In cephalochordates, the circulatory system is peculiar because

Options

A. heart is absent
B. blood corpuscles are absent
C. blood is colourless
D. all the above

Answer

### Step-by-Step Solution:

1. **Understanding Cephalochordates**:
- Cephalochordates are a subphylum of the phylum Chordata. They are characterized by having a notochord, which is a flexible rod-like structure that provides support.

2. **Common Example**:
- A well-known example of a cephalochordate is Branchiostoma, commonly known as the lancelet. This organism is often studied to understand the basic features of cephalochordates.

3. **Circulatory System Characteristics**:
- The circulatory system in cephalochordates is unique and differs significantly from that of vertebrates.

4. **Absence of Blood Corpuscles**:
- One peculiar aspect of the circulatory system in cephalochordates is that blood corpuscles (red blood cells) are absent. This means that they do not have the typical cellular components found in the blood of vertebrates.

5. **Colorless Blood**:
- Another characteristic is that the blood is colorless. This is due to the absence of hemoglobin or any other pigments that would typically give blood its color in other organisms.

6. **Conclusion**:
- Therefore, the circulatory system of cephalochordates is peculiar because it lacks blood corpuscles and the blood is colorless. The correct answer to the question is that both statements are true, leading to the conclusion that "all of the above" is the correct answer.