Chapter 11, Phylum Annelida (Segmented Worms)

Phylum Annelida

Phylum Annelida

Body of annelids have metamerism. Metamerism is a characteristic that features regularly repeating body segments.

This phylum includes earthworms and freshwater worms (Oligochaeta), leeches (Hirudinea), and marine worms (Polychaeta)

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 in leeches)

Fleshy appendages called parapodia in some annelids

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. Others feed on suspended particles that are

trapped on parapodia. Some are predaceous, and hunt by hiding in rocks

and corals ambushing their prey. Others are bloodsucking carnivores (leeches).

Annelid Ecology The importance of earthworms in terrestrial

ecosystems can not be overstated. Without earthworms, plants would not be able to

grow, and the entire food chain would be impacted. Earthworms help aerate, fertilize, mix, and provide

proper drainage to soils. Leeches are sometimes used in the medical field

when fingers or toes are cut off. Leeches can help stop the bleeding at the wound while veins and arteries are still healing.

Earthworms and Medical Leeches

Annelid Anatomy and Locomotion The body of an annelid usually consists of a two-part

head (prostomium and peristomium), a series of body segments (called metameres), and a terminal segment called the pygidium.

Annelids have both longitudinal and circular muscles which create waves of expansion and contraction when they move called peristalsis.

Class Polychaeta

Name 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 worm, scale worms, fireworms, fanworms (or featherdusters).

Class Oligochaeta

Name means “few + long hairs”. Few setae per body segment. Terrestrial and freshwater. No head or parapodia. Coelom divided by septa. Hermaphroditic Earthworms

Earthworms

Earthworms (also known as nightcrawlers) burrow in moist soil and emerge at night to feed on detritus and vegetation and to breed.

Earthworms are usually about 12 – 30 cm long. Giant tropical earthworms can get 3 – 4 meters.

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 Digestive System

Earthworm Anatomy

Earthworms also have a crop and a gizzard, which stores and mechanically digests 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 last one).

Earthworm Circulatory/Excretory System

Earthworm Anatomy

The nervous system in earthworms and all annelids have 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 Nervous System

Earthworm Reproduction Mating earthworms are held together by mucus

secreted by a reproductive organ called a 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 terrestrial. Body usually with posterior and anterior suckers

that secrete anticoagulants and consume blood. No parapodia or setae Clitellum present Hermaphroditic