are false.

**Final Answer:** Option D: Both the assertion and the reason are incorrect.

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