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 "Left systemic arch is absent in", we will analyze the presence of systemic arches in different groups of animals step by step.

### Step 1: Understand the Systemic Arch
The systemic arch refers to the blood vessels that carry oxygenated blood from the heart to the body. In vertebrates, there are typically two systemic arches: the left systemic arch and the right systemic arch.

**Hint:** Remember that the systemic arches are important for the distribution of oxygenated blood in the body.

### Step 2: Identify the Groups of Animals
We need to consider the different classes of animals mentioned in the question: reptiles, mammals, and birds.

**Hint:** Think about the circulatory system of each group and how it varies among them.

### Step 3: Analyze Each Group
- **Reptiles:** Both left and right systemic arches are present.
- **Mammals:** Only the left systemic arch is present.
- **Birds:** Only the right systemic arch is present.

**Hint:** Focus on which groups have only one type of systemic arch and which have both.

### Step 4: Determine Where the Left Systemic Arch is Absent
From our analysis:
- In reptiles, both arches are present.
- In mammals, the left systemic arch is present.
- In birds, the left systemic arch is absent (only the right systemic arch is present).

**Hint:** The question specifically asks for the absence of the left systemic arch.

### Step 5: Conclusion
The left systemic arch is absent in **birds**.

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