Difference Between Prostomium and Peristomium

Key Difference – Prostomium vs Peristomium
 

Prostomium and Peristomium are two parts found in the head region of the annelids. The key difference between prostomium and peristomium is that the prostomium contains sensory organs and a mouth region while the peristomium is present within the mouth region of the prostomium and does not contain any sensory organs.

Phylum Annelida is a large group of ringed worms that are segmented. Earth worms, leaches, and marine worms are included in this phylum. Annelid body is made up of three main segments namely head region, metameres, and pygidium (terminal region). The head region contains prostomium and peristomium.

CONTENTS

1. Overview and Key Difference
2. What is Prostomium
3. What is Peristomium
4. Similarities Between Prostomium and Peristomium
5. Side by Side Comparison – Prostomium vs Peristomium in Tabular Form
6. Summary

What is Prostomium?

The Prostomium is defined as the region of the head of annelids that is present in front of their mouth region. A typical annelids body could be divided into three major parts. These include the prostomium, the trunk and the pygidium. Therefore, the prostomium is the anterior region of the body.

It is composed of the mouth region and sensory organs. The presence of sensory organs provides the unique characteristic feature to the prostomium. The function of these sensory organs is to detect different sensory stumbling from the outside environment.

The prostomium also contains light-sensitive structures known as eyespots. The sensory organs that are present in the prostomium include caruncle and nuchal organ. Also, the prostomium might possess tentacles, cirri and palps in different species of annelids.

What is Peristomium?

The peristomium is defined as the tongue like the segment that is present in the front that surrounds the mouth. The mouth region is located in the prostomium. Therefore, it could be said that the peristomium is present within the prostomium.

The peristomium does not contain any organs including sensory receptors, etc. Since no organs are present, the peristomium is not considered as a true segment. With regards to the function, the peristomium does not possess any unique functions due to the absence of organs.  Some species of annelids possess appendages in the peristomium. These appendages are referred to as chaetae.

It is mentioned that the prostomium contains tentacles, cirri and palps in different species of annelids. Since the peristomium is actually present within the prostomium, these tentacles, cirri and palps are present in the peristomium.

What are the Similarities Between Prostomium and Peristomium?

• Both Prostomium and Peristomium are segments of the annelids body
• Both are present in the anterior region of the body.

What is the Difference Between Prostomium and Peristomium?

Prostomium vs Peristomium
Prostomium is defined as the region of the head of annelids that is present in front of their mouth region.	Peristomium is defined as the segment that is present in the front which surrounds the mouth.
Composition
The Prostomium is composed of annelids sensory organs and mouth region.	The Peristomium does not compose of special organs.
Major Function
It assists the annelids body to be pushed into the soil.	No major function in having the peristomium.
Presence of Sensory Function
Presence of sensory organs in prostomium involves in sensory coordination.	No sensory function since sensory organs are absent in peristomium.
Segmental Age Difference
Comparatively the Prostomium is less aged than the Peristomium.	Peristomium is considered as the oldest segment of the annelids body.
Location
Prostomium is located in the head region.	Peristomium is located in the first part of the body in the front region.
Organs
The Prostomium has caruncle, nuchal organ and eye spots.	No special organs are present in peristomium.
True Segment
The Prostomium is considered as a true segment of the annelids body.	The Peristomium is not considered as a true segment of the body.
Appendages
A mouth region is present in prostomium.	Chaetae are present in the peristomium.

Summary – Prostomium vs Peristomium

The body of the annelid is divided into three main regions. They are the prostomium, the trunk and the pygidium. The prostomium contains a mouth region and sensory organs. The peristomium does not contain any sensory organs. The peristomium is considered as the oldest segment of the annelids’ body. The prostomium is considered as a true segment of the body while the peristomium is not. This is the difference between prostomium and peristomium.

Reference:

1.“Prostomium.” Encyclopædia Britannica, Encyclopædia Britannica, Inc. Available here 
2.Fauchald, Kristian. “Worms, Annelida.” Encyclopedia of Biodiversity, 2001, pp. 831–842., doi:10.1016/b0-12-226865-2/00285-6.

Image Courtesy:

1.’Epilobus prostomium (ie 1st segment) (4482502155)’By Gilles San Martin from Namur, Belgium (CC BY-SA 2.0) via Commons Wikimedia 
2.’Lysidice oele anterior anatomy’By Joko Pamungkas (CC BY 4.0) via Commons Wikimedia