Concept 33.5: Annelids are segmented worms The phylum Annelida is made up of segmented worms, numbering about 15,000 species. Body segmentation, which first appears in the fossil record in annelids, was a major step in the evolution of animals. The name annelida comes from the latin “annelus” meaning little rings. Each segement is called an annulus.

Concept 33.5: Annelids are segmented worms This coelom is divided into a series of repeated parts. This repetition is called metamerism, and each segment is called a metamere.

The phylum Annelida is divided into three classes Classes of Annelids The phylum Annelida is divided into three classes Table 33.4

Classes of Annelids Annelids with a clitellum (a swelling towards the head of the animal, where the gonads are located) are classified in the superclass Clitellata, which is further divided into classes Hirudinea (leeches) and Oligochaeta (including the earthworms). The Polychaeta lack a clitellum and have parapodia, paddle-like appendages with numerous bristles or chaetae.

Classes of Annelids These segmented worms are among the most common marine organisms, and can be found living in the depths of the ocean, floating free near the surface, or burrowing in the mud and sand of the beach. Some, such as Eunice gigantea, may reach three meters long.

Classes of Annelids Polychaetes are known by many names: lugworms, clam worms, bristleworms, fire worms, palolo worms, sea mice, featherduster worms, etc., but all possess an array of bristles on their many leg-like parapodia -- the name polychaete, in fact, means "many bristles". The many common names reflect the wide array of body forms found in this group, unlike the earthworms and leeches which all have the same general appearance.

Polychaetes Class Polychaeta consists of mostly marine annelids. Polychaetes are the largest group of annelids, and have hard bristles that allow the worms to wriggle and move. However, only one type of polychaetes actually move.

Polychaetes Many polychaetes live in tubes that they make from sticky proteins secreted near the mouth. Feathery appendages extending from the tubes trap food in the water. An example of a tube-building polychaete is a Christmas tree worm. Polychaetes usually have at least one set of eyes, one set of sensory organs, and are either male or female.

Members of class Polychaeta Polychaetes Members of class Polychaeta Possess paddlelike parapodia that function as gills and aid in locomotion Parapodia

Polychaetes Resembling giant lipsticks, tubeworms (Riftia pachyptila) live over a mile deep on the Pacific Ocean floor near hydrothermal vents. They may grow to about 3 meters (8 ft) long. The worms’ white tube home is made of a tough, natural material called chitin (pronounced “kite-in”).

Oligochaetes Class Oligochaeta consist of earthworms and other worms that live in terrestrial or freshwater environments. As shown by the class name (meaning few setae, or appendages), these worms lack the bristles that allow movement and increase surface area.

Oligochaetes All oligochaetes are hermaphroditic, neither male nor female. One oligochaete, the earthworm, extracts nutrients from the soil as it burrows, excreted from the anus. Because the nutrients that earthworms dig up are necessary for fertile soil, these earthworms are an intricate part of farms, actually tilling the soil.

Oligochaetes The largest earthworm ever found was in South Africa and was 22 feet long!

Hirudinea The final major class in phylum Annelida, class Hirudinea, consists of the leeches. Leeches are known for sucking the blood of humans, but most are free-living and eat small invertebrates, feeding on their blood. Most live in freshwater environments, although a few can live in terrestrial environments.

Hirudinea Like earthworms, leeches are hermaphroditic and lack appendages. To successfully drink the blood, they have a small sucker in the anterior end and a larger one in the posterior end.

Hirudinea

During the reattachment of severed fingers and ears, or of the detached scalp, the blood flow needs to be reestablished. This is achieved by reconnecting the major arteries and veins. In particular, the veins can be difficult to find. If not enough veins are reconnected, the blood may initially enter the reattached organ but cannot exit. This will prevent fresh, oxygenated blood from entering and, consequentially, the reattachment procedure will fail.

This is where the medicinal leech comes to the rescue. The animals are applied to the tissue and they actively remove blood and secrete numerous compounds that have vasodialator, anticoagulant, and clot-dissolving properties. This prevents the tissue from dying off and allows the body to reestablish good blood flow to the reattached part.