Urochordates are

Options

A. Exclusively marine
B. Exclusively fresh water
C. Terrestrial
D. Amphibious

Answer

### Step-by-Step Solution:

1. **Definition of Urochordates**: Urochordates, also known as tunicates, are a subphylum within the phylum Chordata. They are characterized by their unique body structure and developmental stages.

2. **Notochord Presence**: In urochordates, the notochord is present only during the larval stage, specifically in the tail region. This is a key feature that distinguishes them from other chordates.

3. **Adult Form Characteristics**: As urochordates mature into their adult form, they lose the notochord. This is an unusual characteristic compared to other chordates where the notochord is retained in some form.

4. **Habitat**: Urochordates are exclusively marine organisms. They are not found in freshwater environments, which is an important aspect of their classification.

5. **Exoskeleton Composition**: Urochordates possess an exoskeleton made of a substance called tunicin, which is a defining feature of their anatomy.

6. **Classification**: While urochordates share some similarities with fish, they are not classified as fish due to their distinct characteristics and classification under the subphylum tunicates.

7. **Conclusion**: Based on the above points, urochordates can be summarized as exclusively marine organisms that exhibit a unique developmental pattern, losing their notochord in adulthood and having a tunicin-based exoskeleton.