The most likely ancestors of chordates are considered to be echinoderms because :

Options

A. tunicates are echinoderms
B. there is a direct lineage from sea stars to tunicates
C. embryos of some echinoderms are similar to primitive chordate embryos
D. all of the above are correct

Explanation

To answer the question regarding why echinoderms are considered the most likely ancestors of chordates, we can break down the reasoning into several steps:

### Step-by-Step Solution:

1. **Understanding Chordates**:
- Chordates are a group of animals that possess certain key features at some stage of their life cycle. These features include a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits.

2. **Characteristics of Echinoderms**:
- Echinoderms, such as starfish and sea urchins, are marine animals that also exhibit some primitive features that are similar to those found in chordates.

3. **Common Ancestry**:
- Both chordates and echinoderms are classified as deuterostomes. This classification suggests that they share a common ancestor, which is a significant point in evolutionary biology.

4. **Embryonic Similarities**:
- The embryos of some echinoderms show similarities to the primitive embryos of chordates. This similarity in embryonic development supports the idea that echinoderms could be the ancestors of chordates.

5. **Conclusion**:
- Based on the shared characteristics and embryonic similarities, it is concluded that echinoderms are considered the most likely ancestors of chordates.

### Final Answer:
The most likely ancestors of chordates are considered to be echinoderms because the embryos of some echinoderms are similar to the primitive chordate embryos, and both groups share a common ancestry as deuterostomes.

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