Members of which phylum lack a well- defined circulatory system and respiratory system?

Options

A. Aschelminthes
B. Annelida
C. Arthropoda
D. Hemichordata

Answer

To solve the question regarding which phylum lacks a well-defined circulatory system and respiratory system, we can analyze the characteristics of the given phyla: Aschelminthes, Annelida, Arthropoda, and Hemichordata.

### Step-by-Step Solution:

1. **Identify the Phyla**: The question lists four phyla: Aschelminthes, Annelida, Arthropoda, and Hemichordata. We need to evaluate each one to determine which lacks a well-defined circulatory and respiratory system.

2. **Analyze Aschelminthes**:
- Commonly known as roundworms.
- They can be free-living or parasitic.
- They are found in various environments, including soil and fresh water.
- They have a digestive system with an alimentary canal and muscular pharynx.
- **Circulatory and Respiratory Systems**: Aschelminthes lack a well-defined circulatory and respiratory system. Gaseous exchange occurs through the body surface.

3. **Analyze Annelida**:
- Annelids are segmented worms.
- They have a well-developed circulatory system (closed circulatory system) and a complex digestive system.
- Respiration occurs primarily through their moist skin.
- **Conclusion**: Annelida do not fit the criteria as they have a well-defined circulatory and respiratory system.

4. **Analyze Arthropoda**:
- Arthropods have an open circulatory system and a body cavity known as a hemocoel.
- They respire through various means, including gills or tracheae, depending on the class (e.g., insects, crustaceans).
- **Conclusion**: Arthropoda do not fit the criteria as they have a circulatory and respiratory system.

5. **Analyze Hemichordata**:
- Hemichordates have gill slits that assist in respiration.
- They also possess an open circulatory system.
- **Conclusion**: Hemichordata do not fit the criteria as they have a circulatory and respiratory system.

6. **Final Conclusion**: After analyzing all the phyla, it is clear that **Aschelminthes** is the only phylum that lacks a well-defined circulatory and respiratory system.

### Answer:
The members of the phylum **Aschelminthes** lack a well-defined circulatory system and respiratory system.