Which of the following characteristics is shared by protostomes and deuterostomes?

Options

A. They both have spiral cleavage of the embryo
B. Coelor develops in the same way in both
C. Mouth forms first in both, before the anus
D. They both form a hollow sphere in the embryo that has an indentation called a blastopore

Explanation

To answer the question regarding the shared characteristics of protostomes and deuterostomes, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Protostomes and Deuterostomes**:
- Protostomes and deuterostomes are two major groups of animals that differ in their embryonic development. Protostomes include organisms like mollusks and annelids, while deuterostomes include echinoderms and chordates.

2. **Identifying Common Characteristics**:
- Both protostomes and deuterostomes share certain developmental features. One of the key characteristics is the presence of a blastopore, which is the opening that forms during the early stages of embryonic development.

3. **Formation of the Coelom**:
- Both groups develop a coelom, which is a body cavity that arises during embryonic development. This cavity is important for the organization of internal organs.

4. **Development of the Archenteron**:
- The archenteron, which is the primitive gut formed during gastrulation, is present in both protostomes and deuterostomes. This structure eventually develops into the digestive system.

5. **Mesoderm Formation**:
- Both groups form a mesoderm germ layer, which is crucial for the development of muscles and other internal structures.

6. **Cleavage and Embryonic Stages**:
- Both protostomes and deuterostomes undergo cleavage, leading to the formation of an 8-cell stage and further development into a hollow sphere of cells known as a blastula.

7. **Conclusion**:
- Based on the shared characteristics discussed, we can conclude that a significant characteristic shared by both protostomes and deuterostomes is that they form a hollow sphere in the embryo that has indentations called the blastopore.

### Final Answer:
The characteristic shared by protostomes and deuterostomes is that they form a hollow sphere in the embryo that has indentations called the blastopore.

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