Phylum Annelida: Oligochaeta Class Quiz

Phylum: Annelida

• Annelida is a phylum of segmented worms.
• Annulus means ring, eidos means form.

Classification

• Phylum: Annelida
• Class: Oligochaeta
• Genus: Pheretima
• Species: posthuma

Habit and Habitat

• Pheretima is a common Indian earthworm found in moist places.
• They are abundant during the rainy season.
• The presence of earthworms is indicated by their characteristic coiled castings.
• They burrow during the day and emerge at night to search for food.

External Features

• Pheretima has a long, cylindrical body (15-20 cm long, 0.3-0.5 cm wide).
• The dorsal surface is browner than the ventral surface.
• This coloration is due to porphyrin, a pigment formed from chlorophyll consumed by the earthworm.

Body Structure

• The body is segmented (100-120 segments).
• The anterior end has a mouth below a fleshy structure called the prostomium.
• The first segment surrounding the mouth is the peristomium.
• The dorsal surface has a dorsal blood vessel.
• The ventral surface has a ventral blood vessel.

Body Segments

• The intersegmental groove is the space between segments.
• Dorsal pores are found in the intersegmental groove, after 12 segments.
• Spermathecal pores, located in intersegmental grooves 5/6, 6/7, 7/8, and 8/9,
• Each segment has locomotory structures called setae, developed from a setigerous cell.
• The setae are curved in an 'S' shape, with a thicker middle part (nodule).
• Setae are absent in the first and last segments, as well as in the clitellum.

Clitellum

• Segments 14-16 are deep brown and thick-walled.
• This region is called the clitellum.
• The female genital opening is located on the ventral side of segment 14.
• Male genital openings are on the ventral side of segment 18.
• A pair of genital papillae are found on the ventral surface of segments 17 and 19.
• The anus is at the end of the last segment.

Digestive System

• The digestive system is a straight tube from the mouth to the anus.
• It consists of mouth, buccal cavity, pharynx, esophagus, gizzard, stomach, intestine, and anus.
• The digestive system is involved in food digestion and absorption.
• Parts of the digestive tract include the buccal chamber, pharynx, esophagus, gizzard, stomach, intestine, and intestinal caecum.
• Pre-typhlosolar region extends from segment 15 to 26.
• The typhlosolar region extends from segment 26 to 23–25 in front of the anus.
• The post-typhlosolar region is devoid of typhlosole.
• The anus is in the last segment.

Digestion

• Earthworms eat leaves, seeds, and soil.
• The buccal cavity is everted and the food is drawn into the mouth by the pharynx.
• Pharyngeal glands secrete a juice containing mucin and proteolytic enzymes.
• These lubricate the food and act on proteins.
• The food enters the gizzard, where it is ground by the muscular action of the gizzard wall.
• In the intestine, the food contacts digestive juices to convert proteins to peptones, starches to sugars and fats to split fat.
• The end products of digestion are absorbed into the blood and distributed throughout the body.
• Undigested food is expelled at intervals through the anus.

Excretory System

• The excretory organs are nephridia.
• They are coiled tubes found in all segments except the first three segments.
• There are three types: septal, pharyngeal and integumentary nephridia
• Glandular cells of the nephridia extract water and nitrogenous wastes from the blood (urea, ammonia, amino-acids)
• Nephridial contents are released in the alimentary canal.
• Water and other products are excreted along with faecal matter.

Nervous System

• The nervous system includes the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS consists of a nerve ring and a double ventral nerve cord with paired ganglia.
• The nerve ring forms a brain-like structure (suprapharyngeal ganglia).
• The ventral nerve cord runs along the ventral side of the body.
• Ganglia are located in each segment.
• The peripheral nervous system consists of nerves that extend from the CNS to various parts of the body.
• Four giant fibers are present in the nerve cord, responsible for quick movements.

Reproductive System

• Earthworms are hermaphrodites (possess both male and female reproductive organs).
• Testes (paired) are situated in the 10th and 11th segments.
• Seminal vesicles are located in 11th and 12th segments.
• Spermatheca (paired) are located in segments 6-9 to receive sperms from another earthworm.
• Spermatic funnel collects sperms from the testes.
• Vas deferens is part of the reproductive system that carries sperm.
• The prostate gland secretes fluid for sperm mixture.
• Male genital pores are in the 18th segment.
• Ovaries are located in the 12th and 13th segments.
• Oviducts carry the eggs.
• Spermathecal pores are located in the intersegmental grooves of segments 5-9 to receive sperm from another earthworm.
• Copulation often occurs during rainy season.

Copulation and Cocoon

• Earthworms exchange sperms during copulation.
• A cocoon is formed from the clitellum and contains fertilized eggs.
• Fertilization is external.
• The cocoon is deposited in a moist environment.

Economic Importance

• Earthworms are:
  • Used in agriculture as soil enrichers through castings.
  • They help aerate soil and improve drainage.
  • They are used as bait in fishing.
  • They are used in laboratories as study organisms.
  • They are used as food by some animals (lizards, birds).
  • They were used as medicine in ancient times to treat various ailments.

Test your knowledge about the phylum Annelida, specifically the Oligochaeta class, featuring the common earthworm Pheretima posthuma. Explore their classification, habitat, external features, and body structure in this engaging quiz.