Phylum Annelida: Segmented Worms

• Phylum Annelida includes segmented worms possessing a coelom that functions as a hydrostatic skeleton.
• Annelids exhibit a pattern of repeated segments, except for the first and last portions.
• They feature a "one-way" digestive tract with a mouth and anus, a closed circulatory system, and a nervous system running through the segments.
• Annelids lack specialized gas exchange structures; blood vessels near the body surface facilitate diffusion if the epithelium remains moist.
• Some marine annelids have vascularized extensions on each segment to increase the surface area for diffusion.
• Many marine annelids have copper-based blood, giving it a greenish color.
• Some freshwater earthworms possess iron-based blood, similar to humans, which is red.
• Bloodworms (Tubifex), found in oxygen-poor water, contain hemoglobin, appear red, and are sold as fish food.

External Features

• Earthworms enhance soil porosity through burrowing, improving air and water penetration.
• They enrich the soil by transporting surface material, like dead plant matter, to deeper layers.

Digestion

• Digestion in annelids is extracellular.
• They have a complete digestive system that includes a mouth and anus.
• Marine worms can be filter feeders or scavengers.
• Earthworms extract organic material from the soil.
• The muscular pharynx contracts to draw food particles into the mouth.
• Food travels through the esophagus to the crop, a thin-walled sac.
• The crop leads to the gizzard, which has thick, muscular walls.
• Digestion and absorption occur in the intestine, which extends from the gizzard to the anus.

Transport and Exchange

• Circulation in earthworms occurs through closed vessels.
• Blood flows anteriorly in the dorsal vessel, which is visible as a dark line on top of the digestive tract.
• Blood travels posteriorly in the ventral vessel.
• Segmental branches from the ventral vessel supply blood to the intestine and body wall.
• Capillary beds facilitate nutrient and oxygen exchange.
• Gas exchange occurs between the body surface capillaries and the environment.
• Hemoglobin in the blood carries oxygen.
• Blood from capillary beds is collected into larger vessels that merge with the dorsal vessel.
• Five pairs of aortic arches ("hearts") near the esophagus pump blood from the dorsal to the ventral vessels.

Excretion and Osmoregulation

• Annelids have a pair of metanephridia (tubular excretory structures) in each segment.
• Metanephridia are open to the coelomic fluid and surrounded by capillaries.
• Cilia draw fluid into the metanephridium's internal opening, allowing it to pass out of the body through a pore.
• Metanephridia serve in excretion and osmoregulation.
• The epithelium lining the tubule reabsorbs solutes and returns them to the blood.
• Ammonium and excess water are excreted.

Reproduction

• Annelids are hermaphroditic.
• Sexual reproduction involves cross-fertilization through the exchange of sperm packets.
• Sperm is stored temporarily.
• The clitellum secretes a mucus cocoon that collects eggs and stored sperm as it slides along the worm.
• Some annelids (polychaetes) can reproduce asexually through budding.
• The oviduct can be located with a diagram.