Difference Between Prostomium and Peristomium

Main Difference – Prostomium vs Peristomium

Prostomium and peristomium are the two segments of the annelid‘s body. Annelids are triploblastic animals with a bilateral symmetry. They are protostomes whose blastopore develops into the mouth. Annelids are segmented worms as well. Their body wall, body cavity, nervous system, and the kidneys are segmented. The prostomium is the head portion of the annelids and is a fleshy segment. The peristomium is the first segment of the annelids body. The main difference between prostomium and peristomium is that prostomium is in front of the mouth of the annelids whereas peristomium surrounds the opening of the mouth. The prostomium consists of sensory organs which help annelids to thrust into the soil.

Key Areas Covered

1. What is Prostomium
      – Definition, Characteristic Features, Function
2. What is Peristomium
      – Definition, Characteristic Features, Function
3. What are the Similarities Between Prostomium and Peristomium
      – Outline of Common Features
4. What is the Difference Between Prostomium and Peristomium
      – Comparison of Key Differences

Key Terms: Annelids, Chetae, Eyespots, Mouth, Palps, Peristomium, Prostomium, Pygidium, Segmented Body, Trunk

What is Prostomium

The prostomium refers to a portion of the head region of the annelids. The annelid’s body can be divided into three sections; prostomium, trunk, and pygidium. The anterior end of the body is called the prostomium. The mouth of the annelids is just behind the prostomium. Some annelids possess a tongue-like structure above the mouth. The region which surrounds the mouth is called the peristomium. The trunk consists of many segments, which form the bulk of the body of the annelid. The pygidium is the posterior portion of the annelid. The growth of the annelid occurs by adding segments to the front of the pygidium. The prostomium and the skin of the annelids consist of sensory receptors to gather sensory information from the environment. The prostomium is also composed of light-sensitive structures called eyespots. The prostomium and the mouth of the annelid are shown in figure 1.

Difference Between Prostomium and Peristomium

Figure 1: Prostomium and Mouth of Annelids

The caruncle and nuchal organ are the other types of sensory organs found in the prostomium. Some appendages like tentacles, palps or cirri also occur in the prostomium.

What is Peristomium

The peristomium refers to the anterior portion of annelids. It surrounds the mouth, which is located in the prostomium. Generally, the peristomium is not considered as a true segment since it does not contain any organ. Some annelids have appendages called chaetae in the peristomium. The prostomium and peristomium are shown in figure 2.

Main Difference - Prostomium vs Peristomium

Figure 2: Annelid head structures
a – Palp, b – prostomium, c – Peristomium, d -eye, e – occipital antenna, f – parapodium with chaetae

Since the growth of the annelids occurs from the posterior part, the peristomium is the oldest segment of the annelids.

Similarities Between Prostomium and Peristomium

Prostomium and peristomium are two segments of the body of annelids.
Both prostomium and peristomium are situated at the anterior section of the annelid’s body.

Definition

Prostomium: Prostomium refers to a portion of the head of the annelids, which is situated in front of the mouth.

Peristomium: Peristomium refers to the front segment of annelids, which surrounds the mouth.

Location

Prostomium: Prostomium is the head portion of the annelid’s body.

Peristomium: Peristomium is the first segment of the annelid’s body.

Age

Prostomium: Prostomium is an older segment of the annelids body.

Peristomium: Peristomium is the oldest segment of the body.

Composition

Prostomium: Prostomium is composed of the mouth and sensory organs of annelids.

Peristomium: Peristomium does not contain any organs.

True Segment

Prostomium: The prostomium is a true segment of the body of the annelids.

Peristomium: The peristomium is not considered as a true segment of the annelid’s body.

Function

Prostomium: The prostomium helps to push the body of annelids into the soil.

Peristomium: The peristomium surrounds the opening of the mouth.

Sensory Function

Prostomium: Prostomium consists of sensory organs of annelids.

Peristomium: Peristomium does not contain any sensory function in annelids.

Conclusion

Prostomium and peristomium are the two front segments of the annelids. Since annelids are segmented worms, the organs of the body are arranged into different segments. The prostomium is the head portion of the body of annelids and contains mouth and sensory organs like eyespots. The peristomium is the first and the oldest segment of the annelid’s body and surrounds the mouth. The main difference between prostomium and peristomium is the structure and the function of each segment.

Reference:

1. Wilkin, Ph.D. Douglas, and Jennifer Blanchette. “15.26: Annelid Structure and Function – Advanced DRAFT.” CK-12 Foundation, 16 June 2017, Available here. Accessed 15 Sept. 2017.
2.“Peristomium.” Wikipedia, Wikimedia Foundation, 5 Dec. 2016, Available here. Accessed 15 Sept. 2017.

Image Courtesy:

1. “Earthworm head” By KDS4444 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Lysidice oele anterior anatomy” By Joko Pamungkas – Pamungkas J (2015) Species richness and macronutrient content of wawo worms (Polychaeta, Annelida) from Ambonese waters, Maluku, Indonesia. Biodiversity Data Journal 3: e4251. doi:10.3897/BDJ.3.e4251 (CC BY 4.0) via Commons Wikimedia