Left systemic arch is absent in

Options

A. Reptiles only
B. mammals only
C. birds only
D. both birds and animals

Answer

To solve the question regarding the absence of the left systemic arch, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Left Systemic Arch**:
- The left systemic arch is a blood vessel present in the embryonic development of tetrapods, which include amphibians, reptiles, and mammals. It is responsible for carrying blood from the aorta to the trunk and hind limbs.

2. **Identifying Tetrapods**:
- Tetrapods are a group of animals that have four limbs. This includes amphibians (like frogs), reptiles (like lizards and snakes), and mammals (like humans).

3. **Function of the Left Systemic Arch**:
- The primary function of this arch is to facilitate blood circulation to the hind limbs and trunk of these organisms.

4. **Examining the Presence in Different Classes**:
- In amphibians and reptiles, the left systemic arch is present and functional.
- In mammals, it is also typically present, but we need to identify any exceptions.

5. **Investigating Birds**:
- Birds are a unique class of tetrapods that have adaptations for flight. To reduce their body weight for flight efficiency, certain structures are modified or absent.

6. **Conclusion**:
- The left systemic arch is notably absent in birds. This adaptation aids in reducing weight, which is crucial for flight.

### Final Answer:
The left systemic arch is absent in **birds**.

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