Chapter 11, Phylum Annelida (Segmented Worms)

Phylum Annelida

Phylum Annelida Annelids exhibit metamerism. Metamerism is a characteristic that features regularly repeating body segments. This phylum includes earthworms, leeches, and marine worms. Annelids are sometimes called “Bristle worms”, because of the tiny chitonous bristles that they bear (except leeches) called setae.

Important Characteristics of Phylum Annelida Metamerism or segmentation. Epidermal, chitonous setae (except leeches). Fleshy appendages called parapodia (in polychaetes). Coelomate body cavity.

Annelid Ecology Annelids are found in the ocean, freshwater, and in terrestrial soil. Often live in burrows in the ground and feeds on organic matter in soil (earthworms). Others (polychaetes) feed on suspended particles that are trapped on parapodia. Some are predators, and hunt by hiding in rocks and corals, ambushing their prey. Others are bloodsucking parasites (leeches).

Annelid Ecology Earthworms are extremely important for terrestrial ecosystems. Without earthworms, plants could not grow, and the entire food chain would be affected. Earthworms help aerate, fertilize, mix, and provide proper drainage to soils.

Medical Purposes for Leeches Leeches are sometimes used in the medical field when fingers or toes are cut off. Leeches can help stop the bleeding at the site of the wound while veins and arteries are still healing.

Annelid Anatomy and Locomotion The body of an annelid usually consists of a two- part head (prostomium and peristomium), a series of body segments (metameres), and a terminal segment called the pygidium. Annelids have both longitudinal and circular muscles, which create waves of expansion and contraction called peristalsis.

Class Polychaeta Name polychaete means “many + long hairs”. Many setae per body segment. Mostly marine and dioecious. Most segments with parapodia used for crawling, swimming, feeding and respiration. Examples include the clam worms, scale worms, fireworms, and fanworms (or featherdusters).

Class Oligochaeta Name means “few long hairs”. Few setae per body segment. Terrestrial and freshwater. No head or parapodia. Hermaphroditic Earthworms

Earthworms Earthworms (nightcrawlers), burrow in moist soil and emerge at night to feed on detritus and vegetation, and to breed. Usually about 5-12 inches long. Some giant tropical earthworms can get 3 – 4 meters long.

Earthworm Anatomy Setae project through small pores in the cuticle to provide anchoring points when the earthworm moves or burrows. Food is brought in by a muscular pharynx. The digestive tract is unsegmented and runs continuously the length of the body. The intestine has a u-shaped fold called a typhlosole that increases surface area.

Earthworm Anatomy

Earthworm Anatomy Earthworms also have a crop which stores food, and a gizzard, which mechanically breaks down food. Annelids have closed circulatory systems. Earthworms have five (aortic arches) hearts. The dorsal blood vessel is the main blood vessel. The excretory system consists of a pair of nephridia (similar to our kidneys) in each body segment (except the first three and the last one).

Earthworm Anatomy

Earthworm Anatomy The nervous system in annelids consist of a pair of cerebral ganglia. Posterior to the cerebral ganglia, ventral nerve cords run the length of the body and each body segment has its own pair of smaller ganglia. Earthworms are hermaphrodites and exchange sperm with each other during copulation. When earthworms mate, they come out of their burrows at night and touch ventral surfaces.

Earthworm Anatomy

Earthworm Reproduction Mating earthworms are held together by mucus secreted by an organ called the clitellum, and they are also held together by ventral setae. Sperm are discharged and travel to an opening called the seminal receptacle. After sperm exchange, the worms separate. Each worm then secretes a protective covering over the clitellum that forms a cocoon.

Earthworm Reproduction The cocoon slides forward along the body. As it slides forward, it passes over the oviducts. Eggs from the oviduct and sperm from the seminal receptacles transfer into the cocoon. After fertilization the cocoon slides off the worm. The embryos develop in the cocoon and eventually emerge as juvenile worms.

Earthworm Reproduction

Class Hirudinea Name means leeches Mostly freshwater, some marine and some terrestrial. Body usually with posterior and anterior suckers that secrete anticoagulants and consume blood. No parapodia or setae Clitellum is present Hermaphroditic