Annelids

Earthworms, lugworms, leeches, and polychaetes are annelids. Their physiological characteristics bring them under the Phylum Annelida. Annelids have a different type of body structure with a primitive neurological system. Their segmented body is connected by a simple nervous system with a primitive central organ to control. In this section, we will discuss the nervous system of annelid worms and find out how it acts to control the different physiological functions.

What is an Annelid?

To understand the Annelida meaning, let us discuss the characteristics of this phylum first.

• Long, slender, and segmented body

• These animals are bilaterally symmetrical

• They have three layers of embryonic cells (endoderm, mesoderm, and ectoderm) and are triploblastic in nature

• The worms show the organization of tissues to form primitive organs

• A thin cuticle covers the entire body

• These worms have body cavities or coeloms.

• Annelids prefer living in moist environments, freshwater, moist soils, and marine habitats.

• A nervous system is present

• Digestive and excretory systems are present

• They are invertebrates but have a closed circulatory system

There are many other features that determine the primitive organ systems of annelids. One such system is the formation of a simple nervous system with a brain-like ganglionic structure present near the mouth. There are 17,000 types of annelids present in land and water habitats. These are the common features that all the annelids share.

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Nervous System of Annelids

The phylum Annelida examples are earthworms, leeches, and lugworms. These are the three common examples you will find around you. As per the characters of phylum annelid, all the organisms in this classification have a similar configuration of the nervous system. The organ differentiation can be specifically witnessed in the nervous system. This cites how the complexity in the organ system develops as we move up in the phylum categories of the invertebrates and then in the vertebrates. Here is a brief description of the nervous system of the annelids.

• The Brain

The nervous system of the Annelida species forms a small brain-like structure right near the mouth end. The leeches and earthworms have similar nervous systems where a ganglia structure is present. It is a concentration of nerve tissues and endings developed to control physiological functions.

The brain lies just above the pharynx in the 3rd segment of the Annelida animals. It has two lobes from where the sensory nerves start originating and spreading to the entire body. The formation of the bi-lobed brain shows the complex formation of a nervous system in the animal kingdom. The nerves originate and move to the first segment present at the anterior end and the posterior end of the last segment.

• Sensory Nerves

The sensory nerves are segmented or differentiated into a ventral nerve cord and transverse connectives. The pair of ventral nerve cords extends till the posterior end of the Annelida earthworm. Every segment in the body contains a swelling or collection of nerve endings to form a ganglion. These ganglions are the meeting points of the transverse connectives and the ventral nerve cords. The most prominent among these ganglions are anterior and sub-pharyngeal are the most prominent ones.

Each ganglion leaves 2 to 5 lateral nerve pairs for the innervations of each segment. The subepidermal plexus is scattered throughout the entire body. The stomodaeal, enteric, or sympathetic plexus spreads inside the gut wall. This is a prime annelid feature that makes this phylum different from the rest.

What Polychaetes Nervous System is Different?

Marine annelids are somewhat different from land and freshwater annelids concerning their nervous system. It is more complicated in the polychaetes as they have a primitive brain differentiated into fore, mid, and hindbrain showing the stage-wise development of the system in the animal kingdom.

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Marine annelids or polychaetes have bristles protruding from the circular segments of their bodies. These bristles help them swim or move on the marine floor and are made of chitin. Most of them scavenge the ocean floors but some are good predators. The predators hide and wait for the prey in the cracks, rocks, reefs, etc. Their complex movement and other functions need a complex nervous system.

Functions of the Nervous System of Annelida

The primitive nervous system of the land and marine annelids help them to move. Most annelids on the land move due to a well-coordinated compression and relaxation of the segment muscles. You might have seen the movement of the earthworms and leeches. A part of them squeezes and the rest of it elongates. When they feel threatened, they can increase their speed or squeeze drastically as an evasive manoeuvre. On the other hand, predatory marine annelids have good reflexes. The nervous system helps them to detect prey and catch them when they are in the range.

Conclusion

The nervous system of annelids is the most primitive one that shows how organ differentiation begins in the primitive phyla. The presence of a nervous system means the body has to coordinate its actions throughout the segments. The two lobes of the ganglionic brain are the origin of the sensory nerves continuing from the anterior to the posterior end. The nervous system of the polychaetes, a type of annelids, is more advanced than the rest.