Phylum Annelida

• Name means segmented worms
• Food enters the digestive tract through the mouth, muscular pharynx is used to capture food (carnivores can extend pharynx through the mouth, herbivores‘ pharynx jaws can be used to cut pieces of plants, detritus feeders like earthworms have a sticky pharynx with no jaws covered in mucus, extends through mouth where food sticks to it then pulled back in mouth, parasites’ pharynx functions like a pump
• Examples: earthworms, leeches
• General characteristics: annulus means ”rings”, body segments are separated by the internal walls (septa), almost all body segments are identical, size can be up to 3m in length, digestive tract has 2 openings (mouth and anus) which spans the entire length of body.
• Food travels through the crop where food is stored (if necessary), food travels through the gizzard where mechanical digestion occurs (breaks food into smaller pieces), small food particles are fully digested in the intestine, nutrients enter the worm’s blood by diffusion across the thin intestinal wall
• undigested food (waste) exits through the anus
• aquatic annelids use diffusion to exchange gases (O2 and CO2) to/from the water
• terrestrial annelids: gas exchange occurs through their skin by diffusion, so skin must remain moist for gas exchange to occur (must live in dark or moist environments, epidermis secretes a protective coating called a cuticle (holds moisture in)
• blood is pumped by a. five hearts. thick, muscular ring vessels located in the forward body segments. b. movement. movement contractions cause blood circulation
• Closed circulatory system (blood flows through blood vessels). Composed of two major blood vessels: dorsal blood vessel (moves blood towards the head), ventral blood vessel (moves blood toward the tail)
• cellular wastes/liquid wastes: nephridia (tube-shaped excretory organs) remove wastes, and excrete them directly outside the worm through excretory pores.
• well-developed nervous system and brain, brain is located above the digestive system at the anterior (head) end.
• free-living marine annelids have developed many simple sensory organs: sensory tentacles, chemical receptors, statocysts, eyes, various sensory cells located at random locations on body
• free-living terrestrial annelids (earthworms): typically do not have complex sensory organs
• have two groups of muscles: longitudinal (anterior-posterior muscles that cause worm to change length), circular (muscles that control the thickness of the worm)
• asexual reproduction: very few species can reproduce asexually by fragmentation
• external fertilization (most aquatic species): sexes are separate, sperm and eggs are released directly into the water, fertilized egg develops into a swimming larvae (trochophore)
• internal/external fertilization (earthworms): Hermaphrodites, adult worms swap sperm with each other and store it, clitellum produces a tube made of mucus and chitin, eggs are deposited in the tube, tube slides up worm, sperm added = fertilization
• class Polychaeta ("many bristles"): aquatic, bristles appear to cover the entire body (looks like fur or hair)
• class Oligochaeta ("few bristles"): burrow into, and eat soil (scavengers), bristles help burrow and move through soil
• class Hirudinea (leeches) - have no bristles: most are short, freshwater, external parasites which suck the host's blood or body tissues