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

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

1. General Characteristics of the Phylum Annelida

The phylum Annelida comprises about 12,000 species belonging to 6 classes: Archiannelida, Polychaeta, Oligochaeta, Hirudinea, Echiura, and Sipuncula. Typical representatives include the lugworm, nereis, earthworm, and medicinal leech (Fig. 30). Annelids are the most complexly organized, predominantly free-living worms. Their size ranges from 0.5 mm to 3 m. Annelids inhabit seas, freshwaters, and damp soil. The majority of annelids are inhabitants of the hydrosphere, leading a benthic lifestyle, and are commonly found in the coastal zone.

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Fig. 30. Various species of annelids (after Dogiel, 1981):

a — lugworm; b — nereis; c — earthworm; d — medicinal leech.

Benthic annelids crawl along the bottom or burrow into silt or sand, and sessile forms also occur among them. Relatively few species lead a free-swimming lifestyle, forming part of the plankton. A small number, such as leeches, are ectoparasites of animals and humans. The body of an annelid consists of a prostomium (HEAD lobe), a segmented trunk, and a pygidium (anal lobe). The trunk is divided into segments by means of special partitions called dissepiments (from Latin septum — partition). The trunk segments (annuli) are similar to one another (homonomous segmentation).

The Skin-Muscle sac is much better developed than in Flatworms and Roundworms. The body is covered by a single-layered epithelium that secretes a thin cuticle. Beneath it lie two layers of well-developed Muscles: circular and longitudinal. Auxiliary appendages—setae and parapodia—also participate in movement. Setae are arranged singly or in bundles in regular longitudinal rows. Parapodia are lateral outgrowths of the body with well-developed musculature and bundles of setae, located on the sides of each segment except the head and tail regions. The segmentation of the body, the presence of well-developed muscles, parapodia, and setae ensure a variety of movements for these animals in Water, on the land surface, and in the soil.

Annelids are characterized by a secondary body cavity, or coelom (Fig. 31). This is the cavity between the body wall and the Internal Organs, bounded by the coelomic epithelium. It develops from the mesoderm. In annelids, the coelom is divided into segments by partitions.

Fig. 31. Cytology/practical/72.html">Cross section of an earthworm (after Dogiel, 1981):

1 — cuticle; 2 — epithelium; 3 — circular muscles; 4 — longitudinal muscles; 5 — coelomic epithelium;

6 — coelom; 7 — setae; 8 — metanephridium; 9 — ventral nerve cord;

10 — ventral Blood vessel; 11 — intestine; 12 — typhlosole (intestinal fold); 13 — dorsal blood vessel.

The secondary body cavity is filled with a fluid whose chemical composition is close to that of seawater. Coelomic fluid performs several Functions: it creates pressure that helps maintain a constant body shape (a "hydrostatic Skeleton"), and transports nutrients, oxygen, carbon dioxide, and Metabolic waste products.

The Digestive System of annelids is represented by a digestive tube, typically straight and divided into the following sections: Oral Cavity, Pharynx, Esophagus (which expands into a crop), muscular gizzard, midgut, and hindgut ending in the anus. The ducts of Salivary Glands open into the esophagus, and their secretion AIDS in Processing food. The walls of the midgut contain glandular Cells that produce digestive Enzymes. Other cells absorb digested food.

The excretory organs—multicellular tubules (metanephridia)—function to eliminate metabolic waste products. They begin with a funnel (nephrostome) in the body cavity, from which a convoluted canal extends, opening to the outside in the following segment. Metanephridia are arranged metamerically: There are two in each segment.

The Circulatory system is closed. The main vessels—the dorsal and ventral ones—are connected segmentally by circular vessels. Among them, pulsating vessels that surround the esophagus stand out by their thickness and act as hearts. The circulatory system performs the functions of transporting nutrients and gases, as well as a protective function (blood phagocytes engulf Bacteria and foreign bodies).

Gas exchange occurs through the skin, although many marine forms possess gills.

The Nervous system is represented by supraesophageal and subesophageal ganglia, which connect to form a circumesophageal nerve ring, and a ventral nerve cord with segmentally arranged ganglia that extend nerves to various organs.

Sense Organs are well developed in annelids. Many of them possess eyes, which are located not only on the prostomium but also on the trunk and pygidium. Taste receptors and olfactory cells that perceive odors and chemical stimuli are also present. Hearing organs, structured similarly to locators, are well developed. Some annelids (polychaetes) possess statocysts. The diverse sense organs of annelids enable the animals to receive various information from the environment, which is processed in the cerebral ganglia. This significantly increased the vital activity of the worms and determined complex forms of their behavior.

Among annelids, there are both dioecious forms and hermaphrodites. Reproduction is predominantly sexual, and occasionally asexual (budding, fission). Fertilization can be either external or internal. Development is direct or involves metamorphosis. Annelids possess a high capacity for regeneration.