The first segment of the earthworm, the peristomium (see figure 1), contains the mouth. There is a small tongue-like lobe just above the mouth called the prostomium (see figure 1). Earthworms use the prostomium to see their environment, as earthworms have no eyes, ears, nose or hands.

Peristomium is the correct answer for the segment of an earthworm bearing the mouth. Let's understand why.

The earthworm is a segmented animal with a tube-like body divided into several segments called metameres. Each segment of the earthworm has a specific function and is characterized by certain features.

Peristomium:
The first segment of an earthworm is called the peristomium. It is also known as the buccal segment or the prostomium. This segment is located at the anterior end of the worm and bears the mouth. The peristomium is not involved in locomotion but is responsible for sensory perception and food intake.

Functions of Peristomium:
1. Mouth: The peristomium is the segment where the mouth is located. It contains the earthworm's mouth opening, which is surrounded by a specialized muscular structure known as the prostomial muscles. These muscles help in the movement and manipulation of food into the digestive system.

2. Sensory Perception: The peristomium contains sensory structures like chemoreceptors and mechanoreceptors. These receptors enable the earthworm to detect and respond to its environment, including the presence of food, predators, or other stimuli.

3. Mucus Secretion: The peristomium also plays a role in mucus secretion. Earthworms produce mucus to aid in locomotion and as a protective barrier against harmful substances. The mucus secreted by the peristomium helps in lubricating the mouth region and facilitating food intake.

Importance of Peristomium:
The peristomium is a crucial segment of the earthworm as it is involved in the initial stages of digestion and sensory perception. It allows the worm to detect and consume food, which is then further processed and digested in the subsequent segments of the digestive system.

In conclusion, the peristomium is the segment of the earthworm bearing the mouth. It is involved in food intake, sensory perception, and mucus secretion.

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