**Annelids** are characterized by having a ringlike appearance and their bodies are separated into about 100 segments with each segment containing organs. They also have a true coelom. **Roundworms** have unsegmented bodies with pseudocolom (false or incomplete coelom).

**Annelids** have a ringlike body segment appearance. The body of these organisms is divided into many segments that are separated from each other through a structure called septa. Each segment of their body is composed of similar structures such as eyes and other sense organs. Annelids have a true coelom, and this is covered by a mesoderm. There are three classes of **annelids** and they are **oligochaetes**, **leeches**, and **polychaetes**.

1. **Oligochaetes** – a group of annelids that contain a streamlined body and have fewer bristles (setae) compared to polychaetes. These organisms are usually found in soil or freshwater environments. They can be threadlike or tubelike in forms. Example: Earthworms.

2. **Leeches** – are a group of annelid organisms under class Hirudina. These organisms suck body fluids such as blood from their host organisms, hence they are considered parasites. Leeches contain powerful suckers from both ends of their body. One-fourth of their population feed on soft-bodied invertebrates such as those snails, worms, etc.

Another useful piece of information about these organisms is they are used in the medical field. They are used to treat medical conditions such as reducing the swelling after surgical operations and reducing blood clotting.

Example species: *Herudo verbana*

3. **Polychaetes** – are a group of annelids that are found in marine environments. They have paired, paddle-like appendages that contain setae. These setae appeared to be in brushlike form, and they are numerous. They usually thrive in crevices and cracks of coral reefs, rocks, or ocean sands. Example species: sandworms, and bloodworms.

**Annelids** have developed complex organ systems including their digestive system. These organisms range from being filter feeders to predators eating soft-bodied animals, or sucking body fluids from the hosts.

In earthworms, they use their pharynx to swallow soil and other organic matters into their stomach where nutrients are grind and absorbed. Similarly, leeches use pharynxes to obtain the blood of the host into their stomach where nutrients are turned into energy.

It is beneficial to have light sensitive cells on both ends of the body because the animal can sense predators from both ends, thereby protecting its life. It is like having eyes on both ends. It is an important survival mechanism for them.