[Biology Class Notes] on Annelida Pdf

Annelida meaning, it is a phylum that is derived from the Latin word annellus, which means a small ring. They show a protostome development in embryonic stages and thus annelids are also referred to as “segmented worms”. They are widely found in marine, terrestrial and freshwater habitats. Annelids include earthworms, leeches, Polychaete worms.

Approximately 15000 species are categorized under this phylum of which mostly are polychaetes. Annelids have well-developed neuronal and digestive systems and their reproductive strategies include sexual dimorphism.

Classification of Annelida

Below mentioned is the classification of annelida:-

Kingdom: Animalia

Subkingdom: Eumetazoa

Superphylum: Lophotrochozoa

General Characteristics of Annelida

Annelida species range in length from less than 1 millimeter to over 3 meters. All annelids are bilaterally symmetrical, with an elongated cylindrical body shape divided both externally and internally by regular, linear series of segments. 

The distinctive characters of Annelida include:

1. Metamerism

Annelids have three body regions. The majority of the body consists of repeating units called segments. Each segment is limited by septa, dividing it from the neighboring segments, and has a fluid-filled cavity within it that is referred to as a coelom. 

Segments are formed sequentially in annelids and established during development from growth zones located at the posterior end of the body, so the youngest segment in the body of the annelid is always the most posterior. In the annelid body, the only parts that are not segmental are the head and the terminal region that is post-segmental called the pygidium. The head consists of two units, the prostomium, and peristomium. The post-segmental pygidium includes the zone from which new segments proliferate during growth. Annelids have the amazing capacity to regrow segments that are broken down into pieces, this is called regeneration.

2. Chaetae

A distinctive feature of annelids is a structure called chaetae also called setae are bundles of chitinous, thin-walled cylinders held together by sclerotized protein. Chaetae are defined as cuticular structures that are produced by a microvillar border of certain invaginated epidermal cells and develop within epidermal follicles. Chaetae show a huge amount of variation from long thin filaments to stout multi-pronged hooks.

3. Clitellum

The clitellum is a structure that is present on the anterior portion of the worm that helps to generate the mucus to aid in sperm transfer from one worm to another, it also forms a cocoon within which fertilization occurs.

Anatomy

Anatomical features of annelida species include

• A closed circulatory system is present where the dorsal and ventral blood vessels run parallel to the alimentary canal as well as capillaries that flow to individual tissues.

• An excretory system consisting of tubular metanephridia.

• A complete digestive system.

• Annelida segmentation is metameric.

• A well-developed nervous system that includes a nerve ring of fused ganglia is present around the pharynx. The nerve cord is present in a ventral position, that consists of enlarged nodes or ganglia in each of the segments.

• Sensory organs for detecting light and other stimuli.

• Gills for gas exchange but many exchange gases through their skin.

Reproduction

Most species of annelids can produce both sexually and asexually. However, leeches can reproduce only sexually. Asexual reproduction occurs through budding or fission sexual reproduction varies by species.

In some annelida species, the same individual produces both sperm and egg. This is termed hermaphroditism. Worms exchange their sperm by mating rather than self-fertilizing their own eggs. Fertilized eggs are deposited in a mucous cocoon. Offspring emerges from the cocoon looking like small adults. The larval cannot be found instead they directly grow to adult size.

While in some species like Polychaete there are separate sexes. Major transformation occurs in adult worms to develop the reproductive organs. The fertilization takes place in water by the release of gametes into the water. Offsprings go through a larval stage before developing through adults.

Types of Annelids

Below mentioned are the types of annelids:-

1. Earthworm: They are deposit feeders. They eat soil, extracting the organic matter and they burrow through the ground. Earthworm feces are called worm casts, and are very rich in plant nutrients. Earthworm burrows help aerate the soil, which is good for plants. They are also called farmers’ friends.

2. Polychaetes: They live on the ocean floor. They may be active predators, sedentary filter feeders, or scavengers. Active species in search of food can crawl along the ocean floor.

3. Leeches: They are either predators or parasites. As predators, they capture and eat other invertebrates. As parasites, they feed off the blood of vertebrate hosts. They have a tubular organ called the proboscis for feeding.

Conclusion:

After the roundworms the phylum Annelida came into picture. These are segmented worms who have much more advanced features such as coelomates that allows them to coordinate digestion. They are the first ones to display segmentation and they have a closed circulatory system. They evolve a new excretory system called nephridia. They are well-improved ones compared to Platyhelminthes and lack some of the features from molluscs.