Phylum Annelida

What does Annelida mean and/or refer to?

“many little rings”; indicative of segmentation/metamerism

Polychaete

“many hairs”; organism is covered in setae

Oligochaete

“few hairs”; organism lacks setae

Germ layer(s)

Triploblastic

Body cavity type

Coelomate

Symmetry type

Bilateral

Development

protostome; schizocoelous

Embryo type

Mosaic

Cleavage type

Spiral

Does this phylum exhibit metamerism?

YES! Coelomic cavity is highly developed; each segment has its own

How many currently described (2025) species of Annelids are there?

~15,000

What is the taxonomic structure of Phylum Annelida?

Family Chaetopteridae (parchment worms)

Class Errantia (free-moving polychaetes)

Class Sedentaria (a mix of polychaetes, oligochaetes, and leeches)

• Order Clitellata

• Family Lumbricidae (earthworms)

• Family Hirudinidae (leeches)

Describe the unique metamerism of annelids.

Segments are walled off from each other via a septum. Every segment has its own independent coelom, its own set of metanephridia, and a ganglion with sensory and motor nerve fibers.

What muscle types do annelids exhibit?

Circular and longitudinal

(Note: Circular muscles extend, longitudinal muscles retract and shrink.)

True or False: Annelids have a cuticle over their epidermis that helps to retain moisture.

True

True or False: Blood vessels are located both ventrally and dorsally in annelids.

True

Describe the circulatory and respiratory systems of an earthworm (Lumbricidae).

The blood vessels of an earthworm have valves, pumps, and aortic arches that function like a true heart. Their blood is colorless, containing amoeboid cells and hemoglobin that aid gas exchange.

Their capillary network is close to their body surface. They have no specialized respiratory organs, so they rely on diffusion through their tegument. Moist skin is CRUCIAL to this exchange.

Describe the excretory system of an earthworm (Lumbricidae).

Earthworms utilize metanephridia. Most body segments have 1 pair of nephridia tubes. Waste will leave via the dermal nephridiopore (end of nephridia) after active absorption and secretion of ions, similar to a simplified mammal kidney.

Describe the nervous system and sense organs of an earthworm (Lumbricidae).

Earthworms have a central and a peripheral nervous system. The CNS contains the ganglia and nerve cord, whereas the PNS contains all of the smaller lateral nerves.

Pairs of cerebral ganglia on the dorsal surface of the worm connect around the pharynx to subpharyngeal ganglia on the ventral surface.

They have a basic endocrine system; neurohormones regulate reproduction and regeneration. Most have chemoreceptors, basic photoreceptors, and tactile receptors.

Antiphototaxis

The avoidance of light; exhibited by earthworms

In what 3 ways do annelids breathe?

1. Cutaneous respiration (in earthworms and burrowing polychaetes of Class Sedentaria)

2. Gills (in feather duster worms and lugworms of Class Sedentaria)

3. Parapodia (in tube-dwelling polychaetes of Class Errantia)

Nuchal organ

a type of chemosensor comprised of small ciliated slits with receptors

Which class of annelids have parapodia?

Class Errantia

Describe the reproduction of Class Errantia.

Most species are dioecious and have “seasonal” sex organs that enlarge during spawning. Fertilization results in a trochophore larvae.

Some species can reproduce asexually when cut in half. Each half will go on to live as its own individual.

What are the functions of parapodia in Nereis?

Locomotion, sensing, anchoring (in burrowing taxa), and respiration

Describe the general structure of parapodia.

1 pair of parapodia is found on a segment, each with 2 lobes: a notopodium (dorsal) and a neuropodium (ventral). Each lobe has a chitinous aciculum spine that supports it. Cirri and setae also coat the parapodia, aiding in movement and sensing.

Prostomium (Nereis)

head region bearing the eyes, tentacles (cirri), and sensory palps; sometimes retractile

(Note: NOT considered the first true segment)

Peristomium (Nereis)

First true segment, surrounds the mouth; may or may not have setae, extra palps, and/or chitinous jaws

Pygidium (Nereis)

last segment; bears the anus and 2 outwardly-extending cirrus

Which class of Annelids makes up a majority of the phyla? (Around 2/3ds)

Class Sedentaria

How do Sabella (fan/feather duster worms) eat?

Filter feeding

How do Arenicola (lugworms) eat?

They live in U-shaped burrows and draw in water posterior → anterior. When the water leaves the excurrent canal, food and sediment falls back into the burrow to be consumed.

Describe the feeding behavior of Eunice (Errantia).

Eunice are ambush predators. They have massive jaws that stay locked open when the pharynx is inverted. As soon as the pharynx everts, their jaws snap shut to capture prey.

True or False: Earthworms don’t have setae.

False; they do!

Describe, in detail, the feeding behavior of earthworms (Lumbricidae).

Mouth → pharynx → esophagus → crop → gizzard → intestine → anus

Decaying organic matter is moistened by the mouth and sucked in by the pharynx like a straw. The crop stores swallowed food until it passes into the gizzard to be separated from soil particles and ground up. The intestine does all the digesting and absorbing, utilizing its typhlosole (tissue that extends into the lumen and increases surface area) and specialized cells lining the intestinal wall known as chloragogen (synthesize glycogen and fats that aid in nutrient transport).

Describe, in detail, the reproduction and copulatory behavior of earthworms (Lumbricidae).

Most earthworms are monoecious and utilize simultaneous outcrossing, wherein sperm is deposited in BOTH individuals.

Somites 9-15 contain the reproductive tissues. S15 releases sperm to be received in receptacles by S9 and S10. S14 releases eggs.

In copulation, individuals line up in opposite directions. The sperm they release travels posteriorly in a seminal groove to the receptacles of their mate, where it is stored for fertilization.

Post-copulation, a cocoon forms over the clitellum and moves anteriorly. It passes over S14 to receive eggs. Once the cocoon passes over S9 and S10, the eggs within are fertilized. The cocoon will keep sliding until it drops off the anterior end of the worm, and the zygotes within will eventually mature and hatch.

Leeches are _____-feeders.

fluid (blood)

How many segments do most leech species have?

34, although annuli make it LOOK like more

Annuli

areas on a leech where it looks like there should be an interior septa; typically 3 per segment

True or False: Leeches typically have reduced or no septa.

True; remember, the coelom is filled with connective tissue

What adaptations do leeches have to accommodate being fluid-feeders?

1. Gut specialized for storing fluids and blood (stretchy!)

2. Many pouches/ceca off the intestine

What do leeches use for locomotion?

Leeches lack setae, so instead they use suckers to move around inchworm style

How many suckers do leeches have?

2; one anterior and one posterior

Where are the sensory organs located in a leech?

The central annulus of each segment