In higher chordates, pharyngeal gill slits are

Options

A. Absent
B. Present in the embryo
C. Replaced by abdominal gill slits
D. Replaced by thoracic gill slits

Answer

To answer the question "In higher chordates, pharyngeal gill slits are:", we will analyze the options and provide a step-by-step solution.

### Step-by-Step Solution:

1. **Understanding Pharyngeal Gill Slits**:
- Pharyngeal gill slits are openings that occur in the pharynx (the part of the throat behind the mouth) of chordates. In primitive chordates, these slits serve various functions, including filter feeding and respiration.

2. **Function in Primitive Chordates**:
- In primitive chordates, such as tunicates and lancelets, these gill slits are primarily used for filter feeding. They are located in the mouth area and help in trapping food particles.

3. **Function in Fishes and Amphibians**:
- In fishes and amphibians, the gill slits are adapted for respiration. They allow water to flow over the gills, facilitating gas exchange (oxygen and carbon dioxide) necessary for survival.

4. **Presence in Higher Chordates**:
- In higher chordates (such as mammals, birds, reptiles), pharyngeal gill slits are present only during the embryonic stage. They play a role in the development of structures such as the jaw and inner ear but are not retained in the adult form.

5. **Evaluating the Options**:
- **Absent**: This option is incorrect because gill slits are present during embryonic development.
- **Present in the embryo**: This is the correct option as gill slits do appear during the embryonic stages of higher chordates.
- **Replaced by abdominal gill slits**: This option is incorrect; there are no abdominal gill slits in higher chordates.
- **Replaced by thoracic gill slits**: This option is also incorrect; thoracic gill slits do not exist in higher chordates.

6. **Conclusion**:
- The correct answer is that in higher chordates, pharyngeal gill slits are present in the embryo.

### Final Answer:
In higher chordates, pharyngeal gill slits are **present in the embryo**.

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