Excretory system is absent in
(a) Echinodermata
(b) Mollusca
(c) Annelids
(d) Arthropods

Options

A. Echinodermata
B. Mollusca
C. Annelids
D. Arthropods

Answer

To solve the question regarding which group of organisms lacks an excretory system, we will analyze each option provided:

1. **Understanding the Excretory System**:
- The excretory system is responsible for the removal of nitrogenous waste from the body. This waste can include substances like ammonia, urea, and uric acid.
- Additionally, excretory systems often play a role in osmoregulation, which is the regulation of water and electrolyte balance in the body.

2. **Analyzing Each Option**:
- **(a) Echinodermata**:
- Echinoderms, such as starfish and sea urchins, do not have a traditional excretory system. Instead, they utilize a water vascular system for nutrient uptake and waste removal. Therefore, they do not possess specialized excretory organs.
- **(b) Mollusca**:
- Mollusks, including snails and squids, have an excretory system that includes structures called nephridia. These organs are responsible for filtering waste from the blood and excreting it.
- **(c) Annelids**:
- Annelids, such as earthworms, also possess nephridia as their excretory structures. This indicates that they do have an excretory system.
- **(d) Arthropods**:
- Arthropods, which include insects and crustaceans, have various excretory structures, the most notable being Malpighian tubules. These structures help in the excretion of waste products.

3. **Conclusion**:
- After analyzing all the options, it is clear that the only group that lacks a traditional excretory system is **Echinodermata**. Therefore, the correct answer is **(a) Echinodermata**.

### Final Answer:
**(a) Echinodermata**