AKA Segmented Roundworms Annelids – Part 1 AKA Segmented Roundworms

Annelids The word annelid is derived from the word annulus which means ring. In this way, annelids get their name from their most defining characteristic – their external and internal segments, or rings.

Classes of Annelids Annelids have three main classes based on habitat and differences in body structure. They are: Polychaete (paul-ee-keet) Oligochaete (awl-ee-go-keet) Hirudinea (hir-oo-din-ee-uh)

Polychaete Worms AKA clamworms Size: millimeters to meters long Change in body structure: has many bristles. Habitat: marine environment Types of movement: sedentary or mobile

Mobile Polychaete Mobile worms: Tend to swim around and catch food Have parapodia (little flaps of skin that act as legs) for movement. Parapodia have setae (bristles) and cirri (needle-like projections) on them. Have eyes, sensory tentacles, and a retractable pharynx with jaws. Ex. Nereis virens

Sedentary Polychaete Sedentary Worms Tend to burrow in sand or mud & filter feed. They suck up sand or mud and selectively absorb nutrients. Often lack eyes and have much smaller parapodia. Some secrete their own tube to burrow into. Ex. Arenicola

Oligochaete Worms Change in body structure: few bristles. Habitat: damp terrestrial environments Best known oligochaete is the earthworm

Earthworm The earthworm is mostly nocturnal and tends to live in underground tunnels. They help to develop and add nutrients to the soil. They are considered to be more advanced than polychaete.

Hirudinea Change in body structure: flattened body, with no parapodia or bristles. Habitat: freshwater or terrestrial parasites or predators. They attach to their prey by suckers. Ex. Leeches After having it’s fill of a victims blood they can go for weeks or months without eating.

Annelid Body Structure Bilateral symmetry Coelomates: have a lined coelom (body cavity). This is an advancement over nematodes. Cuticle covered body to keep the worm from drying out, while still allowing movement.

Digestive System More advanced than the nematodes. One way system, in addition to the nematode parts they have: Calciferous glands Monitor acid-base levels of the coelomic and blood fluids. Crop Used to store food. Gizzard Food is ground into digestible pieces. Mouth  buccal cavity  pharynx  esophagus (attached to calciferous glands)  crop  gizzard  intestine  anus

Nervous System Anterior brain with paired ventral nerve cords. Has peripheral nerves that supply the muscle layers and reach the sense receptors of the outer wall. Also have ganglia that allow for movement. Each segment has a nerve cord that passes through a ganglia and attaches to sense receptors. This nerve cord consists of giant nerve fibers and is responsible for coordinating movement of all the segments.

Muscular System Both longitudinal and circular muscles – another advancement over nematodes. Seta retractors are muscles that connect from one set of setae to another. This allows for coordinated movement. Remember, setae are bundles of bristles.

Circulatory System We haven’t seen one of these since the piggy! Two kinds are seen in annelids: Open: no vessels, blood empties into sinuses, or cavitites. (in the leech) Closed: has vessels to hold and direct blood. (in the clam- worm & earthworm)

Reproduction Annelids are hermaphroditic. They reproduce sexually.

Excretory System The excretory system is called a metanepridium. Each segment has a funnel like nephrostome which is covered by tiny hair-like structures (cilia) leading into the coelom of that segment. The segments are connected together by a convuluted tubule which leads from the nephrostome into the bladder of another segment. Wastes are excreted through the nephridiopore.

A note on leeches Leeches are sometimes considered to be parasitic organisms. However, their relationship with the host is relatively brief, so it is more correct to classify them as predatory. #13 on page 99 in your resource book incorrectly states that they are not free-living, but rather are considered to be a parasitic organism.

Annelid Importance Annelids are important to humans for the following reasons: Earthworms help to add nutrients to our soil. Leeches are used medicinally to help promote blood flow to areas. They make good bait for fishing!!