Common Group Name: Annelids

Phylum: Annelida Examples: Oligochaetes (earthworms) Leeches Polychaetes (mostly marine)

Body Plan Annelids have bilateral symmetry Body segments present, separated by septa- or internal walls. Segments are similar and may specialize in function. They have a true coelom Contain a digestive tract with a mouth and anus.

Range widely from filter feeders to predators. Feeding Range widely from filter feeders to predators. More advanced digestive system. In earthworms: food is collected by the 1)pharnyx- 2)esophagus- 3)crop- 4)gizzard- nutrients absorbed in 5)intestine.

Respiration Aquatic annelids breathe through gills- organs specialized for gas exchange underwater. Land annelids exchange gases through their moist skin.

Circulation Typical annelids have a closed circulatory system- blood is contained in a network of vessels. Earthworms have a dorsal vessel and an ventral vessel connected by smaller vessels that supply blood to organs. Blood flow: Dorsal- tail to head, Ventral- head to tail.

Excretion Digestive waste leaves through the anus. Cellular (metabolic)wastes leave through organs called nephridia.

Response Well-developed nervous system with brain and cords. Specialty adaptations: Tentacles Chemical receptors Statocysts- gravity detectors Eyes

Movement Muscles that are Longitudinal- make the worm shorter and fatter. Circular- make the worm longer and thinner. Movement= alternating contraction between the two groups. Marine annelids have appendages for swimming.

Reproduction Most annelids reproduce sexually. Some use external fertilization with separate sexes. Others are hermaphrodites producing sperm and egg. The process: Worms attach to each other- exchange sperm- store sperm- eggs and sperm go into a special ring (clitellum) for fertilization- ring slips off worm body- birth weeks later.

Other Information Most unique feature: Segmented body type. Oligochaetes- streamlined bodies; in freshwater and soil Leeches- external parasites that suck the blood of their hosts Polychaetes- marine annelids with paired paddlelike appendages.

Earthworms and the Environment Earthworms are considered beneficial to the environment because: Their castings contain nutrients. They aerate the soil as they move through it They break up the soil in which they live