ZOOLOGY: A STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 10. PHYLUM SEGMENTED WORMS, OR ANNELIDS (Annelida)

4. Class Oligochaetes, or Microdriles and Megadriles (Oligochaeta)

The Class Oligochaeta comprises segmented worms characterized by reduced parapodia and a lack of gills. There are over 5,000 known species (with about 200 species found in Ukraine).

Representative species of oligochaetes are primarily soil-dwellers (such as the earthworm and the Australian giant earthworm) and freshwater inhabitants (such as the sludge worm).

Oligochaetes vary dramatically in size: some are dwarfs measuring only a few millimeters in length, while others are true giants. For instance, the Australian giant earthworm, which externally resembles a large snake, can reach 2.5 to 3 meters in length. Sludge worms typically range from 0.3 cm to 20 cm.

The hallmark features of oligochaetes include:

1) absence of parapodia, replaced by few stiff setae along the sides of the body used for gripping uneven surfaces;

2) a poorly developed HEAD region lacking additional appendages or lobes;

3) Sensory Organs represented by tactile and photoreceptor Cells scattered across the epidermis;

4) presence of a clitellum spanning several segments near the anterior end of the body;

5) cutaneous Respiration (breathing through the Skin);

6) Hermaphroditism;

7) cross-Fertilization, which is internal;

8) eggs are deposited into a cocoon formed from secretions produced by the clitellum;

9) direct development;

10) a high capacity for regeneration.

The earthworm serves as a typical representative of this class. Its body length ranges from 15 to 20 cm. The anterior end of the earthworm is thicker and more tapered than the posterior end. The Mouth is located on the ventral side near the anterior end, while the anus is situated at the posterior tip. The body has a brownish coloration. Externally, the entire body of the earthworm is divided by shallow grooves into over 160 segments. Spanning segments 31 to 33 is a glandular skin thickening known as the clitellum, which plays a crucial role in reproduction.

The earthworm's body lacks antennae, lobes, or eyes—an adaptation to its subterranean lifestyle, where it burrows extensive tunnel networks. These tunnels help aerate the soil, thereby improving ROOT gas exchange for plants. Each body segment bears four pairs of small, rigid, and sharp setae that allow the worm to anchor itself to soil particles. The setae point backward, facilitating forward movement without slipping.

The earthworm's body is moist and coated in mucus secreted by epidermal glands. This mucus prevents the skin from drying out and eases movement

through the soil. Earthworms move by the alternating contraction of the circular and longitudinal Muscles of the skin-muscular sac.

The circulatory, digestive, excretory, and nervous systems of the earthworm are typical of all Annelids.

The respiratory organ of the earthworm is the skin surface. However, unlike in hydras, oxygen does not diffuse directly into body cells; instead, it first penetrates the extensive capillary network of the skin and is transported throughout the body via the Blood.

Earthworms play a vital role in nature, particularly in soil formation, and therefore these animals are protected by law.