Chapter 18 Annelids

18.10 Annelids are segmented worms The segmented bodies of phylum Annelida Give them added mobility for swimming and burrowing

Ear thworms and Their Relatives Eat their way through soil Have a closed circulatory system Giant Australian earthworm Mouth Brain Dorsal vessel Coelom Digestive tract Mucus-secreting organ Segment wall Anus Bristles Blood vessels Nerve cord Pumping segmental vessels Ventral vessel Excretory Segment wall (partition between segments) Epidermis Circular muscle Longitudinal Intestine Figure 18.10A

What Is an Annelid? What Is an Annelid? Annelids are worms with segmented bodies. They have a true coelom that is lined with tissue derived from mesoderm.

Three Germ Layers of an Annelid What Is an Annelid? Three Germ Layers of an Annelid Annelids are among the simplest animals to have a true coelom that is lined with mesoderm. Annelids are also called segmented worms because the body is divided into many similar segments.

What Is an Annelid? The body of an annelid is divided into segments. Each segment is separated by septum, which are internal walls between each segment.

What Is an Annelid? Body segments may carry eyes, antennae, other sense organs, or be specialized for functions such as respiration. Bristles called setae may be attached to each segment. Annelids have a tube-within-a-tube digestive tract that food passes through from the mouth to the anus.

Form and Function in Annelids Annelids have complex organ systems. Many of these systems are unique because of the segmented body plan of this group.

Form and Function in Annelids Feeding and Digestion In carnivorous species, the pharynx usually holds two or more sharp jaws that are used to attack prey. Annelids that feed on decaying vegetation have a pharynx covered with sticky mucus. Other annelids obtain nutrients by filter feeding.

Form and Function in Annelids In earthworms, the pharynx pumps food and soil into the esophagus. The food then moves through the crop, where it can be stored. It then moves through the gizzard, where it is ground into smaller pieces. The food is absorbed farther along in the digestive tract in the intestine.

Form and Function in Annelids Circulation  Annelids typically have a closed circulatory system, in which blood is contained within a network of blood vessels.

Form and Function in Annelids Blood in the dorsal (top) vessel moves toward the head of the worm. The dorsal blood vessel functions like a heart because it contracts rhythmically and helps pump blood. Dorsal blood vessel

Form and Function in Annelids Blood in the ventral (bottom) vessel runs from head to tail. Ventral blood vessel

Form and Function in Annelids In each body segment, a pair of smaller blood vessels connect the dorsal and ventral blood vessels and supply blood to the internal organs.

Form and Function in Annelids Respiration  Aquatic annelids often breathe through gills. A gill is an organ specialized for the exchange of gases underwater. Land-dwelling annelids take in oxygen and give off carbon dioxide through their moist skin.

Form and Function in Annelids Excretion   Digestive waste passes out through the anus. Cellular waste containing nitrogen is eliminated by nephridia.

Form and Function in Annelids Nephridia are excretory organs that filter fluid in the coelom. Nephridia

Form and Function in Annelids Response   Most annelids have a well-developed nervous system consisting of a brain and several nerve cords. Brain Ganglia

Form and Function in Annelids Movement   Annelids have two groups of body muscles that function as part of a hydrostatic skeleton. Longitudinal muscles run from the front of the worm to the rear and can contract to make the worm shorter and fatter.

Form and Function in Annelids Circular muscles wrap around each body segment and can contract to make the worm longer and thinner. The earthworm moves by alternately contracting these two sets of muscles.

Form and Function in Annelids Reproduction    Most annelids reproduce sexually. Some species use external fertilization and have separate sexes. Other annelids are hermaphrodites. Two worms attach to each other, exchange sperm, and then store the sperm in special sacs.

Form and Function in Annelids When eggs are ready for fertilization, a clitellum secretes a mucus ring into which eggs and sperm are released. A clitellum is a band of thickened, specialized segments. After eggs are fertilized in the ring, the ring slips off the worm's body and forms a protective cocoon. Young worms hatch weeks later.

Groups of Annelids Groups of Annelids Annelids are divided into three classes oligochaetes leeches polychaetes

Groups of Annelids Oligochaetes Oligochaetes contains earthworms and their relatives. Oligochaetes typically have streamlined bodies and relatively few setae compared to polychaetes. Most oligochaetes live in soil or fresh water.

Groups of Annelids Leeches The class Hirudinea contains the leeches. Leeches are typically external parasites that suck the blood and body fluids of their host.

Groups of Annelids Leeches have powerful suckers at both ends of their bodies that help them cling to their hosts. Some leeches force a muscular extension called a proboscis into the tissue of their host. Others slice into the skin with a razor-sharp pair of jaws.

Leeches Most leeches Are free-living carnivores, but some suck blood Figurer 18.10D

Groups of Annelids The leech uses its pharynx to suck blood from the wound. Some leeches release a substance that anesthetizes the wound—keeping the host from knowing it has been bitten. Leeches were once used to treat medical conditions.

Groups of Annelids Polychaetes Polychaetes include sandworms, bloodworms, and their relatives. Polychaetes are marine annelids that have paired, paddlelike appendages tipped with setae. The setae are brushlike structures on the worm.

Polychaetes The polychaetes Form the largest group of annelids Search for prey on the seafloor or live in tubes and filter food par ticles Figure 18.10B, C

Ecology of Annelids Ecology of Annelids Earthworms and many other annelids spend their lives burrowing through soil, aerating and mixing it. Earthworms help plant matter decompose. Earthworm castings are rich in nitrogen, phosphorus, potassium, micronutrients, and beneficial bacteria.