Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Annelids
Class Oligochaeta

There are over 3,000 known species of oligochaete Annelids. They inhabit primarily soil and the bottoms of freshwater bodies. Soil-dwelling oligochaetes include earthworms, the most familiar representative being the common earthworm. The Structure of oligochaetes will be examined using the earthworm as an example.

The body of an oligochaete is greatly elongated, roughly cylindrical in shape, and composed of A large number of similar segments that can nevertheless be grouped into distinct regions: the HEAD, trunk, and anal regions. The head region (Fig. 32) includes 2 segments—the prostomium and the peristomium. The trunk region consists of a large number of trunk segments (typically 90 to 300, and up to 500–600 in some forms). Each segment bears small setae arranged in bundles (a pair of lateral and a pair of ventral bundles). In the earthworm, there are 2 setae per bundle, making a total of 8 per segment. The final region of the oligochaete body is the anal lobe, consisting of a single segment containing the anus.

The body of oligochaetes is covered by a thin, elastic cuticle that is constantly moistened by secretions from mucous glands located in the epidermal epithelium. Glandular Cells are especially numerous in the clitellum region, which becomes clearly visible during the breeding season. Beneath the Skin lies the musculature, represented by layers of circular and longitudinal Muscles (Fig. 33). Dorsal pores are located between the body segments, through which coelomic fluid is expelled to the outside and, together with skin gland secretions, moistens the worm's cuticle, facilitating its movement through the soil.

Class="center">

Fig. 32 External view of the anterior end of an earthworm:

1 — prostomium; 2 — lateral setae; 3 — ventral setae; 4 — male genital pore; 5 — female genital pore; 6 — seminal groove; 7 — clitellum.

Fig. 33 Cytology/practical/72.html">Cross section of an earthworm body:

1 — metanephridium; 2 — nephrostome; 3 — ganglion of the ventral nerve cord; 4 — epidermal epithelium; 5 — circular muscles; 6 — longitudinal muscles; 7 — setae; 8 — dorsal typhlosole (intestinal fold); 9, 10 — dorsal and ventral Blood Vessels; 11 — subneural vessel.

The Digestive System of oligochaetes comprises foregut, midgut, and hindgut regions. The foregut consists of the Pharynx, Esophagus, crop, and gizzard (Fig. 34). In earthworms, the ducts of calciferous glands open into the esophagus; these glands secrete calcium carbonate, which neutralizes the humic acids present in the food. The midgut is the longest section of the digestive tract.

Fig. 34 Longitudinal section of the anterior part of an earthworm body:

1 — septa; 2 — Seminal Vesicles; 3 — metanephridia; 4 — esophagus; 5 — pharynx; 6 — Mouth; 7 — supraesophageal ganglion (Brain); 8 — ventral nerve cord; 9 — crop; 10 — gizzard; 11 — midgut.

In most worms, the midgut forms a dorsal fold that projects into the intestinal lumen, thereby increasing its digestive and absorptive surface area. The short hindgut opens externally via the anus. Earthworms feed on decaying plant remains—leaves, herbaceous stems, and the like.

The respiratory organ for most oligochaetes, including earthworms, is the skin, which features a dense network of fine blood vessels known as capillaries. These are especially abundant on the dorsal side, causing it to appear darker than the ventral side. After rain, Water penetrates the inner layers of the soil and absorbs the oxygen dissolved within it. An oxygen-free environment is fatal to earthworms, prompting them to crawl out onto the soil surface. This behavior is reflected in their common name.

The Circulatory system OF oligochaetes is closed. The primary propulsive organ responsible for driving the blood is the dorsal blood vessel. It connects to the ventral vessel via uniformly spaced annular (ring) vessels. The ring vessels at the anterior end of the body, which encircle the esophagus, are capable of independent pulsation and are therefore referred to as hearts.

The excretory Organs of oligochaetes are typical metanephridia, arranged in pairs within each segment.

The Nervous system of oligochaetes is represented by paired supraesophageal ganglia connected by connectives to subesophageal ganglia, from which the ventral nerve cord extends. Among their Senses, Touch is the best developed in oligochaetes. Tactile cells are scattered across the entire body. Earthworms lack eyes but are highly sensitive to light. Photoreceptor cells are located predominantly at the anterior end of the body. Experiments show that earthworms can perceive odors, and their SENSE OF SMELL helps them locate necessary food.

Oligochaetes are hermaphrodites. The reproductive Organs of the earthworm are located in the region of segments 9–15. Male organs consist of Testes, a system of sperm ducts, and three pairs of seminal vesicles. Female organs consist of Ovaries, oviducts, and 2 pairs of seminal receptacles. Fertilization in earthworms is cross-fertilization. Two individuals align themselves with their ventral surfaces pressed together (Fig. 35). The clitellum of each worm secretes a mucous tube into which Male Germ Cells (spermatozoa) are discharged. Passing through the tube, the sperm enters the seminal receptacles of the other worm. Having exchanged sperm, the worms separate.

Fig. 35 Mating of earthworms

Egg laying and fertilization occur later. During this process, the worm secretes another mucous tube into which the ova are released first, followed by the spermatozoa.

The tube containing the Gametes slides off the worm, seals at the edges, and transforms into a cocoon where egg fertilization and embryonic development take place. Development in earthworms is direct—a sexually mature individual gradually develops from the fertilized egg. Regeneration—The ability to restore the body from separate parts—is characteristic of many annelids.

In the middle zone of the European part of Russia, earthworms reach lengths of up to 25 cm. As one moves further south, they grow larger. For instance, specimens up to 50–60 cm long are found in the Caucasus, and a giant earthworm measuring over 2 meters in length lives in Australia.

Earthworms are beneficial to agriculture. As early as 1837, Charles Darwin noted their positive impact on soil fertility. As earthworms move through the ground, they loosen it, which stimulates The activity of microorganisms and small animals, allows plant roots to penetrate deeper, and improves aeration and water supply. Furthermore, earthworms effectively "plow" the soil by passing it through their digestive tracts, bringing lower layers to the surface. According to expert estimates, earthworms inhabiting an area of 1 hectare bring 10–30 tons of processed soil to the surface annually. By mixing soil layers, earthworms help bury stones. By dragging plant debris into their burrows, they bury it, thereby contributing to the accumulation of organic matter and The formation of humus in the soil.

Earthworms are a valuable feed for fish and poultry. Recently, the artificial breeding of worms has been expanding increasingly.