Hemichordates have now been placed with the chordates close to echinoderms because:

Options

A. Notochord is absent
B. Pharyngeal gill - slits are lacking
C. Dorsal nerve cord is absent
D. Heart is lacking

Answer

### Step-by-Step Solution:

1. **Understanding Hemichordates**:
Hemichordates are a group of marine animals that share some characteristics with chordates and echinoderms. They are primarily known for their body structure and developmental features.

2. **Key Characteristics of Hemichordates**:
- **Presence of Gill Slits**: Hemichordates possess pharyngeal gill slits, which are also a characteristic feature of chordates.
- **Dorsal Nerve Cord**: They have a nerve cord that runs along the dorsal side of their body, similar to chordates.
- **Post-anal Tail**: Hemichordates exhibit a post-anal tail, which is another feature shared with chordates.

3. **Comparison with Chordates and Echinoderms**:
- **Chordates**: Chordates are defined by the presence of a notochord, dorsal nerve cord, pharyngeal gill slits, and a post-anal tail. Hemichordates share some of these features but lack a true notochord.
- **Echinoderms**: Echinoderms, such as starfish and sea urchins, are a separate group but share a common ancestry with hemichordates and chordates.

4. **Phylogenetic Placement**:
- Hemichordates are placed close to chordates in the evolutionary tree because of their shared characteristics, particularly the presence of gill slits and a dorsal nerve cord.
- They are considered to be more closely related to chordates than to other invertebrate groups due to these similarities.

5. **Conclusion**:
Hemichordates are classified near chordates and echinoderms due to their anatomical features, particularly the presence of gill slits and a dorsal nerve cord, despite lacking a true notochord.