Annelida: Examples | Zoology Optional Notes for UPSC PDF Download

Phylum-Annelida: Introduction

The Phylum Annelida comprises a diverse group of segmented worms, with Nereis being a notable representative. This classification falls under the Kingdom Animalia, with specific categorizations, including the class Polychaeta.

Nereis: Habitat and Distribution

Nereis, commonly known as Ragworm or Clam Worm, is a marine, free-living organism with a penchant for temporary burrows in sandy environments. Its cosmopolitan distribution spans the North Atlantic coast, Pacific coast, U.S.A., and Europe.

Key Characteristics of Nereis

Nereis exhibits a cylindrical and elongated body with around 200 similar segments. The mouth is anteriorly located on the peristomium, and locomotory parapodia are present on all body segments except the head and anal segment. The intersegmental groove separates these metameres, and the anal segment contains a pair of anal cirri. Nereis is dioecious, and distinguishing between male and female worms is challenging.

Heteronereis: Another Facet of Nereis

Within the Annelida Phylum, Heteronereis represents the sexual phase of Nereis. It undergoes marked modifications in the posterior segments during the breeding season, leaving its tube to become free-swimming. The body differentiates into asexual anterior atoke and a posterior sexual epitoke, housing gametes. Various anatomical features, such as prostomial tentacles and palps, undergo changes, and the development of large gonads leads to reductions in muscles and the alimentary canal.

Aphrodite: The Enigmatic Sea Mouse

Aphrodite, also known as the Sea Mouse, is a marine worm inhabiting deep-water muddy bottoms. This oval-shaped creature, measuring around 12 cm in length, is comprised of 30 to 35 segments. Its dorsal surface is convex and covered with various setae arising from the notopodium. The ventral surface is flat, segmented, and forms a creeping sole. The pumping action of the dorsal body wall filters seawater through the dorsal felt into the space below.

Sabella: The Peacock Worm

Sabella, commonly referred to as the Peacock Worm, is a tubicolous polychaete found in marine environments, especially along the U.S.A. coast. It lives in an upright tube made of sand grains and possesses a head, trunk, and abdomen. The head contains gill filaments, and the abdomen is the largest part. A dorsal fecal groove extends from the mouth to the anus, aiding in waste elimination.

Serpula: Inhabitants of Calcareous Tubes

Serpula is characterized by its life within hard, shelly, calcareous, and twisted tubes attached to marine objects. The body is elongated and cylindrical, with the anterior end forming a head consisting of prostomium and peristomium. The posterior end remains attached to a mass of calcareous tubes. Serpula's prostomium contains feathered gills, modified palps serving respiratory functions, and a peristome extended forward as a paired membrane.

Pheretima: The Earthworm

Pheretima, commonly known as the Earthworm, is a hermaphroditic oligochaete found in soil. It lives in burrows in the upper layers of slightly damp soils, lawns, and gardens. The segmented body, displaying true metamerism, consists of approximately 100 to 120 ring-like segments. The clitellum, located around segments 14 to 16, plays a crucial role in reproduction.

Hirudinaria granulosa: The Indian Cattle Leech

Hirudinaria granulosa, commonly referred to as the Indian Cattle Leech, is a freshwater blood-sucking ectoparasite found in ponds, lakes, and swamps. With a soft, vermiform body measuring 30 to 35 cm, it exhibits distinctive colors on its dorsal and ventral surfaces. The well-developed anterior and posterior suckers facilitate attachment, and the body is divided into several regions, each containing a specific number of segments.

Chaetopterus: The Paddle Worm with Bioluminescence

Chaetopterus, known as the Paddle Worm, is a marine, tubicolous annelid characterized by its bioluminescence. Living permanently in a U-tube made of sand and mucus, it is commonly found in Europe and the U.S.A. The body, divided into anterior, middle, and posterior regions, features modified parapodia serving various functions. Notably, Chaetopterus possesses a remarkable power of regeneration.

Trochophore Larva of Nereis: Developmental Stages

In the development of Nereis, the trochophore larva is formed after gastrulation. This conical, microscopic larva exhibits fast swimming capabilities. It contains essential structures such as an apical sensory organ, nerve ganglion, open mouth, rounded stomach, intestine, anus, ciliated bands for swimming, coelom, larval nephridium, mesoderm, eye spot, and larval muscles. After metamorphosis, the trochophore larva transforms into an adult Nereis.

In conclusion, the Phylum Annelida encompasses a diverse array of segmented worms, each exhibiting unique characteristics, habitats, and developmental stages. Understanding the intricacies of these organisms provides valuable insights into the broader field of zoology.